724results found.

  • (L2, L3), (M4, M5)…spectra

    Absorption edges characteristic of elements, which appear in an energy region higher than 50 eV in an EELS spectrum. These absorption edges arise due to excitations of inner shell electrons to the conduction band. They are called "K, L, M…" shell excitation spectra depending on the excited inner shell. The inner-shell levels have fine structures due to the spin orbit coupling. The split levels are expressed as K(1s1/2), L1(2s1/2), L2(2p1/2), L3(2p3/2), M1(3s1/2), M2(3p1/2), M3(3p3/2), M4(3d3/2), M5(3d5/2), …. Since the difference of the L2 & L3 energy levels of 3d transition metals is 5 to 20 eV, two spectra with similar shape successively appear with the energy difference in the EELS spectrum. For Si and Al, the L2 & L3 spectra form a non-separated absorption-edge spectrum because the energy separation of the L2 & L3 levels is 1 eV or less. Thus, they are denoted as L2,3. The intensity ratio of the L2 & L3 spectra is expected to be 1:2 from the occupation ratio of the levels. However, the ratio experimentally observed is different from the ratio expected due to non-flat configuration of the conduction band and the core-hole interaction. In the case of the excitation of M shell electrons of 4d transition metals, M4 and M5 spectra appear successively with an energy difference of 2 to 10 eV.

  • (off-axial) coma aberration

    Electron beams exit from a point that is not located at the optical axis on the object plane at various angles with respect to the optical axis do not come to one point on the image plane after passing through the lens but produce a cone-shaped (comet-shaped) image. This phenomenon is called "(off-axial) coma aberration." In this case, the vertex angle of the cone (divergence angle of the tail of the comet) is 60°. This is one of five Seidel aberrations which are inherent to the lens. It is noted that this is different from the axial parasitic coma aberration. Coma aberration is theoretically the next most important aberration to the spherical aberration for the objective lens. Although an example of off-axial coma correction has been reported, the effect of coma aberration is small for high magnification images. The name of "coma" originates from "comet."

  • (oil) diffusion pump

    The (oil) diffusion pump heats the oil to a high temperature. The oil vapor is sprayed out at a high speed from the nozzle. The residual gas molecules are carried with the aid of the jet flow of the vapor. The back side of the pump is evacuated with a rotary pump. The pump mechanism is simple and its price is low. Since the diffusion pump operates from 10-1 Pa and its evacuation speed is high, the pump is used for evacuation of the TEM camera chamber with a large evacuation volume and large gas emission. The working pressure is from 10-1 to 10-8 Pa. When oil-free, high vacuum is required, the pump cannot be used. The pump is not suitable for pumping H2O.

  • (oil) rotary pump

    The (oil) rotary pump rotates a rotor in a case at the hermetic state maintained using oil to aspirate gas molecules into the inlet, and the gas molecules are compressed to open a valve at the outlet, then gasses are ejected to the air. Since the rotary pump operates from atmospheric pressure (105 Pa), the pump is used for rough pumping of a TEM and for evacuation of the back side of the (oil) diffusion pump and the turbo-molecular pump. The working pressure is 1055 to 1 Pa. If a long-time evacuation is continued at the evacuation limit, reflux of the oil vapor of the pump to the vacuum chamber occurs. Thus, the long-time evacuation by the pump should be avoided. When an oil-free pump is required, the scroll pump is used.

  • 5th order aberrations

    "Five Seidel aberrations" are proportional to the cube of α (angle between the incident electron beam and optical axis) and r (distance of the electron beam from the optical axis). "The 5th order aberrations" or so called "nine Schwarzschild aberrations" mean the aberrations of the next order to the Seidel aberrations or those proportional to the fifth power of α and r. Due to the development of Cs correctors, the third-order spherical aberration and third-order axial (parasitic) aberrations have nowadays been successfully corrected. The fourth-order axial (parasitic) aberrations can be corrected by the alignment of the optical axis. Thus, the fifth-order axial aberrations must be considered to decrease blurring of an image. If we have a Cs corrector with transfer lenses, the value of the fifth-order spherical aberration C5α5 is adjustable, thus C5α5 = 0 being possible. In the case of two-stage hexapole type correctors, efforts to minimize the axial (parasitic) 6-fold rotation astigmatism among the 5th order aberrations have been made.

  • Auger electron

    When an atom at an excitation state makes a transition to the ground state, if its energy is not used to emit characteristic X-rays but used to emit an electron in the atom, the emitted electron is called "Auger electron." The Auger electron energy is characteristic of an element and the escape depth of the Auger electron is very small (0.5 nm to several nm). Thus, the Auger electron is utilized for qualitative and quantitative compositional analysis and electronic structure analysis (analysis of chemical-bonding states) on top surfaces of solids. The accuracy of Auger electron spectroscopy is about 10%.

  • Autoradiography (Autoradiography in Electron Microscopy)

    Autoradiography in Electron Microscopy is a method to observe a specific site of a biological specimen labeled with a substance containing a radioactive isotope. The method is implemented in the following procedure.
    A substance containing a radioactive isotope is doped into a biological specimen for labeling specific tissues or cells. The biological specimen is thinned down to an ultrathin section and a photo-sensitive emulsion (silver halide suspension) is applied to the thin section. Silver halides in the vicinity of the labeled sites are exposed with β rays emitted from the radioactive isotope. When photo-developed, silver particles are segregated at the labeled sites. When the section is observed with a transmission electron microscope, the positions of the labeled tissues or cells can be identified from the localized silver particles.
    In order to perform high-resolution observation of the sites of the labeled tissues or cells, tritium (which emits small energy β-rays) is often used as a radioactive isotope because tritium causes small silver segregates in the photosensitive emulsion.
    An example of Autoradiography in Electron Microscopy: Thymidine containing radioactive tritium is applied to label the sites where cell divisions are active in a biological specimen. The labeled sites are revealed from the segregated silver particles by observing the electron microscope image of the specimen.

    Autoradiography

  • B-A (vacuum) gauge

    A thermal-cathode ionization gauge. In the B-A (vacuum) gauge, a heated filament emits electrons and the emitted electrons are accelerated, and these electrons ionize residual gasses, then the produced ion current is measured. The B-A gauge measures the pressure of the middle to high vacuum region. The measurable range is 0.1 to 10-5Pa. The gauge can measure lower pressures than the Penning gauge can. Since the gauge is made of a glass tube, it is likely to be damaged. In addition, the gauge has a gas emission problem. Thus, the B-A gauge adopts, in many cases, a design where the gauge head is directly exposed to the vacuum. This is called a nude gauge (measurable pressure: ~10-9Pa). The output current is proportional to the pressure. For a TEM, the gauge is mainly used to measure the pressure in the ultra-high vacuum specimen chamber.

  • Bethe Ridge

    "Bethe Ridge" is a tail-like peak (ridge) which appears in the expression of energy loss against scattering angle, E(θ) for the collision between incident electrons and quasi-free electrons in a solid. Taking account of a similar phenomenon in the case of X-ray scattering, Bethe Ridge is also called "Compton peak." In a treatment of classical dynamics, the position of Bethe Ridge is expressed as E/E0~sin2θ,where E0 is the energy of incident electrons. Bethe Ridge is observed by taking angle-resolved EELS spectra.

  • Bethe's method

    A method for calculating the intensities of transmitted and diffracted waves at the bottom plane of a crystalline specimen when the incident electron beam interacts with the specimen. In the "Bethe's method," the energy of the electron wave is given, and the states of the electron waves (wave number vectors) allowed in the crystal are obtained by the Schroedinger equation, and finally the amplitudes of transmitted and diffracted waves at the exit plane are obtained by connecting these waves to the incident electron wave using the boundary condition. In this method, multiple scattering (dynamical diffraction) is taken into account. Since the equation to obtain the wave vectors takes the form of a matrix, the method is termed the matrix method or the eigenvalue method.

  • Bloch wall

    "Bloch wall" is one type of the boundary structure of magnetic domains whose magnetization directions are antiparallel or are different by 180°to each other. The magnetic dipoles continuously rotate in planes parallel to the magnetic boundary and finally connect to those at the adjacent magnetic domain with opposite magnetization. This structure is formed in a bulk specimen with a thickness of more than 100 nm. The thickness of the Bloch wall is about 50 nm for iron. In a diffraction pattern formed by two adjacent magnetic domains containing a Bloch wall, a diffuse intensity line appears connecting the diffraction spots from the two domains.

  • Bloch wave

    The incident plane electron-wave to a crystal cannot exist as a single plane wave but as the waves composed of the incident wave and reflected waves due to strong coupling of those waves (dynamical diffraction effect). The waves are called Bloch waves. When the incident wave and one reflected wave are considered (two-beam approximation), two Bloch waves, the linear combinations of the two plane waves, are produced in the crystal. One Bloch wave is localized on atomic columns and another Bloch wave is localized between atomic columns.

  • Boersch effect

    When the current of electrons emitted from the electron gun is increased, Coulomb interactions between the electrons make an increase of the energy spread of the electrons. This phenomenon is termed "Boersch effect," which gives rise to the increase of the chromatic aberration.

  • Born approximation

    If the potential energy in a crystal is much smaller than the incident electron energy, the scattering event can be assumed to occur only one time in the crystal and the amplitude of the incident electron wave is not attenuated in the crystal. To calculate the amplitude of the scattered wave under such an approximation is called Born approximation.
    When the scattered wave in a crystal is calculated as the solution of the integral form of Schroedinger equation, the scattering amplitude is proportional to Coulomb potential at the point where the scattering event occurs, and the amplitude of the electron wave incident at the point. Under Born (the 1st Born) approximation, the scattering amplitude is calculated by replacing the amplitude of the electron wave falling on the point with that of the incident electron wave to the crystal. The scattering amplitude of the electron wave is given by the Fourier coefficient of the crystal potential.

  • Bragg reflection

    When a lattice plane (of a crystal) is situated at a specific angle with respect to an incident electron beam, this lattice plane reflects the electron beam as if the plane acts as a mirror. Incident electrons hit each constituent atom in the crystal and then, these electrons are scattered in various directions and interfere with each other. At this event, only the electrons that satisfy the Bragg condition interfere constructively, resulting into a diffracted wave (reflection line) with a strong intensity in a specific direction. (Electron waves, which travel in other directions, interfere destructively and disappear.) Such electron reflection is termed "Bragg reflection" and a diffraction pattern is formed on a back focal plane of the objective lens in a TEM.

  • Brillouin zone

    "Brillouin zone" is defined in the wave number space (reciprocal space) as a region separated by perpendicular bisector planes of reciprocal lattice vectors drawn from the point of origin. Bragg reflections take place on the boundaries of the Brillouin zones, and the incident electron with a constant energy undergoes dispersion in their wave numbers, thus dispersion surfaces are produced. Since the dispersion surfaces exhibit the periodicity of the crystal lattice, the change of wave numbers (dispersion surfaces) due to dynamical diffraction is enough to be calculated only in a Brillouin zone.

  • Burgers vector

  • CAT(composition analysis by thickness-fringe)method

    The "CAT" method is devised to determine the compositions of layer materials that have the same lattice spacing but have different compositions between the layers, like an artificial lattice material of AlxGa1-xAs. Utilizing the fact that the distance between thickness fringes (extinction distance) is inversely proportional to the crystal structure factor, this method determines the compositions of the different layers by measuring the difference of the extinction distances between the layers of a wedge-shaped specimen.

  • Castaing-Henry filter

    One of the in-column type energy filters, which is installed between the intermediate and projector lenses in a TEM. The filter consists of an isosceles-triangle electromagnet and an electrostatic mirror that reflects incident electrons to reverse their traveling direction. Since the "Castaing-Henry filter" exerts an electrostatic potential, the incident-electron energy is limited to 80 keV. Its energy dispersion is ~1 μm/eV for a 80 kV electron beam.

  • Cc corrector

    The Cc corrector makes the chromatic aberration of the image-forming lens or the probe-forming lens to be 0 (zero) by producing a negative chromatic aberration. To produce the negative chromatic aberration, the corrector uses a quadrupole field formed by the superposition of an electrostatic field and a magnetostatic field. Multi-stage multi-poles are used to create the quadrupole field.
    The cylindrically symmetric convex lens of the magnetic field type or the electric filed type, which is used for electron microscopes, always possesses a positive chromatic aberration. Thus, electrons having a lower velocity are more largely deflected toward the convergence direction (inward).
    On the other hand, the quadrupole field of the Cc corrector exhibits a negative energy dispersion, which is opposite to that of the convex lens. Thus, the Cc corrector makes the electrons having a lower velocity to deflect more largely toward the divergence direction (outward). Therefore, incorporation of the Cc corrector into the convex lens system enables the positive chromatic aberration of the lens system to cancel with the negative chromatic aberration produced by the quadrupole field, making the chromatic aberration of the entire lens system of the electron microscope to be zero.

  • Cherenkov radiation

    When a charged particle running in a material (medium) exceeds the velocity of light in the medium(c/n, c is the velocity of light in vacuum, and n is the refractive index of the medium), light is emitted from the medium. This is called Cherenkov radiation.
    As shown in Fig. 1, electric polarization arises around the charged particle that enters the medium, and light is created when the electric polarization disappears after the particle passes. The light from each point along the particle trajectory forms a uniform wave-front in a specific direction and a light with a sharp directivity (Cherenkov radiation) is emitted when the particle runs faster than the velocity of light in the medium. This phenomenon is similar to the shock wave generated by a supersonic flying object. When the velocity of the charged particle is slower than the velocity of light in the medium, the phases of the light are not matched in any direction and Cherenkov radiation is not generated.
    Cherenkov radiation generated by electron incidence has the following properties. Here, n is the refractive index of the medium, v the velocity of electron, c the velocity of light in vacuum, and β=v/c.

    • The condition for the generation of Cherenkov radiation is given by v>c/n or nβ>1. (Critical velocity)
    • The radiation angle θ is determined by cosθ=1/.
    • The spectral intensity of the radiation I(λ) shows the dependence of λ-2.

    For electrons accelerated at an accelerating voltage of 100 kV (200 kV), β=0.55 (β=0.70), Cherenkov radiation is generated when the refractive index of the medium is larger than 1.8 (1.4).
    Fig. 2 shows the Cherenkov radiation spectra from a mica thin film observed in an electron microscope for different accelerating voltages of the incident electrons. Since the refractive index of mica is 1.59, the acceleration voltage that gives the critical velocity is 146 kV. It is seen that radiation is not observed at an accelerating voltage of 120 kV, but that strong radiation appears at 160 kV and 200 kV.
    It is noted that a counting device using Cherenkov radiation is used for detecting neutrinos and other elementary particles in the field of high energy physics.

    (By Dr. Naoki Yamamoto, Tokyo Institute of Technology)

    Cherenkov radiation

    Fig. 1

    Generation mechanism of Cherenkov radiation.

    Cherenkov radiation

    Fig. 2

    Cherenkov radiation spectra from a mica thin film at different accelerating voltages, (a) 200 kV, (b) 160 kV, and (c) 120 kV.

  • Cliff-Lorimer method

    The Cliff-Lorimer method is a qualitative measurement method of elements in spectroscopic analysis of characteristic X-rays (EDS). It is also called the thin-film approximation method. The method is applied when a specimen thickness is 10 nm or less (though depending on measured elements). For example, when a substance is composed of two elements A and B, characteristic X-ray intensities IA and IB are measured. Then, the concentration ratio of element A to element B (CA/CB) is obtained from equation CA/CB = k・IA/IB, where k is a proportionality factor, which is determined by ionization cross sections, fluorescent yields etc. of the elements. If a specimen is thin, quantitative measurements can be performed with a relatively high accuracy even when corrections for the atomic-number effect, the absorption effect and the fluorescence excitation effect are neglected. On the other hand, if a specimen is thick, the corrections must be executed for the measured intensities (ZAF correction).

  • Cockcroft-Walton high-voltage circuit

    A multi-stage circuit that combines rectifiers and capacitors to generate a stable high DC voltage from an AC voltage. The "Cockcroft-Walton high-voltage circuit (CWC)" is used for reducing voltage fluctuations of the high-voltage power supply (high-voltage generator).

  • Cornu spiral

    "Cornu spiral" is a graphical presentation of the diffraction amplitude produced by Fresnel diffraction (Fresnel integral) on the complex plane (Gaussian plane) with the integration range (length from the source to the observation point) as a variable, resulting in a spiral shape. The diffraction intensity obtained by Fresnel diffraction is calculated as the square of the length of the line drawn from the start point (corresponding to the position of the source) to the end point (the observation point) of the Cornu spiral.

  • Cs corrector

    The "Cs corrector" produces a negative spherical aberration coefficient (Cs) to cancel positive Cs of the objective and condenser lenses, which are axially-symmetric magnetic field lenses. The following Cs correctors are now in practical use. 1) One consists of two hexapoles with opposite polarity and transfer lenses that connect the hexapoles. The first hexapole produces a negative Cs. The unnecessary three-fold distortion produced by the first hexapole is compensated by the second hexapole. The value of negative Cs is doubled by the second hexapole. 2) The other consists of three pairs of elements combining octupoles and quadrupoles. Negative Cs is produced in the X direction by the first element, in the Y direction by the second element and the intermediate direction by the third element, respectively. Cs correction of the objective lens achieves a higher resolution TEM image. Cs correction of the condenser lens leads to a smaller, higher-intensity electron probe, thus, enabling us to obtain a higher-resolution HAADF image and to perform elemental analysis of one atomic column.
    Figure shows the schematic of a Cs corrector and its basic action. The corrector, which consists of two hexapoles with opposite polarity and transfer lenses connecting the hexapoles, produces a negative Cs and cancels a triangle shape of the beam caused by the first-stage hexapole field using the second-stage hexapole field.

    球面収差補正装置(CS corrector)
    Fig.Two-stage hexapole Cs corrector
    Electron trajectory in hexapoles or dodecapoles (upper left) and schematic of lens configuration (lower tight)

    In a two-stage hexapole Cs corrector, the thick hexapole field produces a negative Cs, and this action is used to correct the positive Cs of the objective lens. Upper left figure shows the cross section of the electron trajectory in the hexpoles for every 10 mrad angle (from inside blue line to outside red line). The first-stage hexapole field creates a negative Cs but an unwanted triangle shape trajectory. The second-stage hexapole field with the opposite polarity again creates the same amount of negative Cs and the opposite sense of triangle to cancel the triangle shape trajectory caused by the first hexapole, As a result, the amount of negative Cs is doubled and a cylindrical symmetric trajectory is retrieved. Owing to the negative Cs, the cross section of the trajectory expands outwards (shown in red), compared to the trajectory for the case of no hexapole field (in green). With the use of this negative Cs, the positive Cs of the objective lens is canceled. It should be noted that, the transfer lens (lower right figure) is used to transfer an electron beam emerged from the first-stage hexapole into the second-stage hexapole while preserving the beam shape. In the upper left figure, the action of the transfer lens is omitted.
     

  • Debye-Scherrer ring

    "Debye-Scherrer rings" are concentric diffraction rings produced by Bragg reflections, which are obtained when polycrystalline thin films are illuminated with a highly parallel electron beam.

  • Debye-Waller factor

    A factor that expresses an effect of thermal vibrations (lattice vibrations) of atoms. The intensity of Bragg reflections at high angles is damped by the "Debye-Waller factor."

  • Diffractive imaging

    "Diffractive imaging" is a method to reconstruct a structure image of a specimen from a diffraction pattern of the specimen. Since the diffraction pattern is less influenced by lens aberrations, the resolution of the structure image obtained is determined by the maximum diffraction angle of the diffraction pattern, thus a higher-resolution structure image (amplitude image and phase image) than a HREM image (taken by using lenses) can be obtained. This method has actively been studied in the field of X-ray diffractometry, the method being called "Coherent Diffractive Imaging." In electron microscopy, the method is called "Diffractive Imaging" or "Diffractive Microscopy." The method has been applied to carbon nanotubes, etc., and a spatial resolution around 0.1 nm has been obtained. In addition, the method can be applied to not only crystals but also non-periodic structure specimens such as a single-molecule specimen. To reconstruct the image, the Fourier repetitive phase recovery method is used. That is, the magnitudes of the diffraction amplitudes are calculated by the square root of the diffraction intensities taken from a specimen, and random initial phases are given to the diffraction amplitudes. Then, the diffraction amplitudes with the phases are Fourier-transformed to obtain an approximate structure image. The obtained image exhibits structures even in areas exceeding the specimen area from which the diffraction pattern was taken. When the external shape of the specimen is clearly determined, the image intensities from areas except for the external dimension are set to be 0 (zero) (real-space constraint conditions). (On the other hand, when it is difficult to accurately determine the external shape, a specimen area (called "support") that is slightly larger than the external dimension is defined, and the image intensities from areas exceeding the support are set to be 0. Then, the image is inverse-Fourier-transformed to a diffraction pattern. The diffraction amplitudes that do not match the experimental amplitudes are replaced with the experimental values (reciprocal-space constraint conditions), and then Fourier transform is applied again to obtain a structure image. As this procedure is repeated, the true phases of the specimen are gradually recovered and finally, a true structure image of the specimen is obtained. The number of iterations until the recovery of the true phases exceeds several thousands of times. The accuracy of the obtained image is influenced by the parameters such as inelastic scattering contained around the origin of the diffraction pattern, noise from the detection system including its electronic circuits, and the external dimension of the support. It should be noted that when taking the original diffraction pattern, an area of twice the specimen area (support) must be illuminated with an electron beam. This means that the diffraction pattern is sampled with steps twice as small as those corresponding to the original specimen size, enabling us to extract all information contained in the specimen, which is called the over-sampling condition. In an actual experiment, it is needed to create an area where no specimen exists around the specimen and to record a diffraction pattern so as to satisfy the over-sampling condition.

    Diffractive_imaging

    Conceptual diagram of the Fourier repetitive phase recovery method for diffractive imaging. A crystal structure image of a specimen is retrieved from an experimental diffraction data with a good accuracy by applying Fourier transform to the reciprocal space data and then inverse Fourier transform to the obtained real space data repeatedly with giving constraint conditions at each step. To be precise, 1) First, a wave field in the reciprocal space is created by using the diffraction amplitudes obtained from a diffraction pattern and giving random initial phases to the diffraction amplitudes. 2) The wave field formed in the reciprocal space is Fourier-transformed to an image in the real space. 3) The intensities of the image outside the support area are set to be 0 (zero). 4) The corrected image is inverse Fourier-transformed to a wave field in the reciprocal space. 5) The amplitudes of the wave field are replaced to those of the experimental diffraction pattern. Repetition of this cycle enables us to accurately retrieve the phases of diffraction spots in the reciprocal space and the wave field in the real space. 6) Finally, an accurate structure image of the crystalline specimen is reconstructed.

  • Drude model

    A model that treats oscillations of free electrons in a solid when an electric field is applied from outside. The "Drude model" allows plasma oscillations to be derived.

  • Dwell time

    "Dwell time" means a time for an electron beam to stay per pixel at acquisition of a STEM image by electron-beam scan. The Dwell time is an index for the scan speed of the electron beam.
    When the Dwell time is multiplied by the number of scan pixels in one horizontal line scan, and then the Flyback time is added, the scan time for one horizontal line scan is obtained. When this scan time is multiplied by the number of the vertical scans (horizontal scan lines), the acquisition time of one scanning image is calculated.

  • Dynamic TEM

    In an ordinary TEM, the time resolution of image recording is limited to 1/30 seconds (TV rate). However, the Dynamic TEM (DTEM) achieves a much higher time resolution of the order of nano seconds to femto seconds by the combined use of an electron source generating electron pulses and a fast recording system. A DTEM is equipped with a laser-pulse-driven photoelectron source and a pulse-laser specimen-irradiation system. By synchronizing the electron beam pulse with the pulse laser for the specimen, one can observe chemical reaction processes and crystallization processes with such a high time resolution.

  • Environmental TEM

    The Environmental TEM (ETEM) is a TEM which enables us to observe specimens under a gas-controlled environment. A volume around the specimen is filled with gases so that the pressure of the volume is kept higher than that of the TEM column. The ETEM is classified into two types; isolation-film type and differential pumping type. The former ETEM is equipped with an (gas) environmental cell (EC) in the specimen holder. A passage for gas introduction and evacuation is connected to the EC. There are holes above and below the EC to allow electron-beam transmission. To prevent gas leakage into the microscope column, thin films such as carbon or silicon nitride films (called "insulation films") are sealed to the holes. The latter ETEM is designed to introduce gasses into the specimen chamber in the microscope column. To prevent vacuum deterioration of the column caused by gas diffusion, differential pumping is adopted. For example, multiple orifices are incorporated into the upper and lower parts of the polepieces (on the optical axis). This construction enables differential pumping of spaces isolated by the respective orifices. The ETEM is used for in-situ observation of reaction processes between the specimen and the introduced gasses, observation of water-containing specimens, etc.

  • Ewald sphere

    The Ewald sphere is a sphere of the radius defined as the reciprocal of the wavelength of the incident wave 1/λ, and is drawn with the point as the center, the point being at the length 1/λ from a certain reciprocal lattice point along the direction of the incident wave to a specimen crystal. The Ewald sphere explains what Bragg reflections occur using the relation between the incident wave vector and the reciprocal lattice points. All of the reciprocal lattice points on the Ewald sphere satisfy the Bragg condition. For a high energy incident electron (100 or 200 keV), the Ewald sphere can be approximated as a flat plane because the radius of the Ewald sphere is much larger than the distance between the reciprocal lattice points. As a result, the reflections, which appear as cross sections between the Ewald sphere and the reciprocal lattice points, can be indexed easier than the indexing in X-ray diffraction.

    Ewald sphere

    The reciprocal lattice points of gold (Au) for [001] incidence (lattice spacing d = 0.204 nm) and the Ewald spheres. The small sphere is the Ewald sphere for X-ray of MoKa (λ = 0.07109 nm) and the large sphere (arc) is the Ewald sphere for an electron beam of 200 kV (λ = 0.002508 nm).
    The Ewald sphere for the electron beam is displayed up to ±10°, which corresponds to a usual limitation of the acceptance angle of an electron microscope. Blue reciprocal lattice points approximately satisfy the Bragg condition.

  • Faraday cup

    A tubular metallic electrode to directly receive an electron beam. "Faraday cup" is made to a cup shape to efficiently capture incoming electrons.

  • Fermat's principle

    "Fermat's principle" states that light (electron beam) travels through a path whose optical path length is shortest (extremum). This principle gives the basics for discussing optics and electron optics. The law of reflection and the law of refraction are derived from the principle.

  • Fermi level

    "Fermi level" is the highest energy level occupied by electrons in the ground state of a crystal. That is, at the ground state, no electrons occupy the levels above the Fermi level.

  • Finite element method

    "Finite element method (FEM)" is one of numerical analytical methods to obtain an approximate solution of partial differential equations that are difficult to solve analytically. First, an object of interest is divided into elements that each has a simple shape and a finite size. Next, physical quantities (temperature, stress, etc.) of each element are approximated by a simpler equation, and then the equations for the elements are combined to construct simultaneous equations. By solving the obtained simultaneous equation under the boundary conditions of the physical quantities at surfaces of the elements, the distribution of the physical quantities over the object are obtained. Since an object is subdivided to polyhedrons, FEM can be conveniently applied to complicated-shape objects. In electron microscopy, the method is used for calculation of mechanical strength and thermal distribution, calculation of distributions of magnetic fields and electrostatic fields of magnetic lenses and electrostatic lenses, etc. In the development of lens polepieces, aberration coefficients are obtained by calculation of electron trajectories using the magnetic field distributions obtained by FEM, and then the shapes of magnetic poles are optimized.

  • Five Seidel aberrations

    Aberrations are departures of the path of electron beams from the path of the ideal (Gaussian or paraxial) imaging. The term, "(five) Seidel aberrations," is the generic name of the third-order aberrations (third order with respect to the product of α (angle between the electron beam and optical axis) and r (distance of the electron beam from the optical axis)), which occurs for a monochromatic but non-paraxial electron beam. The five aberrations are (1) spherical aberration (proportional to α3), (2) (off-axial) coma aberration (proportional to rα2), (3) off-axial astigmatism (proportional to r2α), (4) curvature of image field (proportional to r2α), and (5) distortion (proportional to r3). In the case of an electron microscope, since the specimen area magnified by the objective lens, the first-stage lens, is very small (r ~0), it is enough to consider the beams passing through the optical axis. Thus, the spherical aberration is most important for the objective lens. The (off-axial) coma aberration is next most important theoretically. Although an example of coma correction has been reported, the effect of coma aberration is small for high magnification images. Since the magnified images or the objects for the intermediate and projector lenses are not small, the aberrations produced by the beams passing through the positions distant from the optical axis give definite contributions. That is, off-axial astigmatism, curvature of image field and distortion are more important for the lenses at successive stages. In recent years, the spherical aberration has been successfully corrected.

  • Flyback time

    "Flyback time" means the standby time for a scanning electron beam from the end of a horizontal scan to the start of the next scan at swing-back of the electron beam when acquiring a STEM image.
    The electronic circuit responds non-linearly at the beginning of each horizontal line scan, giving rise to the distortion of the scanning waveform in a certain time zone. Only the linear response time zone of the electronic circuit should be used to eliminate distortions of the STEM image. Thus, the Flyback time is set longer than the non-linear response time zone, removing the response time zone with the distorted waveform from the image acquisition. As a result, the STEM image is obtained without distortion.

  • Fourier mask

    The "Fourier mask" is a mask inserted on the diffraction plane to remove the noise of a lattice image or a structure image, allowing only diffraction spots to pass and background noise between the diffraction spots to be cut.

  • Fourier synthesis

    "Fourier synthesis" is a method that adds the phase to the amplitude of diffraction spots and takes the sum of many reflections in crystal structure analysis. This method enables us to obtain the electron density in a real space in X-ray diffraction, and also in electron diffraction where kinematical diffraction can be applied (for example, for biological specimens such as proteins), we can obtain the electron density from the same method. Furthermore, in the structure analysis using electron diffraction, Fourier synthesis enables us to obtain the crystal potential distribution. Finding the crystal potential allows the electron density to be mathematically obtained. In the case of specimens in material science applications, the structure analysis is performed by CBED since these specimens have strong dynamical effects.

  • Fourier transform

    A method that transforms a function of certain variables into a function of the variables conjugate to the certain variables. For example, "Fourier transform" is used for light or sound as a function of time to express it as a function of frequency (spectral analysis), or for scattered waves from an object or a crystal potential as a function of position to transform it into a diffraction pattern or crystal structure factors as a function of wavenumber.

    The imaging lens system of a TEM can be considered as a system in which scattered waves from an object or a crystal potential are Fourier transformed into a diffraction pattern and subsequently inverse Fourier transformed to an image of the object, where the assumption of Fraunhofer diffraction is applied.

    ψ(r) stands for the scattered waves from the specimen, being a function of the real space r, and Ψ(k) for the diffracted waves formed on the back focal plane of the imaging lens, being a function of the reciprocal space k [1/m]

    Here, ψ(r) stands for the scattered waves from the specimen, being a function of the real space r, and Ψ(k) for the diffracted waves formed on the back focal plane of the imaging lens, being a function of the reciprocal space k [1/m]. The first equation expresses Fourier transform. That is, the scattered waves ψ(r) produced by the specimen are transformed to a diffraction pattern Ψ(k). The second equation expresses inverse Fourier transform. That is, the diffraction pattern Ψ(k) is transformed to a specimen’s image ψ(r).

    Fourier transform
    Fig. 1 TEM imaging by a lens (image and diffraction pattern)
    The imaging process of a transmission electron microscope can be considered as a series of Fourier transform from scattered waves from a specimen to a diffraction pattern and of inverse Fourier transform of the diffraction pattern to an image of the specimen. ψ(r) expresses scattered waves of the specimen, being the function of position r, and Ψ(k) expresses diffracted waves formed on the back focal plane of the imaging lens, being a function of the wave number k [1/m]. Φ(x)∝ψ(x/M) expresses the scattered waves on the imaging plane, which is an image of the specimen, being the function of position r. It is noted that Φ(x) is equal to ψ(r) shown in the second equation, except for a magnification M and image inversion caused by the lens.

    Fourier transform
    Fig. 2 (a) Diffraction pattern (Fourier transform pattern) of an Au crystal. (b) Crystal lattice image of the Au crystal and its FFT pattern. Accelerating voltage: 300 kV.

    Periodicities of the crystal lattice appear as bright diffraction spots on the back focal plane of the lens depending on the spacing and orientation of the lattice, by Fourier transform (a). A magnified image of the crystal lattice is obtained from the diffraction spots by inverse Fourier transform (imaging process of the lens) (b).

  • Fourier-log deconvolution

    "Fourier-log deconvolution" is a method to obtain a single scattering spectrum of EELS from an experimental spectrum by eliminating the multiple-scattering effect, where multiple-scattering is assumed to obey the Poisson distribution. R.F. Egerton has introduced the method to the spectrum processing of EELS. Fourier-log analysis has also been used in the signal processing of sound waves.

  • Fraunhofer diffraction

    "Fraunhofer diffraction" means a diffraction phenomenon where both an electron source and an observation point are located infinitely far from an object, thus both incident and exit waves can be regarded as plane waves. When an object is illuminated with a parallel incident beam, a diffraction pattern formed on the back focal plane of the objective lens is called a Fraunhofer diffraction pattern.

  • Fresnel diffraction

    "Fresnel diffraction" means a diffraction phenomenon where either of an electron source and an observation point or both of them located at a finite distance from an object, thus the incident wave or exit wave cannot be regarded as a plane wave. The Fresnel diffraction phenomenon is observed as an interference fringe (Fresnel fringe) appearing at the shadow part of the object by slightly shifting the focus of the objective lens.

  • Fresnel fringes

    "Fresnel fringes" are produced in the following manner. A spherical wave scattered from the edge of a specimen interferes with the incident wave. Then, interference fringes are produced whose period becomes narrow with the distance from the edge of the specimen. The fringes produced at the specimen side cannot be seen practically. The fringe produced just outside the specimen appears bright at an underfocus condition of the objective lens, forming clear edge contrast. On the other hand, at an overfocus condition, a black fringe appears which makes the edge of the specimen image unclear. Thus, it is better to take a low-magnification image at an underfocus condition because the edge of the image is clearly seen at the underfocus. In the broad sense, the Fresnel fringe means the interference fringe produced in the region where Fresnel diffraction is applied.

    フレネル縞:Fresnel fringes
    TEM image of molybdenum oxide mounted on a carbon grid taken at an accelerating voltage of 80 kV.Fresnel fringes appear on the edge of the molybdenum oxide and carbon grid.
    Since the image is taken at an underfocus condition, the Fresnel fringes produced just outside the specimen are observed bright.

  • Friedel's law

    "Friedel's law" states that the intensities of the hkl reflection and the -h-k-l reflection are equal even for polar crystals. This law holds for X-ray diffraction to which kinematical diffraction can be applied; however does not hold for electron diffraction where the dynamical diffraction effect is strong. This fact implies that X-ray diffraction cannot distinguish polar and non-polar crystals unless anomalous dispersion is utilized, but electron diffraction can.

  • GP zone

    "GP zone" means a plate that is composed of solute atoms segregated on a plane. The zone was discovered independently by A. Guinier and G. D. Preston through the analysis of streaks extending from X-ray Laue spots. It is named Guinier-Preston (GP) Zone after the two researchers. In the case of an age-hardened alloy that consists of Al or Mg but includes minute solute atoms (Cu, etc.), when its super-saturated solid solution is rapidly cooled and undergoes an aging treatment at a low temperature, solute atoms (Cu, etc.) are precipitated as one or two atomic planes with lattice matching on the {001} plane of the parent phase. It is noted that they do not appear as precipitates in the equilibrium state. The GP zone causes age hardening of alloys containing Al or Mg. In a bright-field image, the GP zone shows a line contrast due to lattice distortions. In a high-resolution image, the arrangement of solute atoms is directly observed.



    Bright-field image of an AlCu alloy containing GP zones taken at an accelerating voltage of 100 kV.
    GP zones (Cu precipitates) are seen to be bright (white) lines, indicated by white allows, because they do not satisfy Bragg condition.

  • Gatan imaging filter

    One of the post-column type energy filters produced by Gatan, which is installed behind the microscope column. This product consists of a sector analyzer of the magnetic-field type, quadrupole and octupole magnets for image enlargement, and a scintillator and CCD camera for image detection.

  • Gaussian focus

    A focus (imaging) condition for an ideal lens without aberrations. The image plane formed by the ideal lens is called the Gaussian plane.

  • Gaussian function

    "Gaussian function" is a function given by a exp { - (x - b)2 / c2}, where a, b and c are constants. It is used for pre-processing of the background in a spectrum and for fitting of the spectral intensity. Compared with the "Lorentzian function," the Gaussian function damps a little quickly in its tail. Real spectral shapes are better fitted with the Lorentzian function. However, the Gaussian function is conveniently used because it is manipulated mathematically easier than the Lorentzian function.

  • Graphical User Interface

    "GUI" is a user interface that provides intuitive operations using a computer graphics and a pointing device. Since GUI offers user-friendly operation and high visualization, it is widely used as a major interface for commercial OS. In the TEM, GUI is used to facilitate instrument operation and display of image-data. It is, however, noted that GUI is not always effective because different operations cannot be carried out simultaneously.

  • Green's function

    The Green's function, G(r, r'), provides a response at point r from a point scatterer located at r'. The Green's function in the case of the dynamical theory of electron diffraction is (-1/4π)・exp (i k|r - r'|)/|r - r'|.

  • Hough transform

    "Hough transform" is a method to transform a certain line in an image into one point. In analysis of lattice strain by CBED, the lattice strain is determined in such a way that the positions of experimental HOLZ lines are reproduced by computer simulation of the lines with changing lattice parameters. In this determination, if the HOLZ lines are expressed as points (Cartesian coordinates) by the Hough transform, computer programming for fitting can be facilitated.

  • Howie-Whelan equation

    A method for calculating the intensities of transmitted and diffracted waves at the bottom plane of a crystalline specimen when the incident electron beam interacts with the specimen. In the "Howie-Whelan equation," transmission and diffraction are dealt with in the following way. The specimen is divided into many thin layers. The transmitted and diffracted waves are given at the upper surface of a layer (on the top layer, the diffracted wave amplitudes are assumed to be 0 (zero)). These waves undergo transmission and diffraction according to the respective crystal structure factors in the layer. The new transmitted and diffracted waves are obtained at the lower surface of the layer. These waves are incident on the next layer. This process is repeated for successive planes. Finally the amplitudes of the transmitted and diffracted waves at the bottom plane of the specimen are obtained. This method (equation) is used to explain the image contrast due to lattice defects such as stacking faults and dislocations.

  • Image EELS

    "Image EELS" means a TEM image formed by the electrons of a specific energy selected from an EELS spectrum (electron energy-loss spectrum).

  • Kikuchi pattern

    The Kikuchi pattern is a diffraction pattern produced by Bragg reflections of inelastically scattered (thermal diffuse scattering) electrons in a specimen. Since the inelastically scattered electrons distribute over large angles, the Bragg reflections by the inelastically scattered electrons do not form diffraction spots but form a pair of excess and defect lines (Kikuchi lines) respectively by hkl and -h-k-l reflections. The low-intensity (defect) Kikuchi lines appear near side of the direction of the incident beam, whereas the high-intensity (excess) Kikuchi lines appear far side of the direction.
    When low-order reflections hkl and -h-k-l are strongly excited, a high-intensity (excess) band (Kikuchi band) is formed between the reflections due to a strong dynamical diffraction effect. Kikuchi lines appear sharply for a highly perfect and thick crystal. Kikuchi patterns are effectively used for precise adjustment of a crystal orientation by tilting the crystal for the Kikuchi lines to locate on the Bragg reflection spots.

    菊池図形 Kikuchi Pattern

    (a) Kikuch lines: When the electrons inelastically scattered at a certain point O in a crystal cause Bragg reflections from the front face (F) and the back face (B) of a crystal plane quite inclined against the direction of incident electrons I, a pair of defect intensity and excess intensity lines which are called Kikuchi lines (KL) are produced at the positions (directions) of the Bragg reflections 1 and 2. The intensity of Kikuchi line (1) near side of I is lower than that of the surrounding area (directions). To the contrary, the intensity of Kikuchi line (2) far side of I  is higher than that of the surrounding area (directions).
    The amplitude of the electrons inelastically scattered at the point O is large at low scattering angles and becomes small as the scattering angle increases. The Bragg reflection (2) from the front face (F) of a crystal plane due to the electrons inelastically scattered near the incident direction I forms strong excess and strong defect Kikuchi lines. The Bragg reflection (1) from the back face (B) of the crystal plane due to the inelastically scattered electrons far from the direction I compensates the contributions from the front face. However, since the compensation by the latter is small, the low intensity (defect) KL (1) is formed near side of I, and the high intensity (excess) KL (2) is formed far side of I.

    (b) Transformation from Kikuch lines to Kikuchi band: When the inclined angle of the crystal plane becomes small with respect to the direction of the incident electrons I, the difference of the amplitudes of inelastically scattered electrons incident on the reflection planes F and B becomes small. As a result, the bell-shaped intensity of Kikuchi lines becomes low, the symmetrical feature is lost, but an asymmetric (dispersion type) intensity starts to appear.

    (c) Kikuchi band: When the crystal plane becomes symmetric with respect to the direction of the incident electrons (I), the amplitudes of inelastically scattered electrons become equal for the Bragg reflections 1 and 2. As a result, the bell-shaped intensity vanishes and then, the dispersion type intensity is formed due to a strong dynamical diffraction effect. In the angular region between the positions (directions) 1 and 2, the intensity becomes higher than outside the region. This high intensity band is called Kikuchi band (KB).

    (d) Kikuchi pattern obtained from a Si single crystal: Many pairs of Kikuchi lines due to high-order reflections are seen. In the vicinity of a symmetric incidence, a Kikuchi band is seen between “G = 220 reflection” and “G = -2-20 reflection”. The low-intensity (defect (dark)) Kikuchi lines appear near side of the incident beam direction, whereas the high-intensity (excess (bright)) Kikuchi lines appear far side of the direction. (However the Kikuchi pattern below is shown with reversal of bright and dark.)

    Si単結晶からの菊池図形:Kikuchi pattern obtained from a Si single crystal
     

  • Koehler illumination

    A beam illumination method, which illuminates the specimen with a parallel electron beam by increasing the excitation of the condenser mini lens so that the incident electron beam is focused onto the pre-focal point of the pre-magnetic field of the objective lens. This illumination is used for the observation of bright-field image, dark-field image and HREM image. If the parallel illumination onto the specimen is failed, the diffraction condition becomes different depending on the specimen position. This may mislead interpretation of the image.

  • Kramers-Kronig relation

    A formula that gives the relation between the real and imaginary parts of the response function in linear response theory. In EELS, "Kramers-Kronig relation" is used when obtaining a dielectric function from the loss function of a substance.

  • Larmor rotation

    When magnetic-field components vertical to a velocity component of an electron exist, the electron undergoes a rotary motion vertical to the the magnetic-field components. This rotary motion is called “Larmor rotation.” 
    In a transmission electron microscope (TEM), when an electron transmitted through a specimen travels in the vertical direction (from above to below), the electron undergoes Lamor rotation due to the horizontal magnetic-field components of the magnetic-field lenses. Since the imaging system uses multi-stage magnetic-field lenses, the enlarged image of a specimen is formed onto the fluorescent screen with a certain rotation against the specimen. The rotation angle is determined by the accelerating voltage of the electron and the total ampere turns of the magnetic-field lenses.

  • Laue condition

    A condition for a wave of an atomic scale wavelength, such as X-ray, electron and neutron, to cause diffraction by a crystal lattice. The "Laue condition" is equivalent to the Bragg condition. The Bragg condition is an instinctive scalar expression in the real space. On the other hand, the Laue condition is a vector expression in the reciprocal space, which is indispensable for advanced theoretical treatment of diffraction.

  • Laue function

    The "Laue function" expresses the interference effect (diffraction intensity) of an electron (an X-ray and a neutron) due to a periodic arrangement of the unit cells in a crystal as a function of deviation from a Bragg angle. The angular width of diffraction depends on the number of the unit cells in the crystal when kinematical diffraction can be applied. As the number of unit cells in a crystal is greater, the diffraction peak becomes sharper. The Laue function tells us that the subsidiary maxima can be observed together with the Bragg peak (principal maximum) but they cannot usually be seen because the specimen normally used contains many unit cells. The Laue function cannot hold for a thick specimen to which dynamical diffraction must be applied. In this case, the angular width of diffraction depends on the magnitude of the crystal structure factor and strong subsidiary maxima appear.

  • Laue zone

    Laue zones are defined as the reciprocal lattice planes perpendicular to the direction of the incident beam. The Laue zone containing the origin (reciprocal lattice point corresponding to the incidence point) is called the zeroth-order Laue zone, and Laue zones of the n-th-order counted from the origin to the opposite direction of the incident beam are called the n-th-order Laue zones. The diffraction pattern is considered as the section of the reciprocal lattice points on the Laue zones cut by the Ewald sphere. The reciprocal lattice points belonging to the same Laue zone appear in a circular or an arc form in a diffraction pattern. The radius of the circle or arc is larger for higher-order Laue zones.

  • Lichte focus

    When a defocus amount is set at a value larger than Scherzer focus, a spatial frequency that leads the envelope function to 0 (zero) is increased. Then, the contrast transfer function starts to oscillate between positive and negative values at a lower spatial frequency than at Scherzer focus. In the case of reconstruction of an image by electron holography, the amplitudes with negative phase in the oscillation section can be added to the amplitudes with positive phase by converting the sign of the phase. In image reconstruction by holography, a structure image with a higher spatial resolution can be obtained by setting a defocus amount to a value twice or three times more than Scherzer focus. Such a defocus amount is named "Lichte focus" after the inventor of this technique.

  • Line synchronization

    "Line synchronization" means a synchronization of the timing to start electron-beam scan in each horizontal direction with the phase of the AC power wave when acquiring a STEM image. When the line synchronization function is operated at the image acquisition, the influence of external disturbance with the frequency (or the multiples of the frequency) of the AC power-source becomes same on every scan line, and then the external disturbance happening in each scan line apparently disappears. As a result, the fine noise in the horizontal scan line disappears, and the influence of the external disturbance is reduced from the STEM image.

  • Lorentz electron microscopy

    A method to observe magnetic domain structures of a ferromagnetic material using a TEM. Electrons passing through a ferromagnetic material undergo a Lorentz force that depends on the magnetization direction, thus their traveling direction changes (electrons are deflected). Adjoining magnetic domains experience different deflections, thus producing contrast between the magnetic domains. "Lorentz electron microscopy" has two modes: Fresnel mode and Foucault mode. In the Fresnel mode (defocus mode), the deflected electron-beams from the adjoining domains are superposed at the domain boundary, thus boundaries are observed as the light or dark line. In the Foucault mode (infocus mode), diffraction spots produced by the adjoining domains are displaced a little to each other on the back focal plane. When one of the two spots is selected for image formation, a bright image is obtained for the domain corresponding to the selected diffraction spot, whereas the dark image appears for the domain corresponding to the unselected diffraction spot. In an ordinary TEM, since the specimen is placed in a strong magnetic field, the entire specimen exhibits a single magnetic domain. To overcome this problem, a dedicated objective lens that applies nearly no magnetic field to the specimen position is used.

  • Lorentz force

    A force that acts on electrons moving in electric and magnetic fields.

  • Lorentz model

    A model that considers oscillations of bound electrons in a solid when an electric field is applied from outside. The "Lorentz model" gives the classical model of the valence and inner-shell electron excitations.

  • Lorentzian function

    "Lorentzian function" is a function given by (1/π){b / [(x - a)2 + b2]}, where a and b are constants. It is used for pre-processing of the background in a spectrum and for fitting of the spectral intensity. The real spectral shapes are better approximated by the Lorentzian function than the Gaussian function.

  • Miller index

    "Miller index (h, k, l)" specifies the lattice plane of a crystal, which is given by a/u, b/v, c/w, where u, v and w are the intercepts of the lattice plane on the crystal axes or coordinates at which the lattice plane intersects the crystal axes, and a, b and c are the unit cell dimensions of the crystal. If any of the ratios (a/u, b/v, c/w) take fractional number, the elements of the Miller index or the ratios are rewritten by integral number.

  • Moellenstedt analyzer

    One of the in-column type energy filters, which is installed between the intermediate and projector lenses in a TEM. In the "Moellenstedt analyzer," lens aberration is used to get energy dispersion, instead of deflection by a magnetic field or an electric field. The initial Moellenstedt analyzer used an electrostatic lens, but the modern Moellenstedt analyzer adopts a magnetic field lens for a 100 kV or higher voltage TEM.

  • Moire fringe

    When a lattice with a spacing d1 and another lattice with a spacing d2, where d2-d1<d1 or d2, are superposed in parallel, an enlarged lattice with spacing D = d1・d2/(d2-d1) that is parallel to the original lattice appears. The lattice fringes are called "parallel Moire fringe." When two lattices with the same spacing d are superposed with a rotation by angle α, an enlarged lattice with spacing D = d/α emerges in a direction perpendicular (90°different) to the original lattice. The lattice fringes are called "rotation Moire fringe." An example of applications of Moire fringes is observation of an edge dislocation. When a perfect lattice is superposed on a lattice containing an edge dislocation, where the latter lattice is rotated a little with respect to the former at the dislocation, an enlarged dislocation image appears in a direction perpendicular to the original dislocation.

    モワレ縞

    (1a) Bright-field image of mica showing Moiré fringes, taken at an accelerating voltage of 200 kV.
    (1b) Schematic of rotation Moiré fringes. The fringes with an enlarged spacing appear when crystal lattices A and B which have the same spacing are superposed with a small orientation change.

    モワレ縞

    (2) Schematic of an enlarged dislocation image. An enlarged dislocation image is formed when a perfect lattice (B) is superposed on a lattice containing an edge dislocation (A’) with a small orientation change. It should be noted that the enlarged image is perpendicular to the original dislocation.

  • Monte Carlo method

    "Monte Carlo method" is a comprehensive term of random statistical sampling techniques that are used to solve a deterministic or probabilistic problem. This method is used for the simulation of the process where the incident electrons are scattered and spread within a specimen. As the energy of the incident electron beam is larger, the spread of the incident electrons is larger. As the atomic number and density are larger, the spread of the incident electrons is smaller, For example, Monte Carlo method is effectively used to obtain the atomic number effect in quantitative analysis in characteristic X-ray spectroscopy.

  • Neel wall

    "Neel wall" is one type of the boundary structure of magnetic domains whose magnetization directions are antiparallel or different by 180°to each other. This wall is formed in a thin-film specimen. The magnetic dipoles parallel to the specimen surface continuously rotate in planes parallel to the surface at the magnetic wall, and connect to those of the adjacent domain with the opposite magnetization. In a diffraction pattern formed by two adjacent magnetic domains containing a Neel wall, a diffuse intensity arc appears connecting the diffraction spots from the two domains.

  • Peltier cooling

    "Peltier cooling" uses the Peltier effect to cool substances where heat is generated or absorbed when an electric current flows to the contact region of different metals. Peltier cooling is applied to reduce thermal noise of the CCD camera for a TEM. A three-stage Peltier element can cool a substance down to -45 ℃. This cooling technique is usually used for a data-accumulation type CCD camera. The Peltier-cooling device requires water cooling of the heat generation part (air cooling provides low cooling efficiency). Recent cooling devices are made compact in size. To further reduce the temperature, liquid nitrogen is used.

  • Penning (vacuum) gauge

    A cold-cathode ionization gauge. In the Penning (vacuum) gauge, an annular electrode (anode) is placed between a pair of parallel plate electrodes (cathode), and a magnetic field is applied parallel to their axes to lengthen the traveling distance of electrons. As a result, constant discharge occurs even at a low pressure, thus the Penning gauge can measure the pressure of the middle to high vacuum region or 1 to 10-4Pa. The produced current is proportional to the pressure of the vacuum. For a TEM, the gauge is used for switching of the evacuation system from the diffusion pumping (DP) to the sputter ion pumping (SIP) because the gauge is robust and easy to use.

  • Pirani (vacuum) gauge

    A thermal conductivity (vacuum) gauge that utilizes a phenomenon where electrical resistance of a heated element changes with temperature. In the Pirani (vacuum) gauge, a metal (tungsten) wire placed inside a tube is heated by electric current. Thermal conductivity of the metal wire changes with the degree of vacuum through change of the temperature of the wire. The measurable range is 0.1 to 103Pa. For a TEM, the gauge is used to detect the switching pressure from the rotary pumping (RP) to the diffusion pumping (DP).

  • Poendel Loesung

    If two-beam dynamical diffraction is assumed, when one diffracted wave is strongly excited, the energy of the incident wave is completely transferred to the diffracted wave at a thickness of half the extinction distance. When the wave travels another half the extinction distance, the energy of the diffracted wave is completely transferred to the incident wave again. This phenomenon is called "Poendel Loesung." Thus in electron diffraction, the intensity of the diffracted wave is not proportional to the scattering amplitude of the reflection, but periodically changes with specimen thickness.

  • Poisson distribution

    When the probability of A is p and the probability of not A is 1‐p in a certain population, if n pieces are randomly taken from the popuplation, the probability of A being x is nCxpx(1-p)n-x, which is called the binominal distribution. When the probability p is very small, the binominal distribution becomes "Poisson distribution" e・λx/x !. In the case of TEM, the Poisson distribution is applied to the probability of inelastic scattering events. The higher-order plasmon scattering intensities in an EELS spectrum are removed by assuming Poisson distribution. Poission distribution is also effectively used for evaluation of counting errors in a CCD detector.

  • Ptychography

    Ptychography is a method to reconstruct the crystal structure (image) of a specimen from the diffraction patterns obtained from each point (area) scanned over a specimen using a convergent probe so that a part of the illuminated area overlaps. “Ptycho” means “fold” in Greek. This method has been used in X-ray crystal structural analysis.
    Ptychography for Electron Microscopy has attracted attention as one of the method to obtain the structural image (phase recovery) of atomic resolution since about 2012, owing to the advent of a high-speed and high-sensitivity camera that achieves fast acquisition of a two-dimensional (2D) digital image, together with improvement of microscope stability and advancement of the aberration corrector. In particular, it has been reported in recent electron ptychography studies that low-noise and high-contrast structure images are obtained, thus gaining increased attention. In transmission electron microscopy, the following two types of ptycography methods are being conducted.

    1) Method to scan a specimen with a defocused convergent electron probe (Fig. (a))
    A specimen is illuminated with a defocused convergent electron probe to broaden the illumination area. The probe is scanned on the specimen so that the adjacent illumination areas are partially overlapped to each other. The scan points of the probe are normally a few 10 points × a few 10 points or less depending on the scanning area and probe size. The procedure to obtain a structure image (phase image) by means of this type of ptychography is as follows:
    The initial specimen function is assumed equal to 1 and the probe function is assumed to be a box function. The specimen exit-wave function (a product of the specimen and probe functions) is Fourier-transformed to obtain a diffraction pattern, the intensities of which are replaced by those of the pattern acquired experimentally. The updated diffraction pattern is then transformed back to an image in the real space by an inverse Fourier transform, which gives a new revised exit-wave function for this probe position. The probe function obtained is replaced with the correct function or the original one. And the above procedure is repeated. Then, the calculation moves to the next position. Aforementioned procedure is repeated until the difference of the calculated and experimental diffraction patterns becomes sufficiently small.
    The resultant structural image (phase image) of the specimen is shown in Fig. (c). The present method is similar to conventional incoherent diffractive imaging. It should be however noted that non-unique solution problem arising in diffractive imaging is overcome due to the additional constraint, in which the specimen function in the overlapping region of the adjacent illumination areas have to be the same in the calculation for the diffraction patterns recorded. Furthermore, the issue of a limited field of view is cleared by scanning technique.
    2) Method to scan a specimen with a focused convergent electron probe (Fig. (b))
    A specimen is illuminated with a focused convergent electron probe.The convergent beam electron diffraction (CBED) pattern is recorded as a 2D image. The probe is scanned two dimensionally on the specimen. Scan points of the probe becomes a huge four-dimensional (4D) data set (2D CBED patterns +2D scan points), normally exceeding a few 10 thousands points.
    The procedure for reconstructing a high-contrast structure image is as follows. First, the 4D data set RK (2D scan points R + 2D CBED patterns K) is Fourier-transformed with respect to the 2D scan points R to obtain another 4D data set QK (2D spatial frequencies Q + 2D CBED patterns K). In the overlapping area (K’) of the transmitted disc and a neighboring diffracted disc of the CBED pattern for a certain 2D spatial frequency q, the intensity of the Fourier component q of the structure appears (bottom of Fig. (b)). To improve the signal-to-noise ratio of the structure image (phase image) to be obtained, the intensities outside the interference area (K’) are set to be 0 (zero). And the sign (phase) of the intensities in the interference area (K'') located symmetrically with respect to the transmitted disc is reversed. By this processing, the intensities of the two areas (K’ and K''), which are normally canceled out when integrating, can be added positively or enhanced. Then, the intensities of the two areas (K’ and K'') of the 2D CBED pattern (bottom of Fig. (b)) are integrated for each spatial frequency component q of the 4D data set QK, so that a 2D spatial frequency pattern Q' is created. Finally, this 2D spatial frequency pattern Q' is inverse-Fourier transformed to obtain the structure image (phase image) of the specimen (Fig. (d)).

    (Proofread by Dr. Peng Wang, Nanjing University)

    Fig.1
    Fig. 1
    (a) Ptychography in which a specimen is scanned with a defocused convergent electron probe so that adjacent illumination areas are partially overlapped to each other. The illumination area is about a few nm to a few 10 nm in diameter. The number of the scan points is normally a few 10 times a few 10 or less.
    (b) Ptychography in which a specimen is scanned with a focused convergent electron probe (probe diameter: about 0.3 nm or less). The number of the scan points is a few 100 times a few 100 (the total number exceeding a few 10 thousands) like the case of ordinary STEM. A high-speed and high-sensitivity camera (pixelated STEM detector) is used to obtain a series of the CBED patterns.
    (c) Structure image (Phase image) of a mono-layer of MoS2 reconstructed by the defocus method (a). (Data courtesy: Dr. Peng Wang, Nanjing University)
    (d) Structure image (Phase image) of a mono-layer graphene reconstructed by the focus method (b).

    Fig.2
    Fig. 2 Comparison of a reconstructed structure image (phase) obtained by Ptychography in which a specimen is scanned with a convergent probe and a simultaneously-obtained ABF image of a mono-layer graphene acquired at an accelerating voltage of 200 kV.
    (a) Structure image (phase) of graphene reconstructed from 4D data set. Here, the atomic sites appear bright.
    (b) Ordinal ADF image simultaneously acquired with the 4D data set.
    Comparison of the two images elucidates that the reconstructed structure image (phase) has a higher signal-to-noise ratio and provides higher contrast than those of the ADF image.

  • Radon transform

    Fourier transform executes transformation of the function of Cartesian coordinates (x, y) into the function of the variables conjugate to the original Cartesian coordinates. On the other hand, "Radon transform" executes transformation of the function of a two-dimensional polar coordinates (r, θ) into the function of the variables conjugate to the original two-dimensional polar coordinates. This transformation is used for tomography, which reconstructs the three-dimensional structure of a specimen from electron microscope images taken at many different orientations of the incident electron.

  • Reliability factor, R-factor

    In structure analysis using an X-ray beam or an electron beam, Reliability factor (R-factor) is an index that expresses the degree of reliability for the structure obtained from an experimental structure analysis result. When the crystal structure factor obtained from an experimental diffraction pattern is expressed as Fobs (hkl), and the crystal structure factor calculated from the assumed structure model is expressed as Fcal (hkl), the R-factor is defined by the following equation.

    Reliability factor

    Here, (hkl) is an index of Bragg reflection (Mirror index) and the sum of the right side of the equation is taken for all of the experimentally measured reflections.
    In the structure analysis, so as to minimize the R-factor, the parameters (symmetry, lattice constant, atomic coordinate, etc.) of the crystal structure (structure model) are determined. When the experimentally-obtained crystal structure factor coincides fully with the crystal structure factor calculated from the determined structure, the R-factor becomes 0 (zero). When the crystal structure which provides the highest reproducibility of the experimental value is obtained, the value of the R-factor is about 0.05 for X-ray crystallography and is about 0.2 for electron crystallography.

  • Richardson-Lucy method

    The Richardson-Lucy method is used in combination with the deconvolution method which is one of image processing methods. The deconvolution method is based on the concept where an observed image is expressed by convolution of a true image and a Point Spread Function (PSF) that causes degradation of the image. The true image is obtained from deconvolution of the observed image and the PSF. However, if the PSF is not known, the estimation of the PSF is required. The Richardson-Lucy method is effective for estimating the PSF. (Another method that can be used in combination with convolution is the maximum entropy method.) In deconvolution using the Richardson-Lucy method, first the initial true image is assumed. The PSF is obtained from the deconvolution of the observed image using the true image. Then, the second-generation true image is obtained from the deconvolution of the obtained image using the obtained PSF. The final true image is obtained by repeating these processes. That is, these processes are executed iteratively until the image is converged. For example, the method is used for enhancing an EELS spectrum.

  • Ronchigram

    A Ronchigram is a projection image (pattern) of a specimen formed on the diffraction plane with a convergent incident electron beam focused near the specimen using a probe-forming lens. The Ronchigram enables us to determine the optical characteristics (amount of aberration) of the electron probe formed near the specimen using the probe-forming lens. The image is used to obtain the exact focus of the incident electron probe onto the specimen in STEM observations, to know the angular range of the electron probe with no aberration, and to compensate the axial astigmatism.

    When the convergence point of the incident electron probe becomes closer to an amorphous specimen, the magnification of the specimen-shadow image (pattern) seen in the Ronchigram is increased. When the convergence point of the probe is exactly focused onto the specimen, the magnification of the Ronchigram or the shadow image becomes infinite and its intensity becomes uniform. By adjusting the Ronchigram to have a uniform intensity, the exact focus of the incident beam onto the specimen is confirmed. Measurement of the angular range of the area with a uniform intensity in the Ronchigram allows us to determine the angular range of the incident probe with no aberration or to know the quality of the electron-probe. If the third-order spherical aberration is not corrected, the area with a uniform electron intensity is confined to a small extent. When the aberration is corrected, the area is largely extended. This means that the angular range of the electron beam focused onto a point of the specimen is expanded due to vanishing of the aberration.

    The Ronchigram obtained using a crystalline specimen exhibits interference fringes, when the incident beam with an incidence angle larger than the diffraction angle from a lattice plane of the crystal is illuminated onto the specimen with a small defocus. Appearance of the interference fringes means that the probe diameter on the focal plane is smaller than the lattice spacing. From this experiment, the probe size on the focal plane used for STEM observations can be known.

    The term “Ronchigram” is named after V. Ronchi, who proposed the method to originally examine the performance of a lens of light optics. 

    Ronchigram method
    Fig. (a) Ray diagram of Ronchigram.
    Fig. (b), Fig. (c) Ronchigram patterns obtained from an amorphous thin-film specimen. In the case of no aberration correction (Fig. (b)), the area with a uniform electron intensity is confined to a small extent (semi-angle: ~11 mrad). When the aberration is corrected (Fig. (c)), the area is largely extended (semi-angle: ~45 mrad).

  • Rowland circle

    A circle, which is virtually drawn tangent to the center point of a spherical concave grating with a diameter equal to the radius of curvature of the grating. (This circle is half the size of the circle produced by the concave grating.) When a slit is placed at an arbitrary point on the "Rowland circle" and is illuminated with light. The light whose source is at the slit causes diffraction with the concave grating and forms an aberration-free spectrum on a position of the Rowland circle.

  • Rutherford scattering

    "Rutherford scattering" is a phenomenon where a charged particle is scattered by Coulomb force of an atomic nucleus. The intensity of this scattering is proportional to the square of the atomic number Z and to (sinθ/2)-4, where θ expresses the scattering angle. In the case of the electron beam, the scattering approaches to Rutherford scattering at high-angles where scattering caused by the electron cloud around the atomic nucleus can be neglected.

  • S-shaped distortion

    "S-shaped distortion" means the distortion characteristic of the electromagnetic lens for the electron beam. Unlike the light beam in the optical lens, the electron beam suffers rotation in the electromagnetic lens. The rotation magnitude increases as the electron beam deviates from the optical axis, and then an image that should be observed as a line on the image plane (fluorescent screen) is viewed as an s-shaped image. The distortion becomes a problem for the projector lens because there exist electron beams which pass largely apart from the optical axis. Combination of lenses with opposite polarity can cancel the distortion, but this technique has not been used yet.

  • Scherzer focus

    When a high resolution structure image of a phase object is taken in the TEM mode, "Scherzer focus" is used as the defocus condition, which is determined by the spherical aberration of the objective lens so that the phase of diffracted waves is shifted by 1/4 wavelength (or a phase of π/2) of the electron wave over a wide range of spatial frequencies.

  • Schottky effect

    A phenomenon where the potential barrier decreases when a strong electric field is applied to a substance. The Schottky-type electron gun emits sufficient electrons with the aid of a strong electric field at a lower temperature (~1800 K) than the temperature that can effectively emit thermoelectrons. In the actual Schottky-type electron gun, the surface of the tungsten (W) tip is covered with a thin layer of zirconium oxide (ZrO) to further decrease the potential barrier.

  • Schottky-type electron gun

    Schottky effect means a phenomenon where the potential barrier of a substance decreases in a strong electric field, resulting in ease of thermoelectron emission. In the Schottky-type electron gun, the tungsten (W) tip emitter is heated at a lower temperature (~1800 K) than the temperature that can effectively emit thermoelectrons, and a strong electric field is applied to the tip, thus decreasing the potential barrier to emit electrons from the emitter. In the actual Schottky-type electron gun, the surface of the tip is covered with a thin layer of zirconium oxide (ZrO) to make electron emission easy by a decrease of the work function of the tip (~2.7 eV). The energy spread of the emitted electrons is ~0.7 eV. Its brightness is as high as 4×108 A/cm2.sr at 200 kV. The size of the virtual source produced is >10nm. The Schottky type gun is broadly used because of its high stability of the emission current. This type of gun is not the field emission type because the tunnel effect is not used.

  • Si(Li) detector

    One of the energy-dispersive X-ray detector used for EDS. A lithium (Li)-doped silicon single-crystal semiconductor is used as a detector element. When X-rays enter the detector element, electron-hole pairs (their generation energy is approximately 3 eV) are generated in the detector element, whose number is proportional to the energies of the X-rays. The generated electrons are collected to the anode at the bottom of the detector element by applying an external electric field. That is, the X-ray energies are measured as electric voltage pulses. Detectable elements range from boron (B: 0.18 keV @K line) to uranium (U: 3.16 keV @M line). The energy resolution of the detector is approximately 140 eV (@Mn K line). Cooling of the detector by liquid nitrogen is required to prevent diffusion of doped Li and to reduce a dark current caused by thermal noise. Owing to the development of a silicon drift detector (SDD) which has a high count rate and can be used by Peltier cooling, the Si(Li)detector has less been used.

  • Skyrmion

    Skyrmion is a vortex-like magnetic structure formed by the magnetic moments of electron spins. The term “Skymion” is named after Skyrme, a theoretical physicist of UK, who proposed the quantum structure. “Skymion” is also called “magnetic vortex” or “spin vortex”. Its size ranges from approximately several nm to 100 nm, depending on materials and chemical compositions. Skymions align like a two-dimensional crystal by cooling the specimen temperature and controlling the external magnetic field.
     Skyrmion was first observed as a real-space image by Lorentz electron microscopy. Since then, it was also imaged by STEM using the DPC (differential phase contrast) method.

    skyrmion
    Courtesy of figures:
    Senior Researcher T. Matsumoto and Associate Professor N. Shibata
    The University of Tokyo (as of August 2015)
     
    Fig. (a) Schematic of the magnetic structure of skyrmions in a thin film. The directions of the magnetic moments of electron spins are indicated by arrows. The directions of those magnetic moments are perpendicular to the film plane at the core of the respective skyrmions, but swirl in the film plane at the peripheral region.
    Fig. (b) A real-space image of skymions acquired by the DPC (differential phase contrast) method (specimen: an Iron alloy). The colors in the figure depict the directions and magnitudes of the magnetic vectors in a two-dimensional plane.

  • Stobb's factor

    The contrast of a high-resolution (HREM) image is lower than that expected from image simulation. "Stobb's factor" is used to adjust the simulated contrast to agree with the experimental contrast. The superposition of the background intensity due to thermal diffuse scattering (TDS) is considered to decrease the contrast of HREM images formed by elastic scattering.

  • Thon's curve

    Phase contrast from an amorphous specimen (granular structure) remarkably changes with the defocus amount. "Thon's curve" expresses how the spatial frequency (emphasized spacings), in which the phase of the waves are matched, changes with the defocus amount.

  • Umweganregung

    Two or more diffractions occur (dynamical diffraction) to excite a forbidden reflection due to a screw axis or a glide plane (symmetry element of a space group). This phenomenon is termed "Umweganregung." In electron diffraction, kinematically forbidden reflections become allowed reflections due to Umweganregung. Thus, to examine the existence or absence of a screw axis or a glide plane, it is necessary to select a specimen orientation without Umweganregung paths. It should be noted that CBED which takes dynamical diffraction into account enables us to unambiguously determine the existence or absence of the screw axis or the glide plane.

  • Vortex electron wave

    A vortex electron wave is an electron wave that propagates in a space with a spiral wavefront (or a spiral equiphase plane). Since the wavefront of the electron wave is of a spiral shape, only waves which create an advance of (= integer) times the wavelength in the axial direction is allowed for one turn about the central axis along the equiphase plane. Here, is called topological number, topological quantum number or topological charge, and is a parameter characterizing the vortex wave. Since the physical quantity characterizing a rotation is the angular momentum, a phase rotation with respect to the central axis can be considered to have an (orbital) angular momentum. This means that the vortex electron wave is a wave carrying an orbital angular momentum. The orbital angular momentum is given by ℓ×h/2π, where h is the Planck constant.
    In 2010, Uchida and Tonomura experimentally demonstrated the formation of a vortex electron wave using a spiral-shaped phase plate. In their experiment, a plane electron wave was incident onto a graphite with a spiral-shaped thickness change. The transmitted electron wave was interfered with a reference wave which does not pass through the graphite. Then, they confirmed that the transmitted wave has a spiral-shaped wave front or is a vortex wave. Afterwards it was found that the vortex electron wave can be formed also using a fork-shaped grating (Fig.1a), a spiral zone plate (Fig.1b), etc.
    A vortex light wave was discovered in 1992. It has been used for rotation operation of light tweezers by transferring the orbital angular momentum of the wave. For the vortex electron wave, magnetic imaging is expected by utilizing interactions between magnetic material and magnetic moment of the vortex electron wave originating from the spiral rotation of electrons.
    The upper figures of Fig.2 show the equi-phase planes of vortex electron waves. The wave with = 0 expresses a plane wave. A point on the orange plane comes back to the same point for one turn about the central axis. The vortex wave of = 1 creates an advance of one-wave length in the axial direction for one turn on the equi-phase plane about the central axis, which is illustrated by orange planes. The vortex wave of = -1 creates an advance of one-wave length in the opposite direction for one turn on the equi-phase plane about the central axis. The vortex wave of = 2 creates an advance of two-wave length in the axial direction for one turn on the equi-phase plane. Green planes in Fig.2 are drawn for easily recognizing the two-wave length progress of the orange planes. The vortex wave of = 3 creates an advance of three-wave length for one turn on the equi-phase plane. Purple and green planes are drawn for easily finding the three-wave length progress of the orange planes.
    The lower figures of Fig.2 show the intensity distribution of the vortex electron wave for the vertical cross section of the wave. Since the vortex electron wave does not have a fixed phase at the central axis (a phase singular point), the vortex wave is unable to have a finite amplitude on the central axis. As a result, the intensity on the central axis must be zero (dark spot) for the vortex waves (≠ 0). For = 0 (plane wave) the wave does not have any angular momentum and any singular point, and the intensity on the central axis takes a maximum value. The larger is, the larger the radius of the intensity ring becomes.
    In Fig.3, a diffraction pattern taken by illuminating a parallel electron beam onto a fork-shaped grating is shown. The intensities of the diffracted waves at the both sides of the transmitted wave are ring-shaped and have dark spots at the respective centers (see the lower figures of Fig.2). This result confirms that the detected electron wave is a vortex electron wave.

    (Courtesy: Professor K. Saito at Nagoya University)
     
    Platinum mask to form a vortex electron wave by passing a plane electron wave through it.
    Fig.1 Platinum mask to form a vortex electron wave by passing a plane electron wave through it.
    (a) Fork-shaped grating mask and (b) Spiral zone plate mask.
    Schematics of equi-phase plane of the vortex wave (top) and radial intensity distribution of the wave (bottom).
    Fig. 2  Schematics of equi-phase plane of the vortex wave (top) and radial intensity distribution of the wave (bottom).
    Examination of the vortex electron wave.
    Fig. 3  Examination of the vortex electron wave.
    Diffraction pattern taken by passing a parallel electron beam through a fork-shaped grating mask. The diffracted waves at the both sides of the transmitted wave have ring-shaped intensity distributions and dark spots at the respective centers. This confirms that the detected electron wave is a vortex electron wave.

  • Wehnelt electrode

    A cylindrical electrode with a hole of a diameter of 1 to 2 mm, which is installed in the electron gun. By applying a bias voltage induced through a bias resistance, the "Wehnelt electrode" converges a diverging electron beam emitted from the electron gun.

  • Wien filter

    One of the in-column type energy filters. The "Wien filter" uses magnetic and electric fields perpendicular to each other for getting energy dispersion, though the omega filter and the alfa filter use only magnetic fields. The Wien filter is used as a monochromator by incorporating it in the illumination system of a TEM. Since its energy dispersion for a high-voltage 200 kV electron beam is small, the electron beam is introduced into the filter before the beam is accelerated or after the beam is decelerated to several 100 V. In this case, its energy dispersion is ~10 μm/eV. There is a limitation on applying a high voltage to the Wien filter due to discharge between electrodes. Thus, the filter is used for an incident electron beam at a voltage of ~10 keV or less. The advantage of the Wien filter is that the electron trajectory in the filter is parallel to the optical axis or forms a straight line.

  • X-ray absorption spectroscopy

    A spectroscopy method to acquire a spectrum of X-rays absorbed by a substance. In a similar manner as EELS, XAS of the soft X-ray region enables us to obtain the density of states of the unoccupied state (conduction band) of a material.

  • X-ray emission spectroscopy

    A spectroscopy method, which measures the density of states of the occupied states (valence band) by analyzing X-rays emitted from a substance illuminated with an X-ray beam or an electron beam.

  • Young fringe

    When a beam exiting from a point source is passed through two slits, two waves exiting from the slits interfere with each other, resulting in formation of a fringe pattern. This fringe pattern is called "Young fringe(s)." As the distance between the two slits is smaller, the period of the Young fringes becomes larger. The Young fringe pattern is used to easily determine the resolution (information limit) of a TEM. In an actual experiment, two overlapping HREM images of an amorphous specimen are taken by shifting the electron beam within the exposure time to take one-frame CCD image, the images are acquired into a computer, and finally the diffraction pattern is obtained by FFT processing of the images. The Young fringes appear to be superposed on the diffraction pattern of the amorphous specimen. The information limit can be measured from the angular position (the radius of the Young fringe pattern) where the Young fringes disappear. The beam shift is chosen so that the Young fringes are easily seen with an appropriate separation.

    Sperm
    (a) High-resolution TEM image of Au particles on a carbon thin film.
    (b) Fourier transform pattern of (a).
    (c) High-resolution TEM image taken by shifting the field of view using the deflector system. The relative shift and shift direction of the image are indicated by a yellow arrow.
    (d) Fourier transform pattern of (c). Fringes (Young fringes) corresponding to the relative shift are seen. The information limit can easily be measured from the angle at which the fringes disappear (indicated by a yellow circle).

  • ZAF correction method

    In spectroscopic analysis of characteristic X-rays, the "ZAF correction method" is used for quantitative analysis of target elements. As a specimen is thicker (~several 10 nm or thicker though depending on measured elements), the intensity of the emitted characteristic X-rays is influenced by the atomic-number effect, the absorption effect and the fluorescence excitation effect. Corrections of these three effects are required. In actual correction, the relative X-ray intensities obtained from an unknown specimen against those from a standard specimen (usually a compound of a simple composition) are measured. And then, the corrections of the three effects are made to the relative intensities. In the case of EPMA, the three effects are corrected. On the other hand, in the case of TEM, only the absorption effect, which has the largest effect, is taken into account in many cases. Normally, the Cliff-Lorimer method (thin-film approximation method) for a thin specimen is applied, which often provides good quantitative results with a relatively high accuracy.

  • Zernike phase contrast

    In TEM, "Zernike phase contrast" means contrast which is obtained by converting the phase change of electron waves scattered by a specimen into the amplitude change. The conversion is performed by using a phase plate or a combined effect of the spherical aberration of the electron lens and defocus.

    Zernike phase contrast

    (a) Conventional TEM (C-TEM) image.
    (b) Zernike phase contrast (ZPC-TEM) image of an ice-embedded T4 phage taken at an accelerating voltage of 200 kV.
    (c) Schematic of T4 phage.

    Compared with the C-TEM image, the ZPC-TEM image clearly visualizes the fine structures of DNA in a capsid, the hair-like structural objects and the cylindrical structures on the capsid surface.

  • absorption effect

    In spectroscopic analysis of characteristic X-rays (EDS), the "absorption effect" means that part of X-rays generated within a specimen is absorbed by the specimen. Since this effect cannot be neglected when a specimen is thicker (~several 10 nm or thicker though depending on target elements), correction for the detected X-ray intensity is required in quantitative analysis. The correction of the absorption effect is crucial for thick specimens because this effect is larger than the atomic-number effect and the fluorescence excitation effect.

  • absorption potential

    When incident electrons strike constituent atoms in a specimen, the incident electrons undergo inelastic scattering. Inelastically scattered electrons do not interfere constructively with elastically scattered electrons, thus giving rise to absorption or attenuation of elastic scattering. This effect can be taken into account in the elastic scattering theory through imaginary potential or "absorption potential."

  • absorption-edge energy

    The energy to excite an electron bound to an orbit of an atom to the lowest level of the unoccupied state.

  • accelerating (acceleration) voltage center

    When fluctuations are added to accelerating voltage using a high-tension wobbler, a TEM image spirally enlarges and shrinks. The center of this enlargement and shrinkage is called "accelerating (acceleration) voltage center." Alignment of the accelerating voltage center is carried out to bring the accelerating voltage center to the center of the fluorescent screen for viewing the image by the use of a double-deflection coil system. Since the fluctuations of the high voltage are small (<10-6), this alignment is used to minimize the effect of energy spread due to inelastic scattering (plasmon scattering) in a specimen rather than the effect of high-voltage fluctuations. This alignment is required when taking images at a medium magnification lower than 100,000×.

  • accelerating voltage

    A voltage to accelerate electrons, which are emitted from the electron gun and illuminate a specimen. This voltage is a voltage applied between the cathode and the final electrode of the acceleration tube.

  • acceleration tube (accelerating tube)

    The "acceleration tube" consists of acceleration electrodes used for sequentially accelerating an electron beam, which is emitted from the electron gun, up to a required voltage. In a 200 kV TEM, six acceleration electrodes constitute the acceleration tube.

  • acceptance angle

    An angle that accepts electrons or X-rays exiting from the specimen with the objective aperture or the detector when acquiring a TEM image, an EELS spectrum, or an EDS spectrum.

  • accidental reflection

    "Accidental reflection(s)" are reflections excited, other than systematic reflections.

  • achromatic plane

    Let electron beams with different energies, which exit from one point on the object plane and pass through an energy filter, meet at one point on an image plane without energy dispersion. This image plane, on which chromatic aberration does not appear, is called the "achromatic plane." An image formed on the achromatic plane is called the achromatic image.

  • active magnetic-field canceller

    An instrument that detects momentarily (ever) changing magnetic fields over a TEM and generates the anti-phase magnetic fields against the magnetic fields to cancel them.

  • airy disk

  • alfa filter

    One of the in-column type energy filters, which is installed between the intermediate and projector lenses in a TEM. The "alfa filter," whose principle of energy dispersion is the same as that of the omega filter, consists of two electromagnets instead of four electromagnets in the omega filter. Since the trajectory of electrons passing through the alfa filter takes the shape of a letter "alfa," this is called alfa filter. Its energy dispersion is ~0.7 μm/eV, which is smaller than that of the omega filter.

  • alloy

    A metallic substance of a composite of intermetallic compounds or metallic phases, which contains two or more metallic elements.

  • amorphous

    A solid substance in which the arrangement of atoms and/or molecules is irregular and disordered.

  • amorphous ice

    Amorphous ice means ice that does not take a crystalline state. When water is cooled below the melting point, ice crystal is formed and grows until its temperature reaches the re-crystallization temperature.  Since formation of ice crystals can cause destruction of fine structures of a biological specimen, amorphous-ice formation without formation of ice crystals is required when rapid freeze fixation is applied.  Thus, ice-crystal formation should be suppressed by cooling the specimen as rapid as possible down to the re-crystallization temperature.

  • amplitude-phase diagram

    In the case of the description of the propagation of a diffracted electron wave, let us plot the value of the wave function in the complex plane with coordinates x0, y0. The length of the line drawn from the origin to the point (x0, y0) expresses the amplitude, and the angle between the line and the horizontal axis expresses the phase of the wave. The diagram is called the "amplitude-phase diagram." When a crystal is divided into many layers in its thickness direction, the amplitude and phase of the diffracted wave in each layer is successively added until the bottom surface. The resultant intensity of the wave at the bottom surface is obtained as the square of the length of the line drawn from the origin to the final point (x, y) at the bottom surface. In 1960s, Hirsch et al., for the first time, revealed the image contrast of dislocations and stacking faults using the amplitude-phase diagram.

  • analysis region

    The size of the "analysis region" (spatial resolution) in EDS analysis is determined by not only the diameter of the incident electron beam but also the beam broadening within a specimen that depends on accelerating voltage, specimen thickness and constituent elements. For an accelerating voltage of 200 kV, the analysis region is 10 to 50 nm in diameter. The beam broadening decreases with the increase of the accelerating voltage.

  • analytical electron microscopy

    A microscopy method that adds analytical functions, such as EDS and EELS to a TEM, in order to perform qualitative and quantitative analysis of elements and/or electronic structure analysis from micro- or nano-areas subjected to TEM observation.

  • analyzing crystal

    A crystal used for the spectral analysis of characteristic X-rays in the WDS analyzer. An LiF crystal (spacing 0.4 nm) is used for analyzing heavy elements such as uranium (U Lα). An STE(Stearate) crystal (spacing 10 nm) is used for analyzing light elements such as beryllium (Be Kα).

  • anaplat

    A lens that corrects spherical and coma aberrations. A telescope and an optical microscope are equipped with an achromatic (chromatic aberration corrected) "anaplat." The anaplat is accomplished in the following way. The surface at the object side of the lens is finished in a spherical shape to eliminate the spherical aberration. The surface at the image side of the lens is fabricated to suppress the occurrence of the coma aberration (surface satisfying the sine condition).

  • anastigmat

    A lens that corrects all of five Seidel aberrations. Almost all the commercial cameras, which have been recently developed, are fitted with an anastigmat lens.

  • anisotropy

    "Anisotropy" means that physical properties of a crystalline substance are different depending on its crystal orientations.

  • annealing

    "Annealing" is a thermal treatment to heat metallic materials and others at an appropriate temperature and then to cool slowly them for obtaining uniform structures or for removing the internal stress of the materials.

  • annular bright-field scanning transmission electron microscopy

    Annular bright-field scanning transmission electron microscopy (ABF-STEM) is a bright-field high-resolution STEM method which preferentially receives only the ring-shaped circumference (e.g. 12 to 24 mrad) of the direct (transmitted)-beam disk using an annular bright-field (ABF) detector, without using the central part of the bright field disk. The integrated intensities of the received electrons are displayed in synchronism with the incident probe position for acquisition of a high-resolution atomic image. The ABF method enables us to effectively visualize atomic columns composed of light atoms.
    In these light-element atomic columns, the intensities of electrons which travel along the atomic columns parallel to the incident beam, become higher than the scattered electrons due to the electron channeling effect. Thus, the electrons which are incident on the light atomic columns increase to pass through the center hole of the ABF detector. As a result, light elements are effectively imaged as dark spots (image formed by smaller quantity of electrons). On the other hand, the electrons which are incident on the heavy atomic columns increase to scatter at high angles outside of the ABF detector. As a result, heavy atomic columns are also imaged as dark spots. Thus, the ABF method enables us to observe both the atomic columns of relatively heavy elements (transition metals, etc.) and light elements (O, Li, etc.) with the same contrast.
    HREM and BF-STEM have long been used to visualize atomic columns. However, those two methods necessitate image simulations for correct image interpretation because the intensity of the atomic column largely depends on the defocus amount and specimen thickness. In the ABF-STEM image, (except for extremely thin specimen) the atoms are always imaged as dark spots irrespective of the variation of specimen thickness. Even in the image formed with a certain defocus, atoms are still imaged as dark spots because the incident electrons over a certain angular range reduce the interference effect of each electron. This makes an easy interpretation of the ABF image compared to the HREM and BF-STEM images. 


    High-resolution STEM images of SrTiO3 taken at the [100] incidence. (Accelerating voltage: 200 kV, Convergence semi-angle of the incident electron beam: 22 mrad)
    Fig.(a) HAADF-STEM image taken with an acceptance semi-angle of the detector 90 to 170 mrad. Sr columns and Ti + O columns, which are composed of relatively heavy atoms, are clearly visualized. However, the light-atom O columns cannot be observed as bright spots.
    Fig.(b) ABF-STEM image taken with an acceptance semi-angle of the detector 11 to 22 mrad.
    The light-atom O columns, which cannot be seen in the HAADF image, are clearly observed as dark spots.

    Comparison of Ray diagrams of two detectors


    Fig.(a) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for HAADF-STEM. Typical inner and outer semi-angles of the detector are respectively β1 = ~50 mrad and β2 = ~200 mrad, detecting inelastically scattered electrons at high angles. The value of the convergence semi-angle α is approximately 25 mrad for a 200 kV Cs-corrected TEM. Usually, an ABF detector and a LAADF detector are placed below a HAADF detector.
     

    Fig.(b) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for ABF-STEM. The inner and outer acceptance semi-angles of the detector are respectively taken as β1 ≒ α/2 and β2 ≒ α, where a is the convergence semi-angle. It should be noted that the only peripheral part of the bright field disk (without the central part) is used for ABF-STEM. In the case of a 200 kV Cs-corrected TEM, α, β1 and β2 are respectively ~25 mrad, ~13 mrad and ~25 mrad.

  • annular dark-field detector

    An annular detector with an inner diameter of ~3 mm and an outer diameter of ~8 mm, which is used for obtaining a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image. The "annular dark-field (ADF) detector" of a phosphor screen or a YAG scintillator receives scattered electrons and converts them into a light signal. The light signal is directed to a PMT through a light pipe and converted into an electric signal, and the electric signal is amplified.

  • anode

    An electrode, to which a positive potential (voltage) is applied against the facing cathode (electron source). The "anode" receives the flow of electrons (electron beam) emitted from the cathode, and guides the electron flow downward through the hole at its center. In the case of a six-stage acceleration of 200 kV, the voltage applied to the anode is ~33 kV against the cathode.

  • anomalous absorption

    When an incident electron beam travels a crystalline specimen, two kinds of electron waves are produced due to the dynamical diffraction effect. That is, one electron wave runs on atomic columns and the other electron wave runs between atomic columns. The former electron wave undergoes a larger absorption than the average absorption, whereas the latter electron wave undergoes a smaller absorption than the average absorption. This phenomenon is termed "anomalous absorption." The main cause of the anomalous absorption is thermal diffuse scattering of incident electrons.

  • anomalous transmission

    When a crystalline specimen is thick, the electron wave that undergoes a larger absorption than the average absorption is damped rapidly; however the electron wave that undergoes a smaller absorption than the average absorption is damped slowly or has a greater transmissivity. This phenomenon is termed "anomalous transmission."

  • anti-contamination device

    A device that is used for suppressing the deposition of hydrocarbons on a specimen in a TEM. By the condensation action of the cooled fin with liquid nitrogen which is set to surround the specimen holder, contamination on the specimen due to hydrocarbons is largely decreased. In this device, two small aperture holes are placed at the top and bottom to allow the electron beam to pass through the holes. The tip of the specimen holder (specimen part) is inserted into the fin through the opening at its lateral side.

  • anti-phase boundary

    "Anti-phase boundary" separates two adjacent crystals which have the same crystallographic orientation but have a 180°phase shift (a shift of half period) each other. Anti-phase boundaries frequently appear in the ordered phase of a binary alloy.

  • anti-phase domain boundary

  • antiferroelectric material

    An "antiferroelectric material" indicates such a crystal that consists of two sublattices which have anti-parallel dielectric polarizations each other, thus the net polarization of the crystal being 0 (zero). The material is polarized in the direction of an electric field like a paraelectric material when placed in a weak electric field. When the electric field is increased, strong electric polarization is generated in the direction of the electric field like a ferroelectric material. It exhibits a double-hysteresis loop against changes of the electric field.

  • antiferromagnetic material

    An "antiferromagnetic material" consists of atoms having magnetic moments but the neighbouring magnetic moments are aligned anti-parallel to each other, thus the net magnetic moment of the material is 0 (zero). The material is magnetized in the direction of a magnetic field like a paramagnetic material when placed in a weak magnetic field. When the magnetic field is increased, it is strongly magnetized in the direction of the magnetic field like a ferromagnetic material. It exhibits a double-hysteresis loop against changes of the magnetic field.

  • aperture

    The "aperture" includes the condenser aperture, the objective aperture and the selector aperture. The aperture is classified into the fixed type aperture and the movable type aperture.

  • aperture diameter

    A diameter of a lens aperture. The condense-aperture diameter determines the divergence angle of an electron beam. The objective-aperture diameter determines scattering angle of electrons exiting from a specimen (the number of diffracted beams used for forming a TEM image). The selector-aperture diameter determines an area from which a diffraction pattern is obtained.

  • area analysis

    In spectroscopic analysis, "area analysis" is to acquire a spectrum from an area (two-dimensional) scan of an electron beam on a specimen.

  • astigmatic difference

    A measure of the axial astigmatism of an objective lens. This is expressed as the difference of the focal length in the two orthogonal directions. The "astigmatic difference" of an objective lens has been decreased down to 1.5 μm or less.

  • atom form factor

    A scattering amplitude of an incident electron generated by an atom. The scattering angle dependence on the scattering amplitude is given by the form of the atom. Incident electrons are scattered by the electrostatic potential that is created by an atomic nucleus and surrounding electrons. (X-rays are scattered by electrons). The atom form factor is used to calculate the crystal structure factor.

  • atom-location by channeling-enhanced microanalysis

    "Atom-location by channeling-enhanced microanalysis (ALCHEMI)" is a method that determines sites (locations) of impurity atoms in a crystal by utilizing a phenomenon where the incident electrons pass through specific atom sites (electron channeling). Sites occupied by impurity atoms can be distinguished from the intensity difference of characteristic X-rays (EDS spectra) observed at a small positive tilt angle and a small negative tilt angle from an exact Bragg condition, the tilt angle being adjusted by observing the electron diffraction pattern. In this method, EDS spectra are measured at two crystal orientations but recently, a more reliable technique has been prevailing. That is, a technique of two-dimensional rocking of the incident electron beam enables us to obtain patterns of characteristic X-ray intensities over a large angular range.

  • atomic plane

    A crystal is regarded as a stacking of atomic planes, which is created by a regular array of atoms. In a (perfect) crystal, different kinds of atomic planes stack periodically.

  • atomic scattering factor

  • atomic-number effect

    In spectroscopic analysis of characteristic X-rays (EDS), the "atomic-number effect" means that, since the amount of incident electrons which do not contribute to the emission of the characteristic X-rays of a target element (the excitation of the electrons of the target element) due to backscattering of the incident electrons is dependent on the average atomic number, the intensity of the X-rays generated from the specimen is dependent on the atomic numbers of the constituent elements of the specimen. When the atomic numbers of constituent elements are largely different, this effect has to be considered. Since this effect cannot be neglected in quantitative analysis when a specimen is thicker (~several 10 nm or thicker though depending on measured elements), correction for the detected X-ray intensity is required.

  • auto-correlation function

    "Auto-correlation function" is a function to give information about the shape of the function such as sharpness, roundness, periodicity, etc. It is a function (or a pattern) that is acquired by integrating the product (overlap) of two same functions with respect to a variable which are shifted each other by a certain amount about the variable. That is, when we define the object function f, an integral variable of the function X and a relative shift of the function x, the auto-correlation function Rff can be written as the following equation: Rff=∫f(X)f*(X-x)dX. Note that * denotes complex conjugate. In the case of a microscope image etc., f is a real function and then f*(X-x)=f(X-x). If the value of Rff is large even for a large shift, the original function (or the original pattern) is delocalized in the X direction. Contrary, if the auto-correlation function decreases rapidly with increasing the relative shift, the original function (or the original pattern) is localized. If the function is peridic, Rff takes a large value at integral multiples of a certain x. Thus, the periodicity of the function is obtained. Therefore, the auto-correlation function enables us to obtain the knowledge about the shape of the function or the pattern with respect to the variable. For example, by calculating the auto-correlation function of a TEM image of a particle, the amount of deocus is estimated from a blur of the image and the amount of 2-fold astigmatism is obtained from an elongation of the image. For high-speed computer calculation of the auto-correlation function, FFT (fast Fourier transform) is used on the basis of the following theorem: Fourier transform of the product of certain functions is equivalent to the product of Fourier transforms of the respective functions. That is, the auto-correlation function is calculated by the inverse Fourier transform of the power of the Fourier transforms of the respective functions.

  • axial astigmatism

    If an electromagnetic lens (objective lens) does not have perfect axial symmetry, a circular image of the light source (electron source) deforms to an ellipse image. This aberration is called "axial astigmatism." The axial aberration arises from asymmetry of the polepiece bore of the lens, magnetic non-uniformity of the polepiece material, charging on the aperture, etc.

  • axial coma aberration

    "Axial coma aberration" is a parasitic aberration due to incomplete axial symmetry of an electromagnetic lens (objective lens). This aberration deforms a circular image of the electron source to a cone-shaped (comet-shaped) image. The axial coma aberration arises from asymmetry of the pole piece bore of the lens, magnetic non-uniformity of the pole piece material, electron charging on the aperture, etc.

  • axial geometrical aberration

    The axial geometrical aberrations are aberrations depending only on α, angle between the beam and the optical axis. The axial geometrical aberrations are defocus, two-fold astigmatism, three-fold astigmatism, axial coma aberration, spherical aberration, four-fold astigmatism, star aberration, etc. On the other hand, aberrations that depend on α and r, distance of an electron beam from the optical axis, are called “off-axial (geometrical) aberrations.” As an example of the off-axial aberrations, the coma aberration of Five Seidel aberrations is mentioned.
    In the case of high-resolution electron microscopy (HREM), the field of view is very narrow because its magnification is very high. Thus, the off-axial geometrical aberrations can be ignored, enabling us to account for only the axial geometrical aberrations. In the case of scanning transmission electron microscopy (STEM), a converged electron probe on the optical axis scans a specimen area. Only the axial geometrical aberrations are accounted for in STEM image formation as in the case of HREM.

  • azimuth (azimuthal angle)

    In the case of a TEM, the "azimuthal angle" is a rotation angle around the optical axis. That is, an azimuth is defined as an angle between a plane that includes the optical axis and the reference plane also including the optical axis.

  • back focal plane

    A focal plane located in the opposite side of the object (plane) with respect to a lens is called the back focal plane. A diffraction pattern is formed on the "back focal plane," thus the plane corresponds to the reciprocal space of the specimen.

  • back-stream

    "Back-stream" is a phenomenon where the oil mist from the rotary pump or the diffusion pump flows to the high-vacuum side. The back-stream must be prevented.

  • backing pressure

    (When gasses are exhausted to a space with a pressure equal to or below atmospheric pressure (105 Pa),) the pressure at the exhaust port (outlet) is called the "backing pressure."

  • backscattered electron

    When incident electrons travel a specimen, a part of electrons is reflected (scattered) backward and emitted from the specimen surface. These emitted electrons are called "backscattered electron(s)." The intensity of the backscattered electrons is larger as the atomic number of the constituent atoms in the specimen is larger. The energy of the electrons is close to that of incident electrons, indicating that backscattered electrons possess higher energy than secondary electrons. Thus, the backscattered electrons are emitted from a deep region from the top surface (depth: 100 nm or less) compared to secondary electrons. A backscattered electron image provides the difference of the specimen composition and topographic shape. If the specimen is a crystal, the backscattered electron intensity largely depends on the orientation of the incident electron beam due to electron channeling. Thus, a backscattered electron image obtained from a crystalline specimen shows the difference of the crystal orientation in the specimen.

  • backscattered-electron detector

    A detector that is used for detecting backscattered electrons from the specimen surface by electron-beam illumination. In a TEM, a micro-channel plate (MCP) is used as the "backscattered-electron detector." (In a dedicated SEM instrument, a semiconductor detector that uses the p-n junction mechanism is adopted.) To improve the detection efficiency, an annular detector is placed just above the specimen. The MCP detects backscattered electrons and converts the electron signal into an electric signal. Since the energy of backscattered electrons is high, additional electron acceleration that is applied to secondary electrons is not required. A backscattered-electron image is obtained by displaying the intensity of the backscattered electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe. A two-segment annular detector is often used for obtaining a composition (COMPO) image and a topographic (TOPO) image. That is, two signals acquired from each segment are amplified by with the preamplifier, and then processed with the main amplifier in such a way that COMPO image is obtained by addition of the two signals and TOPO image is obtained by subtraction of them. Furthermore, the use of a four-segment detector enables us to obtain a stereoscopic (SHADOW) image. To obtain SHADOW image, the composition signal from two segments and the topography signal from other two segments are synthesized.

  • bakeout device

    A device that bakes out vacuum chambers, such as the FEG chamber, the inside walls of the microscope column and the specimen stage, to obtain a high vacuum in a TEM.

  • baking

    "Baking" is to heat vacuum chambers under a vacuum state, such as the FEG chamber, the inside walls of the microscope column and the specimen stage, to obtain a high vacuum in a TEM. Baking accelerates degas from the vacuum chambers and decreases subsequent outgas. When a TEM equipped with an FEG is used, baking of the FEG chamber is performed for more than 40 h at a high temperature of ~300 ℃ so that stable operation and long lifetime (operating time) of the FEG are achieved and also discharge damage to the FEG is decreased. When the column and stage which use O-rings are baked, the baking is carried out for about 3 days at ~60 ℃ because these parts cannot be baked at a high temperature.

  • band gap

    The band gap means energy width of the forbidden band, which is situated between the valence band and the conduction band and does not allow electrons in a crystal to exist. The larger the band gap, the greater the insulation quality is.

  • beam-rocking technique

    A technique that rocks the incident electron beam at a point on the specimen in the orthogonal (x, y) directions over a certain angular range. Using the double-deflection system, the electron beam is rocked by the first-stage coils and synchronously unrocked to illuminate the same specimen position by the second-stage coils. The technique is used to observe variations of ALCHEMI signals against angular changes in the x and y directions, and to obtain LACBED patterns.

  • bend contour (equal inclination fringe)

    When a crystalline specimen is bent, the dark-field image of the specimen shows strong intensities at positions where a Bragg condition is satisfied, thus the positions being seen to be bright. The bright-field image of the positions is complementarily dark. Since the crystallographic orientation of the positions is equal with respect to the incident beam, they give an equal intensity. The intensity contours are termed equal inclination fringes or bend contours. The bend contours reveal the bend feature of a crystalline specimen. It should be noted that if only one reflection is strongly excited, not only the principal maximum line but also subsidiary maxima lines appear due to a strong dynamical effect.

    bend contour (equal inclination fringe)

    TEM images of bend contours in a mica taken at an accelerating voltage of 200 kV.
    (a)Bright-field image.
    (b), (c), (d), (e) and (f) Dark-field images formed by reflections indicated in diffraction pattern (g).
    (g)Diffraction pattern acquired from the same field.

  • beryllium window EDS detector

    An EDS detector that uses a beryllium film of 8 to 10 μm thickness as a window material to maintain vacuum of a semiconductor detector. Since the window material is robust, the detector is attached to and detached from the microscope column without worry of vacuum leakage. However, the detector is hard to detect elements lighter than sodium (Na) because the beryllium film absorbs low energy X-rays. For this reason, the detector has less been used in recent years.

  • biprism

  • boundary

    In the case of crystal, a "boundary" separates crystals with different orientations or phases, or it separates crystals with different compositions or structures. Boundaries include stacking fault, twin boundary, inversion domain boundary, anti-phase domain boundary, grain boundary, and multi-layer boundary.

  • boundary condition

    The boundary condition is the condition which should be satisfied by the solution function of the differential equation of a scattering problem at the boundary of a crystal. The boundary conditions in the case of the Bethe' s method for the amplitudes of transmitted and diffracted waves are as follows: 1) The amplitudes of the incident electron wave and those inside the crystal are the same at the entrance surface. 2) The tangential components of those electron waves are the same at the entrance surface.

  • bremsstrahlung

    "Bremsstrahlung" is an electromagnetic-wave radiation that is produced when an electron is rapidly decelerated by the Coulomb field of an atomic nucleus at a collision event of the electron with the atomic nucleus. Bremsstrahlung forms the background in an EDS spectrum.

  • bright-field image

    An image that is produced by the transmitted wave (the wave that undergoes no diffraction) in a diffraction pattern formed on the back focal plane of the objective lens, using the objective aperture. In the image, a location where diffraction takes place appears dark, whereas a location where diffraction does not take place appears bright. The bright-field image, together with the dark-field image, is used for analysis of lattice defect and measurement of specimen thickness.

    bright-field image
    Fig. Bright-field image of lattice defects (dislocation lines) in an FeAl alloy. The image was taken in such a way that the distorted area caused by the dislocations does not satisfy the Bragg diffraction condition. Thus, the dislocation lines appear dark. The zigzag contrast of the dislocation lines is created by a dynamical diffraction effect, which depends on the depth of the dislocations in the specimen.

  • brightness

    "Brightness" means the current density per unit solid angle, which is a measure of the quality of an electron source. As the tip of the cathode is smaller, the brightness is higher. The brightness is kept constant at any stage of an optical system if the optical system is free from aberrations.

  • calibration curve

    A "calibration curve" provides the responses of a physical property to a certain variable as a curve. For example, if a calibration curve between the concentration of an element and the intensity of the X-ray emission from the element is provided in advance, the quantitative measurement of the element in a substance can be carried out from a measured X-ray emission intensity.

  • calorimeter

    A device that measures the total energy of a neutral particle, a charged particle or an electromagnetic wave through absorption. An XES device equipped with a "calorimeter" provides a high energy resolution (~10 eV (recent highest data is 4.5 eV)). However, the calorimeter needs liquid helium (He), resulting into large in size. The solid angle of signal acquisition for the calorimeter is ten times or much smaller than EDS. Thus, the calorimeter is used for SEM but not for TEM.

  • camera constant

    The product of the camera length and the wavelength of the incident electron is called "camera constant." The camera constant is equal to the product of the distance from the central spot produced by a transmitted wave to a certain diffraction spot and the lattice spacing corresponding to the diffraction spot. Thus, if the camera constant is calculated using a standard specimen whose lattice spacing is known, measuring the distance from the transmission spot to a certain diffraction spot makes it possible to calculate the lattice spacing of the corresponding diffraction spot.

  • camera length

    An effective distance from a specimen to a plane where an observed diffraction pattern is formed.

  • cathode

    An electrode, to which a negative potential (voltage) is applied against the facing anode. The "cathode" means the filament of the thermionic-emission electron gun or the emitter (electron source) of the field-emission electron gun.

  • cathode-ray tube

    The cathode-ray tube (CRT) displays a two-dimensional image on the tube surface in such a way that an electron beam is accelerated, focused and deflected by electric voltages and magnetic fields to scan the tube phosphor surface.

  • cathodoluminescence

    Electrons in a solid are excited by electron-beam irradiation leaving holes. The electrons recombine with the holes to emit light (ultraviolet to infrared). This phenomenon is called "Cathodoluminescence". Cathodoluminescence is utilized as a method to analyze the electronic structure of solids in an electron microscope. The method can measure local electronic states (energy levels) of impurities and defects, the electronic states being formed between the valence band and the conduction band (in the forbidden band). Thus, the method enables the evaluation of inorganic materials containing the structural defects with a high spatial resolution (better than 1 μm) by the combined use with structural information obtained from SEM / STEM / TEM images. Applications to biological specimens, such as detection of specific proteins, have also been studied.

    • In the case of semiconductors, electrons in the valence band are excited to the conduction band by incident electrons with generating holes in the valence band (referred to as electron-hole pair generation). The excited electrons and holes form a pair (free exciton (FX): It occurs remarkably at low temperatures) bound by Coulomb force. Free excitons are unstable and recombine at arbitrary places and emit light. At room temperature or higher temperatures, the free excitons cannot be formed, thus the generated electrons and holes diffuse independently in the semiconductor as carriers. When they are trapped by impurity atoms creating donors and acceptors, radiative recombination occurs. When they are captured by lattice defects such as dislocations, non-radiative recombination occurs. For example, the distribution of impurity atoms can be detected from a CL image using a specific wavelength.
      CL measurements of semiconductors have to be conducted at liquid nitrogen temperature or lower because the luminescence intensity weakens due to the increases of non-radiative recombination through the lattice vibrations. Also, the CL measurements have to be performed with an accelerating voltage less than 100 kV. : When the accelerating voltage of the incident electron beam exceeds the threshold voltage (approx. 100 kV), the generation of point defects increases. The point defects create deep defect levels within the forbidden band and non-radiative recombinations via those levels increase, causing decrease of the luminescence intensity.
    • In the case of insulators, impurity centers are formed by d-electrons of transition elements and f-electrons of rare earth elements when they are added in oxides or sulfides, and color centers are formed at vacancies in alkali halide crystals. These electrons trapped at such localized centers produce a ground level and an excited level within the forbidden band. Light emission occurs at the electronic transition between those levels, enabling positional information on additive elements to be obtained. Restriction on the accelerating voltage of the incident electron beam is not severe.
    • In the case of organic materials, light is emitted by electronic transition from the lowest unoccupied molecular orbital (LUMO) of the organic molecule to the highest occupied molecular orbital (HOMO). Local information cannot be obtained, but information on degradation and aging of organic molecules can be obtained. Since the organic specimen is susceptible to electron-beam irradiation, observation at a low accelerating voltage is required.

    (By Dr. Naoki Yamamoto, Tokyo Institute of Technology)

    cathodoluminescence
    Fig.1 Schematic diagram of cathodoluminescence (CL) detection system.

    Light emitted from a specimen in a STEM (SEM) by an incident electron beam is collected and reflected by a parabolic mirror, and becomes parallel light, then guided outside the electron microscope. This light passes through the lens 1 (L1) and is focused by the lens 2 (L2) onto the slit of the spectrometer in front of the CCD detector. The spectrum of the light passing through the slit is recorded with the CCD. The incident electron beam is two-dimensionally scanned using the beam deflector controlled with a computer, and the emission spectra from each beam position are successively acquired by the CCD. After measurement, a two-dimensional monochromatic CL image is displayed by selecting a specific wavelength. From the STEM or SEM image and the CL image of the specimen, the optical properties of the structural defects are obtained.
    The luminescence ranges from infrared light (1 to 2.5 μm in wavelength) to ultraviolet light (200 nm to 380 nm). Specific gratings and detectors are used for ultraviolet-, visible- and infrared-light depending on the wavelength range. The wavelength resolution of the detector is approximately 1 nm (less than 10 meV in energy).

    cathodoluminescence
    Fig.2 TEM dark field image, CL image and CL spectra of GaN epitaxial film containing the dislocations penetrating the specimen (accelerating voltage 80 kV)

    A GaN epitaxial film grown by the MOCVD (metal organic chemical vapor deposition) method on a sapphire substrate (film thickness 4 μm) was thinned from the substrate side by ion milling. A transmission electron microscope (TEM) image and a CL image and CL spectra of a thin edge region containing the dislocations penetrating the specimen are shown.

    (a) TEM dark field image. Arrows in the image indicate the dislocations.

    (b) Monochromatic CL image of the same area in (a) taken at room temperature using an emission light with wavelength of 336 nm (3.39 eV).
    It is seen that free exciton (FX) emission occurs in the whole area of the specimen (bright part). Dark contrast appears at the dislocations because the carriers are captured there and non-radiative recombination occurs.

    (c) CL spectra measured at different specimen temperatures. The main peak (P1) is FX emission of GaN, the P2 peak is light emission associated with impurities, and the P3 peak is donor-acceptor pair emission (D, A). The FX emission intensity, which is important for elucidating the dislocation positions, decreases with increasing temperature, i.e., the intensity at room temperature is two orders of magnitude smaller than that at 19K. Thus, measurement at low temperature is necessary, especially for a thin specimen with a low emission intensity. From the analysis of the CL intensity distribution around a dislocation, the diffusion length of the carrier can be measured.

  • caustic surface

    In the ideal (aberration-free) lens, all the electrons that exit from a point on the object plane converge at a point on the image plane, and trajectories of adjacent electron beams do not intersect each other. However, in an actual lens with aberration, the adjacent trajectories intersect and the trace of intersections forms a bright envelope surface. The bright surface is called the "caustic surface."

  • centro-symmetry

    "Centro-symmetry" means that a crystal is symmetric with respect to a certain point.

  • characteristic X-ray

    When one of inner-shell electrons is excited, an electron vacancy is formed in the inner-shell states. Then, an outer-shell electron, whose energy level is higher than that of the inner-shell electron, falls on the vacancy while emitting X-rays. The energy of the emitted X-rays corresponds to the energy difference between the outer-shell electron and the inner-shell electron. The emitted X-rays are called "characteristic X-ray(s)" and are characteristic of individual atoms. The characteristic X-rays are utilized for qualitative and quantitative element analysis on a micro-area.

  • charge and orbital ordering

    "Charge and orbital ordering" states the charge-ordered state and the orbital-ordered state for 3d electrons in transition metals in a crystal. The charge-ordered state means that two charge states are alternately and regularly arranged. The orbital-ordered state means that orbitals, for example eg orbitals, with one orientation and the other orientation are alternately and regularly arranged. These states emerge in perovskite oxides containing transition metals when composition or temperature is changed. Such super structures in electronic systems are reflected in lattice systems, thus these structures are observed by superlattice reflections in an electron diffraction pattern.

  • charge-coupled device

    "CCD" is a two-dimensional digital semiconductor photoelectric conversion device. For electron beam detection, electrons are converted into light by a fluorescent material or a YAG crystal. When the CCD is irradiated with light, electron charges are accumulated in the depletion region (potential wells). Then, these charges are transferred to successive wells. Finally, these charges are taken out as electric signals. Since the CCD contains a dark current, the device is cooled to suppress this current (cooled to -30 ℃ by Peltier cooling). A normally-used CCD has a size of 2K × 2K (a square of approximately 3cm) with a spatial resolution (pixel size) of 14 μm (7 μm for visible light available). Compared to the imaging plate, CCD has a smaller dynamic range of 4 digits or a gray scale of 16 bits than an imaging plate. However, the most advantage of CCD is that it can be used online (only offline use in the imaging plate). CCD is used for acquisition of a high-resolution image and for detection of X-rays in WDS. Recently, a CCD with a larger area (4K × 4K) have been getting popular.

  • charging

    If a specimen has no conductivity, part of incident electrons accumulate on the specimen. This phenomenon is called "charging." Charging disturbs normal transmission and scattering of the incident electrons, thus abnormal contrast and distortion of the image occur. To prevent charging, conductive coating is performed.

  • chemical fixation

    Chemical fixation is a technique to fix a specimen with chemicals to prevent autolysis by the action of enzymes and deformation of morphologies during specimen preparation.  Biological tissues start autolysis caused by their enzymes immediately after stopping the activities of them. For the TEM observation of a biological specimen, the specimen must be prepared by removing water to preserve its morphologies under vacuum in the microscope column, and by thinning the specimen to transmit an electron beam.  For these requests, dehydration, resin embedding and thin sectioning are applied, but the procedures cause deformation of fine structures of the specimen. Chemical fixation is carried out to preserve the morphologies and physical properties at the living states of the specimen as much as possible.  This technique prevents the autolysis and deformation by cross-linking the proteins or lipids of biological materials using chemicals.  Chemical fixation for a TEM is performed by the two steps, prefixation and postfixation.

  • chemical order/disorder

    An order about the arrangement of constituent elements in a compound or an alloy. For example, in the case of a compound containing two atoms A and B, a "chemical order" implies that each atom is alternately arranged in an ordered fashion. When each atom is arranged in a random fashion, this state is called a "chemical disorder." The chemical disorder takes place when the chemical properties of the constituent atoms are similar to each other. Compounds or alloys can take a chemical disorder state at high temperature and undergo a chemical order state at low temperature.

  • chemical polishing

    "Chemical polishing" is a technique used for thinning inorganic materials of semiconductors and insulators. In this technique, a specimen is immersed in a polishing solution that is mainly composed of strong acid or strong alkali. Then, the specimen is thinned with its surface kept smooth. The advantage of chemical polishing is that thin specimens can be made with no mechanical distortion stress. Normally, a polishing solution that does not cause selective dissolution (etching) is used. When a particular layer in a multi-layer semiconductor is required to observe, a specified solution is used, which dissolves other layers while leaving the particular layer.

  • chemical shift

    "Chemical shift" in solid state physics means that the energy level of an inner-shell electron changes when the valence number (chemical state) changes. For example, if one valence electron is removed from an atom, inner-shell electrons are further attracted to the nucleus of the atom and the energy level of the inner-shell electrons shifts to a lower level. Thus, the energy difference between the inner-shell level and the bottom of the conduction band becomes larger. As a result, the onset energy of the EELS core-loss spectrum shifts to a higher energy. The removal of one valence electron can cause an energy shift of about 2.5 eV.

  • chemical-bonding state

    "Chemical-bonding state" means the energy and momentum of electrons which connect atoms to each other in a molecule or crystal. EELS enables us to perform detailed analysis of the chemical-bonding state.

  • chirality

    As in the case of the human left-and-right hands, "chirality" means that two certain structures having mirror-image relation to each other cannot coincide through rotation operations. These structures are possible for crystals belonging to the point groups which do not have mirror symmetry and centro-symmetry.

  • chromatic aberration

    If there is energy (wavelength) spread of the incident electron beam or the electron beam passing through a specimen, the refraction angles of these electron beams are different depending on wavelength. Thus, a blurred image is produced on the image plane. This phenomenon is called "chromatic aberration." The energy spread of the electron beam arises from instability of the accelerating voltage, the spread of the initial speed of electrons emitted from the electron gun, the Boersch effect and change of the focal length caused by fluctuations of the excitation current of the lens coils. In addition, as a specimen is thicker (~10 nm or more), energy loss of electrons (change of wavelength) due to inelastic scattering gives rise to the chromatic aberration.

  • cleaving

    "Cleaving" is a technique used for specimen preparation for a TEM, which cleaves a solid to expose a particular orientational section of a crystal.

  • coating

    "Coating" is to form a thin layer (ex. metallic layer) on the surface of a target substance.

  • coherence

    "Coherence" is used to indicate the degree of the capability of interference of the electrons emitted from an electron gun. The spatial coherence length of the electrons exiting from the electron source, i.e., the interference distance along the direction perpendicular to the traveling direction of the electrons is determined by the source size and the wavelength of the electrons. A highly coherent electron source is essential for electron holography, where the electron waves are directly brought into interference. Such an electron source or a small-sized source is also required for taking a high-contrast crystal structure image formed. The temporal coherence length, i.e., the interference distance along the traveling direction of the electrons is determined by the monochromaticity and wavelength of the electrons exiting from the source. The temporal coherence has not become a matter of discussion yet in the case of TEM.

  • cold (cathode) field-emission electron gun

    The cold (cathode) field-emission electron gun (CFEG) emits electrons from the tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is kept at room temperature in a strong electric field. Since the energy spread of the emitted electrons from the CFEG is narrower (~0.3 eV) than the Schottky type, the CFEG provides a superbly high energy resolution in EELS. Since the size of the virtual source produced is as small as ~10 nm, the electron beam has a high coherence, suitable for electron holography. Its brightness is as high as ~1×109 A/cm2.sr at 200 kV. The CFEG can produce a small-sized probe. Since the emitter surface is likely to be contaminated by residual gases, the emission current is likely to fluctuate. It is necessary to flash the emitter tip in a commercially-available CFEG at intervals of about 8 hours. Recently, the stability of the beam current has greatly been improved by acquiring a better vacuum. Thus, the barrier for EELS, EDS and WDS experiments by using CFEG is lowered.

  • column approximation

    "Column approximation" is an approximation method to calculate the amplitudes of transmitted and diffracted waves at the bottom of a crystal, which assumes the crystal to be composed of columns. The top specimen plane, which affects the formation of an electron wave at a point on the crystal bottom plane, is deduced by Fresnel zones. The radius of the first Fresnel zone, R, is given by √(λt), where, λ is the wavelength of the electron and t is the specimen thickness. When it is assumed that λ = 0.0026 nm and t = 100 nm, the diameter of the Fresnel zone, 2R, is ~0.5 nm. If Fresnel zones are taken up to the third zone, its diameter is ~1.5 nm. That is, when the amplitude of an electron wave at a certain point on the crystal bottom plane is calculated, it is enough to consider transmission and diffraction of the electron wave in a column of 2 nm or less. When a distorted crystal is treated, the amplitudes of the transmitted and diffracted waves are calculated inside such columns, in each of which the crystal is assumed to be a perfect, and between which the crystals are shifted each other corresponding to the distortion.

  • coma-free axis

    "Coma-free axis" is an axis where the axial parasitic coma aberration of the objective lens becomes 0 (zero). Alignment of the coma-free axis means to align the incidence direction of the electron beam with the coma-free axis. The axial coma aberration is a parasitic aberration that is originated from mechanical disagreement between the axes of lenses. It should be noted that this aberration is different from the (off-axial) coma aberration among five Seidel aberrations, which is intrinsic to the lens. In the case of an electron microscope without a Cs corrector, the axial coma aberration due to the disagreement of the axis of the objective lens is corrected by the alignment coils above the specimen chamber. The coma-free axis is not so different from the accelerating voltage center. On the other hand, in the case of an electron microscope equipped with a Cs corrector, the axial coma aberration due to the disagreement of the axis of the Cs corrector is the matter of concern. A double-deflection coil system is provided for the alignment of the coma-free axis other than the alignment coil system for accelerating voltage center. The coefficient of the axial coma can be corrected to a value of a few nm. In actual alignment, using an amorphous carbon (C) thin foil or an amorphous germanium (Ge) thin film, the incident beam is tilted to more than some 10 mrads and 4 to 16 micrographs of the thin foil are taken at 0°to 360°in the azimuth directions to form a diffractogram tableau. Then, the tilt of the electron beam to the Cs corrector placed after the objective lens is adjusted so that a set of the tableau becomes centro-symmetric with respect to the center of the tableau (position at a tilt angle of 0°). This alignment is important when taking high-resolution images.

  • combination aberration

    If two thin optical elements (lenses, multi-poles, etc.) are placed at a free space, higher order aberrations than those inherent to the elements can arise due to a synergetic effect of the inherent aberrations. The higher order aberrations induced are called "combination aberrations." A thick hexapole spherical aberration corrector produces a third-order negative spherical aberration by a self-conbination effect of two three-fold astigmatism fields (second-order aberration). With the use of this third-order negative spherical aberration, the third-order positive aberration of the objective lens can be corrected. In the case of the combination between a spherical aberration (Cs) corrector and an objective lens with a free space (incomplete image transfer) between them, a positive or negative fifth-order spherical aberration is induced by a cross combination between the third-order negative aberration of the Cs corrector and the third-order positive aberration of the objective lens. Using the fifth-order spherical aberration produced, the residual fifth-order spherical aberration of the system can be removed.

  • composite crystal

    A "composite crystal" is composed of two or more crystals whose structures have incommensurate periods (not expressed by an integral ratio) to each other.

  • concave grating

    A kind of reflection grating that is designed in such a way that many grooves are cut in parallel on the surface of a spherical or parabolic concave mirror. The concave grating is used for not only spectroscopy but also for focusing a beam or producing a parallel beam.

  • condenser aperture

    The "condenser aperture" is classified into the fixed aperture and the movable aperture. As for the former case, a fixed condenser aperture with a diameter of ~0.5 mm to 1 mm is inserted into the first condenser lens to cut unnecessary beams. As for the latter case, about five movable apertures with different diameters ranging from ~10 μm to 200 μm are inserted into the second condenser lens, which determine the divergence angle and the dose of the incident beam.

  • condenser lens

    The "condenser lens" consists of two lenses: The first lens demagnifies the crossover of the electron beam emitted from the electron gun to ~1/10 in size. The second lens transfers the demagnified beam onto the object plane of the objective lens with a magnification of ~1×.

  • condenser mini lens

    The "condenser mini lens" is placed between the condenser lens and the objective lens for producing an electron beam with an appropriate convergence angle suitable for respective observation modes. The condenser mini lens does not have a polepiece that strengthens the magnetic field as the objective lens does. When the excitation of condenser mini lens is weak, convergent illumination of an electron beam for a nano-area for STEM, CBED and analytical use is achieved. On the other hand, when the lens excitation is strong, parallel illumination of electron beam for observation of the bright-field image, dark-field image and HREM image is achieved.

  • condenser-objective lens (C-O lens)

    The "condenser-objective lens (C-O lens)" makes the pre-magnetic field of the objective lens act as a condenser lens so that the electron beam is focused onto the specimen, and makes the post-magnetic field act as an objective lens to form the specimen image at the image plane. Since the electron beam is demagnfied to ~1/100 by the pre-magnetic field on the specimen, the C-O lens is essential for various purposes, such as CBED that requires sub-nanometer-area illumination, improvement of STEM image resolution, and decreasing of analysis region for EDS and EELS. To produce a parallel beam for HREM in the TEM mode, a condenser mini lens is used.

  • conductive coating

    "Conductive coating" is to coat a nonconductive specimen for TEM with conductive carbon by arc discharge so that charging on this specimen is prevented and observation is properly performed.

  • contamination

    Hydrocarbons deposited on a specimen surface condense at an electron-beam irradiation area by the electrostatic force of the electron beam and are polymerized by the electron beam. This phenomenon is called "(specimen) contamination." Contamination is particularly severe when a small specimen area is irradiated with a focused, high-density electron beam, for example CBED or STEM.

  • continuous X-ray

    "Continuous X-rays" are X-rays having continuous wavelength distribution, which are produced when electrons are rapidly decelerated by the Coulomb field of an atomic nucleus. An X-ray tube used in a laboratory utilizes this phenomenon (Bremsstrahlung).

  • contrast transfer function

  • convergence angle

    When a cone shaped, convergent electron beam illuminates a specimen, the semi-angle of the cone is termed "convergence angle."

  • convergence illumination

    "Convergence illumination" is to illuminate a specimen with a converged beam.

  • convergent-beam electron diffraction

    A method for qualitative and quantitative analysis of crystal structures from a disk diffraction pattern, acquired by illumination of a cone-shaped convergent electron beam on a small specimen area with a diameter of 10 nm or less. The CBED disk displays the intensity distribution corresponding to changes of diffraction conditions (rocking curve). The method enables us to determine not only specimen thickness, lattice parameter, crystal symmetry (point group and space group) and characteristics of a lattice defect, but also a crystal structure (refinement of atomic coordinates, Debye-Waller factors, low-order structure factors (potential distribution)). When the large-angle convergent-beam electron diffraction (LACBED) technique is applied, the characteristic feature of a lattice defect is easily and unambiguously identified and the strains (and a dislocation) at interfaces of a multi-layer material are determined with a high accuracy. The use of an energy filter allows us to conduct more accurate structure analysis.



    (a) Conventional electron diffraction (Selected-area diffraction)
    Illuminating a parallel electron beam onto a specimen forms a spot diffraction pattern on the back focal plane of the objective lens. This diffraction pattern is magnified with the imaging lens system and then displayed on a screen. In the figure, the ray path with the imaging lens system is omitted.

    (b) Convergent-beam electron diffraction
    Illuminating a cone-shaped electron beam onto a specimen forms a disk diffraction pattern on the back focal plane of the objective lens. This diffraction pattern is magnified with the imaging lens system and then displayed on a screen.
    In conventional CBED, the convergence semi-angle a is limited at the maximum to the Bragg angle q to avoid the overlap of adjacent diffraction disks.



    (c) Example of Conventional electron diffraction (Selected-area diffraction) pattern.
    Diffraction pattern of Si [111] taken at an accelerating voltage of 200 kV.

    (d) Example of Convergent-beam electron diffraction pattern.
    CBED pattern of Si [111] taken at an accelerating voltage of 200 kV by eliminating energy loss electrons (Zero-loss pattern).

     

  • coolant (refrigerant)

    A substance that is used for cooling components of instruments or other materials. For cooling, the latent heat of the "coolant" is used, which is emitted or absorbed at the transformation from the liquid state to the gas state or vice versa.

  • core-hole interaction

    Core-hole interaction means a Coulomb interaction between a hole created in an inner shell (a core-hole) and an excited electron, and the effect on the electronic structure caused by the core-hole. In the case of metals, the effect of the core-hole interaction is small because the free electrons screen the effect of the hole. However, in the case of oxides, the effect is large because oxides have no free electrons.
    It has been known that conventional theoretical calculations cannot provide a good agreement with the experimental spectrum of an energy-loss near-edge structure (ELNES) in an EELS (electron energy-loss spectroscopy) spectrum obtained from an insulator (oxides, etc.). To solve this problem, the core-hole interaction has been introduced. When the ELNES spectra are calculated by taking account of the core-hole interaction, the experimental spectra are well reproduced.
    The core-hole interaction causes the absorption edge of the EELS spectrum to shift to a lower energy side. The effect of the interaction is large when the energy difference between the excited electron and the hole is small.
    Fig. 1(a) shows an experimental O-K ELNES spectrum of magnesium oxide (MgO). Fig. 2(b) shows a calculated spectrum of the same material without core-hole interaction. The two spectra do not show a good agreement in their shapes. Fig. 1(c) shows a calculated spectrum with core-hole interaction, where a core-hole is created in the inner shell (1s) of oxygen (O) and a strong Coulomb force between the hole and the excited electron is taken into account. For this spectral calculation, the core-hole interaction was taken into account, where a Coulomb force is strong against the excited electron. This spectrum is seen to show a much better agreement with the experimental spectrum. This enables us to understand the importance of the core-hole interaction for explaining the experimental ELNES spectra.
    Fig. 2 compares the wave functions of the conduction electrons between the two cases (a) without a core-hole and (b) with the core-hole in the 1s orbital of the central oxygen atom (O*). It is seen that the intensity of the wave function near the central oxygen in (a) is higher than in (b). This is because Coulomb force received by the conduction electrons in the vicinity of the oxygen atom is stronger due to creation of the core-hole. Thus, the core-hole interaction is seen to affect the local spatial distribution of the wave function.
    (By Professor Teruyasu Mizoguchi, The University of Tokyo)

    O-K ELNES spectra of MgO.
    Fig. 1 O-K ELNES spectra of MgO.
    (a) Experimental ELNES spectrum.
    (b) Calculated ELNES spectrum without the core-hole interaction.
    (c) Calculated ELNES spectrum with the core-hole interaction.
    The calculated ELNES spectrum which takes account of the core-hole interaction shows a much better agreement with the experimental spectrum.

    Fig.2
    Fig. 2
    Squares of the wave functions of the conduction electrons of MgO (001) with and without the core hole.
    (a) Square of the wave function without the core-hole interaction.
    (b) Square of the wave function with the core-hole interaction.
    The square of the wave function of the conduction electrons in (b) is seen to be larger near the central oxygen atom than that in (a).

  • core-loss spectrum

    The high energy region (more than about 50 eV) of an EELS spectrum is called "core-loss spectrum." This spectrum, which is produced by exciting inner-shell electrons to the conduction band, enables us to perform qualitative and quantitative analysis of constituent elements, and to obtain the density of states of the conduction band of a material. The spectrum shows a structure specific to the material.

  • correlation method

    In three-dimensional tomography, the "correlation method" takes the correlation between each image and performs positional adjustment of these images so that the correlation becomes maximum.

  • counting rate

    When the average value of the occurrence of an event is measured by a counter, "counting rate" means the number of counts per unit time.

  • critical backing pressure

    The maximum backing pressure at which the diffusion pump can operate correctly is called the "critical backing pressure."

  • critical-voltage effect

    "Critical voltage effect" means that the intensity of the second order reflection from a crystal lattice plane vanishes at a certain accelerating voltage of the incident electron beam when it is increased. The use of the effect enables us to refine (determine) the value of the crystal structure factor of the first order reflection with high accuracy.

  • cross-correlation function

    "Cross-correlation function" is a function expressing to what extent arbitrary two functions are similar or to what extent the two functions are shifted. It is a function (or a pattern) that is acquired by integrating the product (overlap) of arbitrary two functions with respect to a variable which are shifted by a certain amount about the variable. That is, when we define the object functions f, g, an integral variable of the functions X and a relative shift of the two functions x, the cross-correlation function Rfg can be written as the following equation: Rfg=∫f(X)g*(X-x)dX. Note that * denotes complex conjugate. In the case of a microscope image, etc., the functions f and g are real, and then g*(X-x)=g(X-x). If the two object functions are the same, the cross-correlation function is reduced to the auto-correlation function. If the value of Rfg is large, it indicates that the two functions (or patterns) are similar to each other. If Rfg takes a large value for a certain value of x, the relative shift between the two functions is obtained. For example, when the cross-correlation function is calculated for two TEM images taken successively, it gives the knowledge about the image drift during the acquisition of the images. (If the value of the correlation function is large for a small value of x, this indicates a small image drift.) For high-speed computer calculation of the cross-correlation function, FFT (fast Fourier transform) is used on the basis of the following theorem: Fourier transform of the product of certain functions is equivalent to the product of Fourier transforms of the respective functions. That is, the cross-correlation function is calculated by the inverse Fourier transform of the power of the Fourier transforms of the respective functions.

  • cross-linking

    “Cross-linking” means that target chain-like macromolecules are additionally bounded to each other with other molecules by constructing inter- or intra-molecular bridges.

  • crossover

    An electron beam emitted from the cathode is converged by an electrostatic acceleration lens in the electron gun, and then forms the minimum cross section ahead of the cathode. The minimum cross section is termed a "crossover." The crossover is formed in the case of an electron gun fitted with a conventional tungsten emitter or a LaB6 tip. The brightness of the electron gun refers to that of the crossover.

  • crossover point

    A point where the cross section of the electron beam becomes minimum when the beam is converged with the electron lens.

  • crushing

    "Crushing" is a technique used for preparing a thin film on an oxide, a ceramic material, etc. In this technique, a bulk specimen is crushed in an agate mortor and the crushed specimen is dispersed in an organic solvent (methanol, acetone, etc.). After this process, an obtained suspending solution is dropped onto a microgrid for TEM and then thin specimen fragments are deposited.

  • cryo pump

    The cryo-pump produces a cryogenic surface using a coolant such as liquid helium. Residual gas molecules are adsorbed to this surface resulting in a high vacuum. The working pressure is from 10-2 to 10-13 Pa. However, the pump is not suitable for adsorption of hydrogen, helium and neon. In the specimen chamber in a TEM, a fin cooled by liquid nitrogen is used to produce high vacuum.

  • cryo-electron microscopy

    "Cryo-electron microscopy" is a microscopy method used for the observation of biological specimens at the temperature of liquid nitrogen or liquid helium. The biological specimens of purified proteins, viruses, lipid molecules, etc. are prepared by various freezing methods (ice embedding, freeze sectioning, etc.) without staining, and are inserted into a microscope with the specimens kept frozen. Since biological specimens are mostly composed of light elements, scattering contrast is extremely weak to observe. Thus, the specimen is observed using phase contrast produced at a defocus of a few μm.
    For inserting the low-temperature (frozen) specimens, two techniques are available: the use of a cryo-transfer holder or a dedicated cryo-electron microscope which has an automatic specimen transfer mechanism.
    3D structure analysis methods using cryo-electron microscopy include single particle analysis and tomography.

  • cryo-transfer holder

    A specimen holder used for biological specimen observation in cryo-electron microscopy. A biological specimen on a microgrid is loaded on the specimen holder in the cryo-workstation under liquid nitrogen temperature. The holder is connected to a Dewar for Liq. N2 and the tip of the holder is maintained at the liquid-nitrogen temperature. When the holder is inserted into the electron-microscope column, the tip of the holder is exposed to the air (though in a short time), thus the holder incorporates a shutter to prevent frost deposition to the specimen.

  • crystal field splitting

    "Crystal field splitting" means that the energy level of 3d electrons is split depending on a crystal field surrounding the 3d electrons. In the case of the perovskite structure, a transition metal having 3d electrons is located at the center of the oxygen octahedron, the energy of the eg orbital of the 3d electrons, which is faced to the oxygen atoms, is higher than that of the t2g orbital, which is pointing between the oxygen atoms. In an EELS spectrum, two peaks corresponding to the t2g and eg states due to the crystal field splitting appear when 2p electrons are excited to the 3d unoccupied band and when 1s electrons are excited to the 2p band hybridized with the 3d state.

  • crystal growth

    "Crystal growth" means that a single crystal or a polycrystal grows from a crystalline substance through various processes such as solidification from melt, solidification from vapor, and precipitation from solvent.

  • crystal orientation

    "Crystal orientation" is defined by the plane (Miller) indices of the lattice plane of a crystal. In observation of an electron microscope image using a TEM, the particular crystal orientation (usually, orientation expressed by the low-order indices) is aligned to the direction of the incident electron beam. The use of a Kikuchi pattern enables us to easily align the crystal orientation with high accuracy.

  • crystal structure

    "Crystal" is an object in which a unit composed of atoms and molecules is periodically arranged. The unit is called a unit cell. If we know the atomic arrangement in the unit cell, we obtain the entire knowledge of the crystal. The arrangement of atoms and molecules in the unit cell is called "crystal structure." In TEM, convergent-beam electron diffraction (CBED) is used for the detailed analysis of the crystal structure.

  • crystal structure analysis

    For crystal structure analysis, the electron microscope (TEM) image is not used, but the electron diffraction is used because the spatial resolution of the TEM is around 0.1 nm, but the electron diffraction pattern achieves a spatial resolution of 0.001 nm. In structure analysis, there are two methods; one is to use kinematical diffraction, the other is to use dynamical diffraction. The former is applied when a crystalline specimen is thin and consists of light elements and the dynamical diffraction effect can be neglected. Actually, this method is used for protein crystal analysis. The intensities of each reflection are measured from a diffraction pattern. The phases of the reflections are obtained from the real and imaginary parts of the scattering factors which are obtained by Fourier transform of the corresponding TEM image. Then, the structure is obtained by Fourier synthesis of the intensities and phases. The latter method, which uses convergent-beam electron diffraction (CBED), is applied to structure analysis of nano-scale crystals in the field of materials science. The CBED method has more advantage to study the secondary structure of solid materials, or local structures due to lattice defects and lattice strain than to study the primary structure of crystals. A disk diffraction pattern is acquired by illumination of an electron beam with an incidence angle of several 10 mrad on a small specimen area of a diameter of 10 nm or less. The acquired disk diffraction pattern (CBED pattern) exhibits a two-dimensional rocking curve (intensity distribution) corresponding to the spread of the incidence beam angle. (The CBED pattern appears to be complex due to the dynamical diffraction effect, thus the pattern is greatly different from the Laue function or the rocking curve which is expected from kinematical diffraction.) The crystal structure is solved by the fitting between the simulated CBED pattern obtained by the full dynamical calculation and the experimentally-acquired CBED pattern. Since the phases of the diffracted waves are reflected in the diffraction intensities due to multiple diffraction effects, separate determination of the phases of the diffracted waves is not necessary, which is needed in the case of kinematical diffraction. In addition, an energy filter is effectively used to remove inelastic scattering. The third method is the pre-session method. In this method, to avoid the strong dynamical diffraction effect using illumination of a cone-shaped incident beam on a crystalline specimen with a tilt angle of several degrees from a zone axis, the intensities produced by the cone illumination are added for each reflection. The crystal structure is solved by applying the kinematical theory using the obtained intensities, where the direct method for X-ray diffraction is used to estimate the phases of the diffracted waves.

  • crystal structure factor

    The crystal structure factor gives the amplitude and phase of a diffracted wave from a crystal. The factor is determined by the atom species and their positions in a unit cell.

  • crystal structure image

    HREM, which allows wave interference between transmitted and diffracted waves to be caused, enables us to obtain an image exhibiting the crystal structure of a thin crystalline specimen (thickness <10 nm). This image is obtained at the defocus condition, which is determined by the spherical aberration of the objective lens and the accelerating voltage of the incident beam (Scherzer focus). The image is taken by setting the electron beam parallel to a low-order zone axis and by passing a transmitted beam and many diffracted beams through the objective aperture. When the image is not taken at Scherzer focus, it is called "lattice image," which does not always correspond to the crystal structure. Three important factors to take an image corresponding to the crystal structure are: (1) Preparation of a sufficiently thin crystalline specimen, (2) High-accuracy adjustment of the crystal orientation, and (3) Adjustment of the Scherzer focus condition.

  • crystallographic point group

    In crystal symmetry, all the combinations of symmetries that exist with respect to a fixed point in a crystal are called "crystallographic point groups." Symmetry elements consist of rotation(1-, 2-, 3-, 4- and 6-fold), rotary inversion(4-), mirror(m) and inversion(i). 32 independent point groups are constructed by the combinations of the 8 symmetry elements.

  • crystallographic space group

    Since the unit cells are arranged repeatedly in a crystal, symmetry operations by which a point in a crystal is transformed to an equivalent point of a neighboring unit cell are allowed as symmetry operations leaving the crystal unchanged. These symmetry elements are screw axes and glide planes. "Crystallographic space groups" are constructed by the combinations of the screw axes and the glide planes with the symmetry elements of the crystallographic point groups. There exist 230 space groups.

  • curve fitting

    "Curve fitting" is to decompose an entire spectrum into the sum of the component spectra. It is frequently used in spectroscopic analysis.

  • damping

    In wave propagation, "damping" means that the amplitude of a wave is decreasing as the wave is traveling.

  • dark-field image

    An image that is produced by one diffracted wave in a diffraction pattern formed on the back focal plane of the objective lens, using the objective aperture. A location in the image, where the selected diffracted wave takes place, appears bright. The dark-field image, together with the bright-field image, is used for analysis of lattice defect and measurement of specimen thickness.

    dark-field image
    Fig. Dark-field image of lattice defects (dislocation lines) in an FeAl alloy.
    The image was taken with a diffraction spot (indicated by an arrow) of the lower-right diffraction pattern at satisfying the Bragg diffraction condition only at the dislocation lines. Thus, the dislocation lines appear bright.

  • de-scan

    De-scan means to bring the electron beam deflected from the optical axis back to the optical axis using a two-stage deflector coil, the deflection being caused by illumination position or illumination angle of the incident electron beam onto a specimen. De-scan is applied mainly to the following two cases.

    1) When the incident electron beam is scanned over a wide area of a specimen, the electron beam can stray off the optical axis at the peripheral part of the scanned area. Then, the incidence position or the incidence angle of the electron beam to a STEM or EELS detector whose center is adjusted to the optical axis deviates from the axis.
    In the case of STEM; if an electron beam passing through a scan point of the specimen deviates from the STEM detector, the STEM image intensity is varied compared with the correct intensity which should be detected.
    In the case of EELS; if an electron beam passing through a scan point of the specimen does not run on the center of the EELS detector or runs obliquely to the optical axis of the detector, the energy value of the EELS spectrum cannot be correctly measured.
    To avoid such an undesirable electron-beam position or angle to the STEM or EELS detector, a two-stage deflector coil in the image-forming lens system is operated synchronously with the beam scan to always bring back the electron beam to the center of the detector and/or parallel to the optical axis.
     
    2) In the case of precession electron diffraction, to precess the incident electron beam onto a certain point on a specimen, a two-stage deflector coil in the illumination lens system is used. The first deflector coil tilts the incident electron beam to a certain angle (up to approximately 5°), and then the second deflector coil compensates the displacement of the tilted incident electron beam from the optical axis. Then, the electron beam passing through the specimen runs oblique to the optical axis. A two-stage defector coil in the image-forming lens system is used to bring back the electron beam to run on the optical axis. A precession diffraction pattern is obtained by such operation of the two-stage defector coils.  

  • dead time

    "Dead time" is the required time interval between the measuring equipment receiving one signal and this equipment being able to accept the next signal.

  • dead time

  • deconvolution

    "Deconvolution" is a method to eliminate spectral blur due to an instrument function. An observed spectrum is given by the convolution of a true spectrum and an instrument function. Fourier transform of the observed spectrum is expressed by the product of the Fourier transform of the true spectrum and the Fourier transform of the instrument function. Thus, the resultant Fourier transform of the true spectrum is obtained by dividing the Fourier transform of the observed spectrum by the Fourier transform of the instrument function. Then, the true spectrum is obtained by the inverse Fourier transform of the obtained Fourier transform of the true spectrum. The deconvolution method is used by combining the Fourier-log method, the maximum entropy method (MEM) or the Richardson-Lucy method.

  • deflection coil

    Coil(s) that produce a magnetic field to deflect an electron beam.

  • defocus

    In TEM image observation, the focus of the objective lens is shifted for observing Fresnel fringes or for taking a lattice image or a structure image. This focus shift is called "defocus."

  • deformation

    "Deformation" means changes in shape or size of a substance due to contraction or expansion arising from changes of stress, temperature or chemical condition.

  • degas

    "Degas" is to emit gasses from an object by artificial operation. In the case of a TEM, degas is carried out by evacuation of the electron-gun and the column and by heating these parts to accelerate gas emission. On the other hand, natural gas desorption is termed "outgas."

  • delocalization

    "Delocalization" means a phenomenon by which local information cannot be obtained in image observation or EELS using a small probe. This phenomenon is caused by (1) lens aberrations and (2) inelastic scattering. (1) When a high-resolution TEM image is observed, lattice fringes smaller than the point resolution of a TEM sometimes appear at a location different from its true position due to objective-lens aberrations (mainly spherical aberration). For example, when a crystal grain boundary is observed, lattice fringes appear across the boundary. This phenomenon is called the delocalization due to lens aberrations. (2) Incident electrons can be inelastically scattered even when they do not hit the specimen but travel near the edge of a specimen. This phenomenon is called delocalization due to inelastic scattering. This delocalization appears conspicuously in EELS. As an energy loss in inelastic scattering is smaller, delocalization is larger. For instance, surface plasmon excitation can occur even when an electron beam travels a few nm apart from the specimen. In core excitations, delocalization can be larger than the inter-atomic distance when the energy loss is small. As a result, when a specimen is scanned by a probe smaller than an atomic distance, the atom position may not be identified. Electrons inelastically scattered at small scattering angles produce lattice fringes and diffraction contrast similar to elastically scattered electrons (contrast is preserved). Such a behavior (the wave nature of inelastically scattered electrons is similar to that of elastically scattered electrons) is termed "nonlocality."

  • density of states

    The number of electronic states per unit volume and per unit energy in energy band. The density of states (DOS) of the conduction band can be investigated by high energy resolution EELS. (The energy resolution is better than 1 eV for a TEM equipped with a thermal FEG and is about 0.3 eV for that with a cold FEG.) The DOS of the valence band is obtained by XES, which analyzes X-rays emitted from a specimen by the illumination of the incident electron beam. An energy resolution better than 1 eV is required for the analysis of the valence band. A recently-developed WDS analyzer provides an energy resolution of about 0.5 eV, giving us good DOS of the valence band.

  • deposition

    "Deposition" is to form a thin film on a solid surface by physical sputtering, chemical vapor deposition, etc.

  • depth profile

    "Depth profile" means a result of compositional analysis along the depth direction in a specimen, which is obtained by milling a specimen step by step from the surface and by analyzing the milled surface.

  • detection efficiency

    "Detection efficiency" is defined as the ratio of the output signals to incident electrons, X-rays or photons to a detector (input signals). A detector with high detection efficiency meets various requirements, such as high conversion efficiency from input to output, short dead time, and low noise.

  • detection limit

    "Detection limit" is defined as the minimum quantity of a detectable specific component in various analyses. If the amount of signals falls short of the detection limit or the level of disturbance of the noise, the signals cannot be detected.

  • detective quantum efficiency

    "Detective quantum efficiency (DQE)" is the ratio of (S/N)2 of the output (detection) signal to the (S/N)2 of the input signal in various analyses. In the case of the ideal detector, DQE reaches 1.

  • diamagnetic material

    A material that is magnetized in the reverse direction against a magnetic-field direction when placed in the magnetic field, and demagnetized when the magnetic field is removed.

  • diamond knife

    A diamond knife is used to prepare an ultrathin section in ultramicrotomy. The diamond knife was originally developed to cut hard inorganic materials, which cannot be cut with a glass knife. Today, the diamond knife is used also for resin specimens because the glass knife is damaged even by a resin specimen, that is, the blade edge of the knife is likely to become dull. Thus, the diamond knife currently plays an essential role in preparing a uniform ultrathin section.

  • dielectric constant

    A quantity to express the degree of polarizability of a material when a voltage is applied to. Or a quantity to express the amount of electrical energy to be stored in a material.

  • dielectric function

    A function that expresses the dielectric constant of a substance as a function of frequency. In general, the "dielectric function" is expressed to be a complex quantity as a function of frequency and wave number vector. From the dielectric function, optical properties of a substance (such as refractive index and reflectivity as a function of frequency) are derived. From an EELS spectrum, the loss function is obtained which is proportional to the imaginary part of the reciprocal of the complex dielectric function.

  • dielectric material

    "Dielectric material" is equivalent to "insulator." In a dielectric material, no free electrons exist or all electrons are bound to atoms or molecules. When an electric field is applied to a dielectric material, no electric current flows because of no free electrons in the material, but positive and negative charges are created on one surface and the other surface due to dielectric polarization, electric energy being stored.

  • differential interference contrast

    Contrast formed by the difference of optical density or refractive index. It is noted that a substance with different refractive indices does not show any contrast in the transmission microscope. In the case of an optical microscope, "differential interference contrast" is obtained by the following way. Light is split into two rays with different polarizations by a polarizer and these two rays are passed through different specimen areas. At this event, a phase difference arises between the two rays depending on the difference of the optical density of the areas. Then, the two rays with the different phases are put back unpolarized by the polarizer to make interference between the two. As a result, the differential interference contrast is produced. When an additional bias phase is given between the two rays, an image with a shadow on the object edge is formed. In the case of TEM, an attempt has been made to obtain a sharp contrast image by giving an additional bias phase to a half of the electron beam that passes through an object specimen.

  • differential phase contrast imaging

    Differential phase contrast imaging is a STEM method to visualize an electromagnetic field in a specimen by measuring the deflection of an electron beam due to the field at each beam-scan point. The beam deflection is measured with a segmented detector or a pixelated detector. When a segmented detector composed of four segments is used (see Figure below), the angle and the direction of the beam deflection (beam shift on the detector plane) are measured from the difference between the signal amounts acquired with the two segments opposed to each other.
    It is noted that the naming “differential phase contrast” of this imaging method is originated from that the deflection of the electron beam causes the differential or gradient of the phase of the electron wave.
    Differential phase contrast imaging is utilized for observations of micrometer to nanometer scale magnetic domains. In recent years, this imaging method has been applied to analysis of electric fields, and the electric field at the atomic scale has been observed using a transmission electron microscope equipped with a Cs corrector.

    Schematics of a segmented detector
    Fig.(a) Schematic of detection of the electron beam deflection in a specimen using a segmented detector (In case that the electron beam is not deflected by the specimen).
    Fig.(b) A STEM detector in this case is composed of four segments. The shadow of the condenser aperture is projected onto the detector.Top view of the detector and the electron beam seen along the incident beam direction. The signal amounts are the same for the four segments. Thus, there is no difference between the signal amounts acquired from the two segments opposed to each other.


    Fig.(c) Schematic of detection of the electron beam deflection in a specimen using a segmented detector (In case that the electron beam is deflected by a specimen). 
    Fig.(d) Top view of the detector and the electron beam seen along the incident beam direction. When the beam is deflected in the positive direction of the x axis, the signal amounts obtained by subtracting Idet3 from Idet1 becomes a negative value, whereas there is no difference between the signal amounts Ide2 and Idet4. As a result, the beam is found to be deflected in the positive direction of the x axis, and the deflection angle is measured from the absolute value of the signal difference.

  • differential pumping

    "Differential pumping" means that two chambers connected through an orifice or a fine tube with high evacuation resistance are evacuated by separate pumps. Each chamber reaches the pressure determined by the performance of the pumps, without influenced by each other. The camera chamber and the column achieve differential pumping because a fixed aperture (0.5 to 2 mm in diameter) or an orifice is inserted between them. Differential pumping is also executed between the specimen chamber and the electron gun chamber.

  • diffracted wave

    A wave that undergoes Bragg reflection (diffraction) in a crystalline specimen. In the two-beam dynamical theory, the intensity of the diffracted wave periodically changes with specimen thickness.

  • diffraction contrast

    Scattered electrons in a crystalline specimen are not continuously distributed with scattering angles but discontinuously distributed as diffracted waves. Diffraction contrast means the intensity change in an electron microscope image that is formed when the diffraction condition is changed with areas of the specimen. In the bright-field image (formed by the transmitted wave), the area where diffraction takes place loses its image intensity, thus getting dark. In the dark-field image (formed by a diffracted wave), the corresponding area gains image intensity, thus getting bright.

    回折コントラスト:diffraction contrast
    TEM images and a diffraction pattern of polycrystalline Si (a semiconductor interconnect) taken at an accelerating voltage of 200 kV.

    (a) Bright-field TEM image. The specimen is composed of many crystalline particles with size of a few 10 nm to a few 100 nm, which exhibit different crystal orientations. In the crystalline particles indicated by red allows and a green arrow, the diffracted waves are intercepted by the objective aperture. As a result, the particles are observed with dark contrast.

    (b) Dark-field TEM image taken from the same area in (a). The objective aperture is inserted so that the diffracted wave from the crystalline particles indicated by the green arrow in (a) is acquired. As a result, the particle is observed with bright contrast.

    (c) Diffraction pattern. Since the specimen is polycrystalline, the Debye-Scherrer ring is observed. A green circle encloses the diffracted wave from the crystalline particle indicated by the green arrow in (a) and (b).
     

  • diffraction limit

    The diffraction limit is the resolution limit due to diffraction of an electron wave for the optical system with no aberrations. Even in the aberration-free optical system, electron waves exiting from one point on the object do not form an infinitesimal point on the image plane but these electron waves are focused into a finite size spot (airy disk) due to their diffraction phenomenon. The radius r of the airy disk is given by the equation r = 0.6λ/sinα, whereλis the wavelength of the electron and α is the divergence angle of the electron. From the equation, it is seen that the size of the airy disk is small for a large divergence angle of the electron beam. In real TEMs, the achievable divergence angle is ~5×10-2 rad. This limitation makes it impossible to produce an infinitesimal point resolution even for ideal lenses.

  • diffractogram tableau

    "Diffractogram tableau," which is also called "Zemlin tableau," displays two-dimensionally Fourier transform patterns of high-magnification images taken from an amorphous thin film while the azimuthal angle is sequentially changed by a sequential tilt of the incident beam (tilt step: 1 to 2°). Utilizing the degrees of ellipse and symmetries of the patterns, axial astigmatism correction, coma-free axis alignment and three-fold astigmatism correction are executed. When a Rose-Haider-type Cs corrector is installed in a TEM, the use of the diffractogram tableau enables us to correct spherical aberration and four-fold astigmatism and optimize fifth-order spherical aberration. Software that automatically executes these corrections has already been developed.
    Diffractogram tableau
    Figs. (a) and (b) show two diffractogram tableaus without Cs correction and with Cs correction, respectively. The diffractogram tableaus are displayed in the following manner. At the center, a diffractogram obtained without tilt of the incident electron is placed, and around the center, the diffractograms obtained by tilting the incident electron beam are placed according to the tilt angles and azimuth angles.
    When the incident beam is tilted, the shapes of the diffractograms are distorted depending on the magnitudes and symmetries of the axial (geometrical) aberrations. In Fig. (a), each diffractogram for the tilted electron beam is largely distorted from a perfect circle, which is caused mainly by the third-order spherical aberration. In Fig. (b), the diffractograms taken at the tilted incident beams keep almost circular shape and show small difference between the patterns of the diffractograms, indicating the aberrations being almost corrected.

  • diffuse streak

    "Diffuse streak" means the streak intensity appearing in a (selected-area) diffraction pattern. The streak is produced from planar lattice defects (stacking faults, etc.) or linear lattice defects (atomic columns of different species or vacancy columns). Disorder of crystal structures can be deduced from the analysis of the diffuse streak.

  • dimple grinder

    "Dimple grinder" is a mechanical polishing device. It creates a dimple (dip) with a thickness down to 10 μm or less on a parallel plate specimen prepared by polishing using a rotational polishing device (thickness: 100 μm or less). It uses a polishing agent, such as corundum particles having different particle sizes (0.1 μm to several μm). The specimen with the dimple is further thinned by ion milling.

  • dipole approximation

  • direct electron detector

    The direct electron detector is a detector in which accelerated electrons are directly received by an image sensor of CCD or CMOS instead of using a scintillator. Those electrons are converted directly into electric signals, whereas in a usual case, the accelerated electrons are received by a scintillator, converted to light and then the light is transferred to a CCD or CMOS using a lens system or an optical fiber system. Very large electric signals are produced in the case of the direct detector because the scintillator converting electron signals to light signals is not used and the incident electrons possess a very high energy than the light signals produced in the scintillator. The direct electron detector has 10 to 100 times higher sensitivity for electron detection than the scintillator - CCD system. Taking advantage of this high sensitivity, the direct electron detector is effectively used for cryo-electron microscopy (observation of biological materials with cooling), which requires a very low electron dose to avoid specimen damage. In addition, nonuse of the scintillator enables the spatial resolution of a TEM image to be improved. Furthermore, the advantage of its high read-out rate is utilized as a detector taking scanning images.
    However, damage to the detector due to electron-beam irradiation is unavoidable. Thus, replacement of the sensor is necessary if the total electron dose reaches approximately 109.

  • disk of least confusion

    Since the electron lens has spherical aberration, electron beams, which exit from a point at the optical axis on the object plane while traveling in various directions, do not come to one point at the optical axis on the ideal image plane (Gaussian plane). An exiting beam, whose angle to the optical axis is small, nearly comes to the optical axis on the ideal image plane. An exiting beam, whose angle with respect to the optical axis is large, intersects the optical axis above the ideal image plane, thus deviates from the optical axis on the ideal image plane. Adding these formed images produces a least circle (disk) image at a position shifted a little from the ideal image plane to the objective lens. This circle is called "disk of least confusion." The diameter of the disk of least confusion, ds, is given by ds = (1/2)Csα3, which is 1/4 of blur on the Gaussian plane. Here, Cs is spherical aberration coefficient, α is the angle between the electron beam and the optical axis.

  • dislocation

    "Dislocation" is a linear or one dimensional defect in a crystal. When a part in a crystal is displaced against the other part, the line at which the displacement is starting is called a dislocation or a dislocation line. There are two types of dislocations, or edge dislocations and screw dislocations. The crystal around the dislocation is highly strained. Plasticity of a crystal is explained by motion and multiplication of dislocations.
    Fig. 1(a) schematically shows the atomic arrangement near an edge dislocation of a cubic crystal. An extra lattice plane (called an extra half plane) is shown by a black line. The end line of the extra half plane is the edge dislocation or edge dislocation line. Suppose a circuit along the crystal lattice around the dislocation line (Burgers circuit: indicated by yellow lines with allows), the end of the circuit after one round does not come back to the original lattice point but comes to the next lattice point. This displacement is termed the Burgers vector (indicated by a red allow), which characterizes the direction and amount of the atomic displacement caused by the dislocation. The Burgers vector (shown by a red line) of the edge dislocation is perpendicular to the dislocation line.
    Fig. 1(b) schematically shows the atomic arrangement near a screw dislocation. The screw dislocation (line) exists at the start (or end) line of the displacement and runs from the top to the bottom of the figure. The Burgers circuit in this case is seen to be spiral. The Burgers vector of the screw dislocation is parallel to the dislocation line (indicated by a red allow).
    Fig. 2 shows a bright-field image of dislocations in a silicon crystal. The dislocations (lines) are seen to be dark (black) lines because the regions near the dislocations come into a Bragg reflection. Fig. 3 shows a [001] atomic-resolution dark-field STEM image of a small-angle grain boundary of SrTiO3. Edge dislocations are seen to be arranged along the grain boundary, creating a small orientation change between both sides of the crystal.

    dislocation
    Fig. 1.(a) Schematic of edge dislocation. (b) Schematic of screw dislocation.

    dislocation
    Fig. 2. Bright-field image of dislocations in a silicon crystal taken at an accelerating voltage of 200 kV.
    (a) A black line (indicated by arrow "a") shows a dislocation line running parallel to the specimen surface.
    (b) A black zigzag line (indicated by arrow "b") exhibits a dislocation running oblique to the specimen surface. The zigzag contrast is created by a dynamical diffraction effect.

    dislocation
    Fig. 3
    (a) [001] atomic-resolution dark-field STEM image of dislocations on a small-angle grain boundary of SrTiO3 taken at an accelerating voltage of 200 kV. (Specimen courtesy: Prof. Y. Ikuhara, The University of Tokyo)
    (b) Edge dislocations are clearly visualized by selectively displaying crystalline planes along the grain boundary.
    (c) Schematic of the edge dislocation (gray circles indicate atomic columns).

  • dislocation loop

    When oversaturated point defects (vacancies or interstitials) accumulate in a plate form, a closed dislocation is produced at the edge of the plate. The closed dislocation is called "dislocation loop." In the case of the vacancy plate, when the atomic planes neighboring to the vacancy plate collapse to retrieve the inherent atomic distance, a stacking fault is produced. Whether the dislocation loop is the vacancy type or the interstitial type is determined by examining whether the dislocation image is formed inside or outside of the dislocation for the diffraction conditions of the positive and negative deviations from the Bragg condition.

  • dispersion surface

    When the electron waves inside the crystal are considered by the two-beam dynamical theory, the incident and reflected waves change their wave numbers near the Bragg position. At this event, a dispersion sphere splits to produce new two surfaces. These surfaces are termed "dispersion surfaces." Near the Bragg position, the incident and reflected waves do not exist independently, but exist as two Bloch waves that are their linear combinations.

  • displacement

    In crystallography, "displacement" means atomic displacement of an atom from a lattice point due to dislocation or stacking fault, and atomic displacement of an atom arising from (crystal) structural change at phase transformation.

  • distortion

    An aberration where an image does not exhibit rectilinear projection of an object. This aberration, "distortion" produces pin-cushion and barrel distortion for a square object. It should be noted that this aberration distorts the image but does not produce a blurred image as other aberrations do. In a TEM, the distortion becomes a matter of concern for the projector lens. In TEMs of previous generations, a distortion correction method was adopted, in which the barrel distortion of the intermediate lens was compensated by the pin-cushion distortion of the projector lens by setting the excitations of the intermediate and projector lenses to a similar value. In modern TEMs, however, the technique is not used to fix the projector lens to a strong excitation and to vary the excitation of the intermediate lens. In a real TEM, the image distortion is not serious because the distance from the projector lens to the viewing plane is large.

  • divergence angle

    The "divergence angle" expresses the divergence of an electron beam as a semi-angle. The divergence angle of the electron beam to a specimen is determined by the excitation of the second condenser lens and the condenser mini lens, and the diameter of the condenser aperture, when the objective-lens excitation is fixed.

  • dose

    A "dose" is the amount of the degree of (electron-beam) irradiation. The dose is defined by the irradiation beam energy per unit area on the specimen.

  • double diffraction

    When two crystals overlap, which have slightly different lattice parameters to each other, a wave diffracted by the upper crystal is further diffracted by the lower crystal. This phenomenon is called "double diffraction." In addition to the diffraction spots from the upper and lower crystals, spurious spots due to double diffraction appear. Thus, care must be taken to identify a specimen substance. When the upper crystal rotates slightly with respect to the lower crystal (or vice versa) or when the two crystals form twins, many spurious spots also appear due to double diffraction.

  • double-deflection system

    The "double-deflection system" is designed in such a way that two pairs of deflection coils are arranged in the vertical direction. The "double-deflection system" is composed of two pairs of deflection coils. It is placed between the objective lens and the second condenser lens. The electron beam is deflected by the first-stage coils and then, the deflected electron beam is deflected again by the second-stage coils. The system is used for various purposes, such as adjustment of the accelerating voltage center, acquisition of a dark-field image without blurring due to spherical aberration of the objective lens, taking a STEM image, and hollow-cone beam illumination.

  • double-tilt beryllium holder

    A specimen holder made of beryllium, which can tilt a specimen about the X and Y axes and enables high-sensitivity EDS analysis. Since beryllium absorbs hard X-rays that form background for characteristic X-rays, the detection efficiency is improved. It should be noted that the holder cannot be handled with bare hands due to strong poisonous property of beryllium.

  • double-tilt cooling holder

    A specimen holder in which the specimen can be cooled and also tilted about the X and Y axes (double axes). The temperature variable range and the minimum temperature holding time of the liquid-nitrogen cooling holder are -175 ℃ (~100 K) to +50 ℃ and 2 to 3 hours, respectively. The temperature variable range and the minimum temperature holding time of the liquid-helium cooling holder are 20 K to 100 K and about 1 hour, respectively.

  • double-tilt heating holder

    A specimen holder which can heat the specimen and also tilt about the X and Y axes (double axes). The maximum temperature attained is ~800 ℃. Some holder achieves ~1000 ℃. The range of tilt angle is ~±20°.

  • double-tilt holder

    A specimen holder in which the specimen can be tilted about the X and Y axes (double axes). Specimen tilt angle depends on the objective-lens polepiece, ranging between ±20° and ±60°.

  • drift correction (cancellation) system

    A system to detect and correct image shift that arises when the specimen is moved or when the specimen temperature is changed.

  • dry-pumping system

    An evacuation system that does not use oil or water. The "dry-pumping system" includes the scroll pump, the sputter ion pump and the turbo-molecular pump.

  • dynamic observation

    "Dynamic observation" is to continuously observe a phenomenon that occurs in a specimen under various treatments (heating, cooling, tensile stress, etc.) in a TEM. This is also called "in situ observation."

  • dynamic range

    A quantity that expresses a reproducibility capability of the signal intensity. The "dynamic range" is defined as the intensity range obtained by subtracting the noise level from the maximum signal level. The dynamic range of the digital signal is expressed by the number of bits.

  • dynamical diffraction

    An electron that passes through a crystalline specimen is reflected (diffracted) by lattice planes satisfying a Bragg condition. As a specimen is thicker, the reflection occurs many times. At a certain depth, the amplitude of the reflected wave becomes greater than that of the incident wave. Then, the reflected wave is reflected in the direction of the incident wave. Dynamical diffraction takes accounts of such interactions between the incident and diffracted waves. Based on the two-beam dynamical theory, the diffraction intensity is proportional to the crystal structure factor (scattering amplitude) of the reflection. If a specimen is very thin (<3 nm), kinematical diffraction is applied, in which the Bragg reflection is assumed to occur only one time.

  • dynamical extinction

    Reflections that are forbidden due to a screw axis or a glide plane (symmetry element of a space group) become allowed (not forbidden) owing to the dynamical diffraction effect (Umweganregung). However, if two symmetric Umweganregung paths to the forbidden reflection take the same excitation errors, the waves that travel these two paths cancel out. This cancellation of the waves is termed "dynamical extinction." The dynamical extinction appears as a dark band in the forbidden reflection disks in a CBED pattern. The dark band is utilized for identifying a screw axis or a glide plane.

  • eikonal

    "Eikonal" is defined as the line of constant optical path length of a wave, which is obtained by multiplying the line of constant phase by λ/2π,whereλis the wave length. The constant plane of eikonal S expresses the plane of the same phase of the wave. The gradient of eikonal, ∇S gives the traveling direction of the wave. |∇S | gives the refractive index at the corresponding position. The concept of eikonal, instead of the refractive index, has been applied to describe the local change of the amplitudes of Bloch waves in a distorted crystal.

  • einzel lens

    An electrostatic lens that has the same potential at the entrance and exit of the lens. Normally, the "einzel lens" consists of three electrodes. Two electrodes at both ends are set at the ground potential, and a positive or negative potential is given to the central electrode. The lens with a positive potential at the central electrode is called the acceleration-type einzel lens, whereas the lens with a negative potential is called the deceleration-type einzel lens. Both are convex lenses. The electrons passing through the einzel lens undergo lens action without acceleration and deceleration at the exit of the lens. This lens is used for ion-beam instruments, for example, an ion mass analyzer.

  • elastically scattered electron

    Elastically scattered electrons are defined as the electrons which are scattered (change in their traveling direction) by constituent atoms in a specimen without losing their energy. When a specimen is a crystal, elastically scattered electrons become diffracted waves that travel in specific directions given by the Bragg condition. As a specimen is thinner, the intensity of a TEM image or a diffraction pattern is explained by elastically scattered electrons. As a specimen is thicker (~10 nm or thicker), the effect of inelastically scattered electrons must be taken into account.

  • electrolytic plating

    "Electrolytic plating" is a technique to form a thin metal layer on a surface of a solid specimen by electrolysis of a conductive solid (ex. metal) in a suitable electrolyte solvent. Actually, a specimen (acting as an anode) and a platinum (Pt) plate or a stainless-steel (acting as a cathode) are immersed in a suitable electrolyte solvent. By the flow of electric current between these electrodes, the eluted Pt or stainless-steel is deposited on the specimen surface.

  • electrolytic polishing

    "Electrolytic polishing" is a technique used for preparing a thin film on metal, alloy, etc. In this technique, a specimen (acting as an anode) and a platinum (Pt) plate or a stainless-steel (acting as a cathode) are immersed in a suitable electrolyte solvent. Then, an electrostatic potential is applied between these electrodes to elute the atoms of the specimen surface, and finally the specimen surface is thinned with its surface kept smooth. The advantage of electrolytic polishing is that a thinned specimen can be prepared with no mechanical stress.

  • electromagnetic (solenoid) valve

    A valve that operates by an electromagnet (solenoid). The "electromagnetic (solenoid) valve" is used to control the flow of gas or liquid through a tube, and to open or close a vacuum valve.

  • electromagnetic lens

    A lens that converges electron beams by a magnetic field. The magnetic field in the lens, which bends electron beams, is generated by a solenoid magnet. By changing the electric current to the solenoid, the generated magnetic field is changed, leading to changes of the focal length and magnification.

  • electromagnetic wave

    An "electromagnetic wave" is a wave, which consists of an oscillating electric and magnetic fields perpendicular to each other, to the propagation direction. The wave travels in a space at the velocity of light (300,000 km/s). If we list electromagnetic waves in order of wavelength, radio waves have the longest wavelength, followed by infrared rays, visible light rays, ultra-violet rays, X-rays, and gamma rays.

  • electron backscatter diffraction

    A Kikuchi pattern, which is produced by inelastically backscattered electrons emitted from a specimen, sensitively changes with specimen orientation. "Electron backscatter diffraction (EBSD)" is a method for obtaining crystal-orientation distribution images of crystal grains of a polycrystalline specimen. To obtain the image, the incident electron beam (probe) is scanned over the specimen surface, and the orientation change of the Kikuchi pattern is observed. EBSD provides an image with a spatial resolution of ~0.1 μm and an angular resolution of ~1°. The angular range acquired by EBSD is ~20°. Normally, EBSD is performed in a SEM though this may be performed also in a TEM.

  • electron biprism

    An "electron biprism" is an electron-wave interferometer to obtain an electron hologram in the first step of electron holography. The electron biprism consists of a fine string electrode placed at the center of the incident electron beam (perpendicular to the electron beam) and parallel-plate ground electrodes placed at the both sides of the string electrode (parallel to the electron beam). The biprism is located below the objective lens in a TEM. A positive voltage is applied to the string electrode, so that the scattered wave transmitted through the object (object wave) passes through one side of the electrode whereas the wave directly coming from the electron source passes through the other side of the electrode. These two waves are attracted each other by the positive potential, and are superposed to form interference fringes (hologram). The interference fringes contain information on the change of the amplitude and phase of the object wave.

  • electron channeling

    When the electron beam enters a crystalline specimen to cause the Bragg reflection of a low-order reflection plane, two Bloch waves are produced. One Bloch wave is localized on atomic columns and the other Bloch wave is localized between atomic columns. Since the latter Bloch wave does not strike atomic columns, this wave is better transmitted in the crystal than the former Bloch wave. This phenomenon is termed "electron channeling."

  • electron diffraction

    A method for obtaining information on crystal structures from diffraction patterns, acquired by electron-beam illumination on a specimen. When a specimen is a crystal, the information on various crystal structures is obtained, which include periodicities, symmetries and ordering of atomic arrangements, and defects of crystal lattices. When a specimen is an amorphous material, the distances between the first, second, ... nearest neighbor atoms and the number of these atoms are obtained. Kinematical diffraction theory, which is used for X-ray diffraction, can be applied in geometrical analysis of electron diffraction patterns. However, dynamical diffraction theory needs to be applied in quantitative analysis of diffraction intensities. Since the electron beam is much sharper than the X-ray beam, electron diffraction enables us to obtain crystallographic information on local areas.

  • electron energy-loss spectroscopy

    When incident electrons strike constituent atoms in a specimen, some electrons are scattered while losing part of their energies (traveling speed becomes slower) through interactions with electrons and crystal lattices in the specimen. These electrons are called inelastically scattered electrons. A spectroscopy method, which obtains the energy spectra of the inelastically scattered electrons to perform qualitative and quantitative analysis of elements and electronic structure analysis from micro- or nano-areas, is called "electron energy-loss spectroscopy (EELS)." Inelastic scatterings analyzed by EELS are classified into three categories: (1) inner-shell electron excitations or core excitations: (50 to 2000 eV), (2) interband transitions due to valence-electron excitations (0 to 10 eV), and (3) plasmon excitations due to collective oscillations of free electrons (10 to 50 eV).

  • electron gun

    An electron-beam generator, which corresponds to a light source of the optical microscope. The "electron gun" emits electrons from the cathode. The electrons are accelerated toward an anode to produce an electron beam.

  • electron hologram

    An "electron hologram" is a record of interference fringes between the transmitted wave through an object and the wave directly coming from the electron source, those waves being interfered by an electron biprism. Information on change of the phase (and also amplitude) in the object is recorded in the electron hologram.

  • electron holography

    "Electron holography" is a technique to reconstruct phase changes suffered by a specimen by utilizing the coherency of electron waves.
    The scattered wave transmitted through the specimen (object wave: suffering a phase change) and the wave traveling from the electron source and passing through vacuum (reference wave: not influenced by the specimen) are deflected by an electron bi-prism and superposed each other, and then interference fringes or an electron hologram is formed. By applying Fourier transform to the obtained hologram using a computer, a spectrum of the hologram is obtained. Then, the equally spaced, main interference components forming the background in the present case are removed by masking, and modulation components due to the specimen or the diffracted waves (the side-band spectra) are extracted. The modulation components are subjected to inverse-Fourier transform. As a result, the phase changes at the bottom plane of the specimen are reconstructed.
    A field-emission electron gun with a small electron source is required because a highly coherent beam is crucial to obtain the hologram. This technique was proposed by Gabor (awarded Nobel Prize in Physics in 1971) to remove aberrations in a TEM. However, nowadays the electron holography technique is widely used for observation of electric and magnetic fields in a very small (micro- to nano-meter scale) specimen area.

    電子線ホログラフィー electron holography
    (a) Lorentz TEM image, (b) Electron hologram and (c) Phase reconstruction image of a rapidly frozen magnetic material Fe73.5Cu1Nb3Si13.5B9.

    In the Lorentz TEM image (a), the boundaries of a magnetic domain (magnetic domain walls) appear as bright or dark lines. Increasing the defocus broadens the width of the bright and dark lines. When the sign of defocus is reversed, bright lines and black lines are interchanged. In the electron hologram (b), interference fringes are observed, which are obtained by deflecting the wave transmitted through the specimen (object wave) and the wave transmitted in vacuum (reference wave) using an electron bi-prism so as to be superposed each other.
    The large square at the left shows an enlarged image of the small square area. Interference fringes between the object wave and the reference wave are seen, where the bend and spacing change of the fringes are found. A large square at the bottom right shows the enlarged image of the small square area, where the interference fringes at the vacuum region are seen. Since there is no phase change due to the specimen, these interference fringes are equally spaced and straight.
    Bright or dark lines in the phase reconstruction image (c) show equi-phase lines. The directions of the lines indicate the directions of magnetic flux (shown by white arrows) and fringe spacings show the magnitudes of the magnetic flux. A region where the equi-phase lines are almost straight and arranged in the same direction exhibits one magnetic domain. Magnetic domain walls exist at the sudden bends (~90 degrees) of the equi-phase lines. It is found that the sudden bends of equi-phase lines in the electron hologram (c) correspond to the bright or dark lines in the Lorentz TEM image (a).
    (Courtesy of the images: Professor D. Shindo, Tohoku University)

  • electron lens

    A lens that performs action of convergence on electron beams similar to the lens action of an optical lens on light rays. The "electron lens" is classified into the magnetic-field type and the electric-field type. The former is called the electromagnetic lens, and the latter is the electrostatic lens.

  • electron optical system

    A system composed of basic elements, such as electromagnetic lenses,electrostatic lenses and deflection coils to execute the magnification, demagnification, energy dispersion etc. of the electron beam.

  • electron optics

    A field of optics that discusses electron trajectories in electromagnetic fields in analogue of geometrical light optics.

  • electron prism

    An "electron prism" is a (spectroscopic) analyzer to disperse energies of electrons similar to that a glass prism disperses the differences in wavelengths of light. The electron prism is essential for EELS analyzers and is effectively used for energy filtering of TEM images and diffraction patterns. They include the Wien filter, the omega filter, the alfa filter and the Castaing-Henry filter.

  • electron staining

    Electron staining means to adsorb heavy metals of a high scattering power to biological specimens or polymer materials composed of light elements which exhibit a small scattering power for electrons, for enhancing the TEM image contrast of these specimens.
    When observing a biological specimen, uranium or lead is adsorbed to proteins, etc., in the specimen. For a specimen of polymers (e.g. polypropylene, polyethylene) consisting of crystalline and amorphous states, ruthenium can be selectively adsorbed to the amorphous regions because ruthenium tetroxide enters only to the amorphous regions. Those heavy metals which strongly scatter incident electrons give rise to enhancement of the image contrast.

  • electron trajectory

    An "electron trajectory" is defined as the path of motion of an electron in an electromagnetic field, where the electron is regarded as a mass point having a negative charge.

  • electron-probe microanalyzer

    An instrument that performs element identification, quantitative element analysis and element distribution analysis in a specimen by illuminating a specimen surface with a fine electron probe and measuring characteristic X-rays generated. Since the "electron-probe microanalyzer (EPMA)" usually has an electron optical system similar to a SEM, backscattered electron images and secondary electron images are used for searching a specimen position to be analyzed. An optical microscope is also installed in an EPMA for searching a specimen position. Multiple wave-dispersive spectrometers (WDS) are incorporated in the EPMA for spectroscopic analysis of characteristic X-rays to achieve a high detection efficiency. The energy resolution of WDS is about 10 eV, whereas that of an energy dispersive spectrometer (EDS) is about 130 eV. The EPMA with the WDS is possible to conduct electronic structure analysis for favorable cases. The EPMA has a high performance of a detection limit of several tens of ppm and an error of 1% for quantitative composition analysis. The minimum specimen area to be analyzed is about 1 μm in diameter. A recent instrument equipped with an FE gun enables composition analysis of a specimen area of 0.1 μm in diameter.

  • electronic structure

    "Electronic structure" means the electronic states in atoms, molecules and materials. In a solid, the orbits of outer-shell electrons of the atoms overlap each other, creating the valence band and the conduction band. The electronic structure is illustrated by the energy of the electron as a function of momentum and by the density distribution of the energy states in the bands.

  • electronic structure analysis

    Analysis for revealing electronic structures (energy and momentum) in a substance. Bonding features between atoms are analyzed.

  • electrostatic lens

    A lens that converges electron beams by an electrostatic field. Since the "electrostatic lens" has larger aberration than the magnetic field lens, the former lens is not used for imaging but used for acceleration and deceleration of the electron beams.

  • electrostatic potential

    An electrostatic field is produced by static charge distributions. The electrostatic potential is the potential of the electric field, which is produced by charge distributions. Or the electric field is given by the space derivative of the electrostatic potential. The Fourier transform of the electrostatic potential that is created by the nucleus and orbital electrons of an atom gives the atom form factor.

  • element (elemental) mapping

    "Element (elemental) mapping" is carried out by using EELS spectra and EDS spectra. In the case of EELS mapping, the loss energy characteristic of each element in a core-loss spectrum is selected with the energy slit at the EELS mode, and then the mapping of the element is obtained by switching to the image mode. (This explanation is based on TEM-EELS but there is STEM-EELS method which uses the scanning technique like EDS mapping.) In the case of EDS mapping, the X-ray intensities characteristic of each element are measured while the electron beam is two-dimensionally scanned on the specimen, and then brightness modulations corresponding to the X-ray intensities are displayed on a computer monitor synchronized with the scanning signals. As a result, a two-dimensional distribution image of an element is obtained.
     

    element (elemental) mapping

    (a) High spatial resolution elemental maps acquired by EDS. Specimen: SrTiO3, Accelerating voltage: 80 kV.
    X-ray spectra are acquired by two-dimensionally scanning the electron beam over the specimen, and intensities of characteristic X-rays of O-K, Ti-K and Sr-L are displayed on the corresponding scanning points. RGB map is the overlay of each elemental map.

    (b) High spatial resolution elemental maps acquired by STEM-EELS. Specimen: SrTiO3, Accelerating voltage: 80 kV.
    Core-loss spectra are acquired by two-dimensionally scanning the electron beam over the specimen, and intensities of energy-loss electrons of O-K, Ti-L and Sr-M are displayed on the corresponding scanning points. RGB map is the overlay of each elemental map.
    Elemental maps acquired by TEM-EELS using an Omega filter.

    (c) Elemental maps acquired by TEM-EELS using an Omega filter. Specimen: SiC/Si3N4, Accelerating voltage: 1250 kV.
    Core-loss spectra (bottom-right figure) are acquired by TEM-EELS. Energy-loss intensities of C-K, N-K or O-K which are characteristic of respective elements are chosen with the energy selection slit. Then, the energy filtered images of the elements C, N and K are displayed as elemental maps. RGB map is the overlay of each elemental map. Elemental mapping using TEM-EELS enables to acquire a wide-field low-magnification elemental map image over an area of a few μm.

  • en bloc staining

    En bloc staining is one of electron staining techniques. A biological specimen (block tissue) is immersed in a solution of uranium acetate or an aqueous solution of lead aspartic acid after postfixation and before dehydration, for electron staining. En bloc staining enhances the TEM image contrast of membrane structures or fibrous structures in cells. 

  • energy analyzer

    An energy analyzer is used to disperse energies of characteristic X-rays or soft X-rays in WDS, and those of inelastically scattered electrons in EELS. In the case of WDS, analyzing crystals and gratings are respectively used for characteristic X-rays (element analysis) and for soft X-rays (measurements of the density of states of the valence band). In the case of EELS, an omega filter is used as an incolumn type energy analyzer, and a sector analyzer is used as a post-column type energy analyzer.

  • energy contrast

    Image contrast produced by differences in loss energies of inelastically scattered electrons. When the absorption-edge energy of a certain element is chosen with the energy-selection slit, mapping of the selected element in the specimen can be performed. In the future, it will be possible to perform electronic state mapping by selecting the excitation energy of a specific electronic states.

  • energy filter

    Instrument that selects electrons with only specific energies in electrons exiting from a specimen. When only elastically scattered electrons are selected by the energy filter, the background due to inelastically scattered electrons is successfully removed. Thus, a clear microscope image or diffraction pattern is obtained, which provides high-precision structural information. When the absorption-edge energy of a specific element is selected, the corresponding element is mapped. The energy filter is classified into the incolumn type incorporated in the TEM column and the post-column type attached below the column. There are various energy filters with different designs.

  • energy resolution

    "Energy resolution" is the minimum energy (eV) of a spectral peak that can be resolved in spectroscopy. In EDS, the detector performance determines the energy resolution to be 130 to 140 eV. In WDS, it is about 10 eV; however recently, some WDS analyzers can produce an energy resolution below 1 eV. In EELS, the energy spread of the incident electron beam almost determines the energy resolution. Standard EELS resolution using a field-emission electron gun is about 0.7 eV. When an electron gun is equipped with a monochromator, a higher energy resolution of about 0.2 eV is obtained.

  • energy spread

    "Energy spread" means an energy width of an electron beam. This is determined by fluctuations of the initial speed of electrons emitted from the cathode and by inelastic scattering of electrons in a specimen.

  • energy-dispersive X-ray spectroscopy

    Energy-dispersive X-ray spectroscopy (EDS) is an element analysis method. Characteristic X-rays generated from a specimen are detected by a semiconductor detector and converted into electric signals. In the EDS analyzer, the pulse currents that are proportional to the energies of the detected characteristic X-rays are generated, and then these currents are measured with a multi-channel pulse-height analyzer. EDS has higher detection efficiency of X-rays than WDS, but the analyzing power of light elements is lower than WDS. (EDS cannot analyze elements from boron (B) on down.) Since the illumination current of the electron beam for EDS can be decreased from several pA to several nA compared with WDS, the beam damage to a specimen is small. Normally, the resolution of EDS is ~140 eV for Mn Kα emission at 5.9 keV. The resolution determined by statistical counting error is around the square root of energy E of the generated X-ray × √3. (The number of created electrons n is obtained as n~E/3 by taking the band gap energy as ~3 eV. Since the statistical error Δn is ~√n, the energy spread (error) is obtained to be ~Δn・3 = √E・√3.) Recently, an EDS detector that can resolve a beryllium (Be) peak has been developed. Its quantification accuracy is 0.5 to 5%. Compared with EPMA that uses analyzing crystals, EDS provides high spatial resolution, 100 times better than EPMA but shows 10 times worse quantification accuracy than EPMA. "EDX" is also used as the abbreviation of energy-dispersive X-ray spectroscopy.

  • energy-loss near-edge structure

    A fine structure appearing in an energy region of about 30 eV above the absorption-edge energy in the EELS core-loss spectrum. This structure is called "energy-loss near-edge structure (ELNES)." The ELNES is produced by the transitions of electrons from the inner-shell state to the conduction band (unoccupied state), enabling us to obtain the density of states of the conduction band of a substance.

  • energy-selection slit

    The slit inserted into the energy dispersive plane of an energy filter, which selects electrons having specific energies.

  • envelope function

    The phase-contrast transfer function plays an important role for the resolution of an HREM image contrast. Another important factor is the envelope function. The phase contrast is damped with increase of scattering angle due to chromatic aberration of the objective lens caused by energy difference in the electron beam, the divergence angle of the electron beam, stability of the objective lens, etc. The function that expresses the damping is called "envelope function." The spatial frequency at which the envelope function becomes practically 0 (zero) is called "information limit."

    envelope_function

    Example of an envelope function (black line) at an accelerating voltage of 200 kV. The horizontal axis stands for the spatial frequency and the amount of information on the crystal structure of a specimen transferred to a TEM image. The closer the value of the envelope function to 1 (or –1), the more structural information on the specimen contributes to form a TEM image. The closer the envelope function to 0 (zero), the more structural information is lost.
    The envelope function expresses the degree of coherence of the scattered waves, which is determined by chromatic aberration, angular spread of an incident electron beam, etc. When the chromatic aberration or angular spread is large, the degree of coherence of scattered waves deteriorates quickly with increase of spatial frequency, or the envelope function attenuates quickly to 0. The phase-contrast transfer function (PCTF) should incorporate the effect of the envelope function, then resulting to an actual PCTF as indicated by a gray line. This PCTF shows that the contribution of the scattered waves to the image formation damps with the spatial frequency.

     

  • epitaxy

    "Epitaxy" is a technique to deposit and grow a crystalline film on a crystalline substrate using vacuum evaporation (deposition) or chemical vapor deposition (CVD), etc. In epitaxy, the crystalline film formed has a specific orientation relation with the substrate crystal.

  • equal thickness fringe

    When one diffracted wave is matched to the Bragg condition in a wedge-shaped crystalline specimen with a constant orientation, both the bright-filed image and dark-field image of this specimen exhibit periodic fringes depending on the change of the thickness. This pattern is termed (equal) thickness fringes. Two beams whose wavelengths are slightly different are formed in the transmitted wave and the diffracted wave due to the two-beam dynamical diffraction effect. They interfere with each other to cause beats or thickness fringes. The thickness change between the neighboring fringes (extinction distance) is proportional to the reciprocal of the crystal structure factor of the diffracted wave. The fringes are used for the estimation of the specimen thickness. The composition change of a multilayer semiconductor film can be measured from the difference of the extinction distances because the value of the crystal structure factor depends on the composition of each layer.



    Bright-field image of an AlCu alloy taken at an accelerating voltage of 200 kV, where 200 reflection is excited.
    The AlCu specimen is wedge-shaped and its thickness increases toward a far direction (upward in the image) from the vacuum side (left bottom of the image). Equal thickness fringes exhibiting thickness changes are seen.

  • escape depth

    The "escape depth" is the depth of a specimen from which secondary electrons, characteristic X-rays, or Auger electrons can escape.

  • escape peak

    In EDS analysis, when characteristic X-rays emitted from a specimen are detected with a semiconductor detector, part of the energies of the X-rays entering the detector is used to excite inner-shell electrons of silicon (Si) that is a constituent element of the detector. As a result, a small peak appears in an EDS spectrum at an energy lower than that of characteristic X-rays by the excitation energy of Si. This peak is called an "escape peak" and therefore, care must be taken for spectral analysis.

  • etching

    "Etching" is to selectively remove specific surface atoms of a specimen by utilizing chemical or physical reactions.

  • eucentric goniometer

    A specimen stage, which is designed in such a way that the tilt axis of the stage is placed on the specimen plane. Thus, the center of the image of the specimen does not shift during specimen tilt.

  • evacuation system

    The evacuation system of a TEM is composed of the electron gun chamber, the column, the specimen chamber and the camera chamber, and various pumps, valves, and exhaust tubes. In the normal TEM, the electron gun chamber and the column are evacuated by the (oil) rotary pump-the (oil) diffusion pump-the ion pump system, whereas the camera chamber is evacuated by the (oil) rotary pump-the (oil) diffusion pump system. When the electron gun is of the field-emission type, an independent ion pump is used for the electron gun chamber to obtain a high vacuum. Recently, due to increasing requirements for oil-free evacuation system, a system using scroll pump-turbo molecular pump is becoming popular for the evacuation of the camera chamber and the column.

  • excitation current

    An electric current flowing through the lens coil to generate a magnetic field. The "excitation current" is varied to change the magnification and the focal length of the lens. Fluctuations of the excitation current give rise to chromatic aberration.

  • excitation error

    "Excitation error," expressed as sg, means a deviation from a Bragg condition for a certain reflection g. The excitation error is defined as the distance from the reciprocal lattice point g to a point on the Ewald sphere measured along the vertical direction to the upper surface of the specimen. When the Bragg condition is accurately satisfied, sg = 0. When the reciprocal lattice point is located outside the Ewald sphere, sg>0, whereas the reciprocal lattice point is located inside the Ewald sphere, sg<0. sg is an observable quantity with dimension of [length]-1. A dimensionless parameter (tilt parameter) "w = sg ・ξg," whereξg is the extinction distance, is used instead of sg as a useful parameter in the theoretical treatment. However, it should be noted that w is a non-observable quantity.

  • extended energy-loss fine structure

    A fine structure appearing in an energy region of about 40 to 200 eV above the absorption-edge energy in the EELS core-loss spectrum. This structure is called "extended energy-loss fine structure (EXELFS)." The EXELFS is produced due to the scattering of electrons, which are excited from the inner-shell state to the conduction band (unoccupied state), by the adjacent atoms. When the EXELFS spectrum is Fourier-transformed, information on local atomic arrangements is obtained. EXELFS corresponds to EXAFS in X-ray spectroscopy.

  • external disturbance (interference)

    Noises from external electro-magnetic fields and external mechanical vibrations, which degrade the stability and performance of an electron microscope, are called "external disturbance (interference)."

  • external magnetic field

    "External magnetic field(s)" are magnetic fields that affect an electron microscope from the outside. Measures to prevent an adverse influence of the external magnetic fields on the instrument performance are needed. The tolerable external magnetic field for the installation of the electron microscope is 1 mG or less.

  • extinction distance

    When electrons pass through a crystalline specimen and cause one Bragg reflection to occur (two-beam approximation), if an incident wave reaches a certain depth (t), its amplitude becomes 0 (zero) and the reflected wave becomes maximum. Furthermore, if the incident wave reaches a depth (2t) twice as deep as the previous depth, the amplitude of the reflected wave becomes 0 again and the amplitude of the incident wave becomes maximum. Thus, the amplitudes of the incident and reflected waves exhibit beats with the depth. The distance of one periodicity of the beats is called "extinction distance." The extinction distance is inversely proportional to the crystal structure factor of the operating reflection and the wavelength of the incident electron beam.

  • extinction rule

    A rule expressing in what case forbidden reflections occur is termed the "extinction rule." (Forbidden reflection means the extinction of reflections due to scattering factor being 0 (zero) even when the Bragg condition is satisfied.) The extinction of the reflections occurs due to the type of space lattice and due to the symmetry element of the space group. In the former case, reflections vanish even in both the kinematical diffraction case and the dynamical diffraction case. In the latter case, i.e. due to glide planes or screw axes (symmetry elements of the space group), the extinction of the reflections occurs only when kinematical diffraction applies. When the dynamical diffraction effect occurs, the forbidden reflections can have intensity. However, even when there exists the dynamical diffraction effect, extinction occurs at a part of the reflections under a specific beam incidence condition. The extinction appears as a dark line in kinematically forbidden reflections in a CBED pattern.

  • extraction electrode

    An electrode in a field-emission electron gun, to which a positive potential (voltage) is applied for extracting electrons from the emitter. A voltage of 2.5 to 3 kV is applied to the cathode.

  • fast Fourier transform

    A computing method to decrease the calculation time of a Fourier transform. Since "fast Fourier transform (FFT)" is executed using a computer, calculations are performed by a discrete finite sum, instead by an integration. It greatly decreases the computation time by dividing the calculation into certain sub-groups and by changing the order of the calculation. In a discrete Fourier transform with a period of N, the number of operation is normally proportional to N2 but it is decreased to N log N in FFT. This method is used for analysis of high-resolution images.

  • ferroelectric domain

    The ferroelectric material has spontaneous electric polarizations under no external electric field. A "ferroelectric domain" means a region where the directions of all the polarizations are the same.

  • ferroelectric material

    A material that has spontaneous electric polarizations, which can be reversed by the application of an external electric field. In a ferroelectric domain, all the polarizations take the same direction. When an external electric field is applied to a ferroelectric material, strong polarizations appear in the direction of the electric field, and residual electric polarizations exist even after the electric field is removed, thus exhibiting a hysteresis loop of polarization against the external field. Certain materials show ferroelectricity below a phase transformation temperature, called the Curie temperature Tc, and are paraelectric above this temperature.

  • ferromagnetic material

    A material that consists of atoms having magnetic moments, which align parallel to each other, thus exhibits strong spontaneous magnetic polarizations without any external magnetic field. In a magnetic domain, all the magnetic moments take the same direction. When an external magnetic field is applied to a ferromagnetic material, strong magnetic moments appear in the direction of the magnetic field, and residual magnetic moments exist even after the magnetic field is removed, thus exhibiting a hysteresis loop of the magnetic moment against the magnetic field. Certain materials show ferromagnetism below a phase transformation temperature, called the Curie temperature Tc, and are paramagnetic (disordering of the moments) above the temperature.

  • fiber pattern

    A "fiber pattern" is a diffraction pattern produced from fiber structures in which molecules are oriented regularly only along one axis. The diffraction pattern with the fiber-axis incidence forms Debye-Scherrer rings. As the incidence direction is tilted from the fiber axis, the ring-shaped intensity distributions on the successive Laue zones intersect with the Ewald sphere. A distorted arc diffraction pattern or a fiber pattern appears.

  • fiberoptic plate

    A "fiberoptic plate" is a plate bonded with bundled optical fibers with each diameter of severalμm. It is used as a tool to transfer optical signals of a TV camera system.

  • field emission

    "Field emission" is a technique to emit electrons from the tip of the cathode. A sharpened cathode made of a material (normally, tungsten) that has an appropriate work function is placed in a strong electric field produced by the extraction electrode to emit electrons from the cathode tip.

  • field extraction

    "Field extraction" is a technique to extract electrons in a solid to the outside without heating the solid, by applying a strong electric field (107 V/cm or more) to the surface of the solid.

  • field-emission electron gun

    There are two types of field-emission electron gun (FEG); the cold cathode type and the thermal (thermally assisted) type. The FEG emits electrons from a sharpened tip of a cathode by applying a strong electric field. Compared with the thermionic-emission electron gun, the beam current of the FEG is small but its brightness is as high as 107 to 108. The FEG produces a small electron probe with a small energy spread. The FEG is essential for electron holography.

  • fifth-order spherical aberration

    Among the aberrations of rotationally symmetric magnetic and electrostatic lenses, the aberration proportional to α5 (α: an angle between an electron beam and the optical axis) is called "fifth-order spherical aberration." Not only the third-order spherical aberration, but also the fifth-order spherical aberration inevitably exists in an electron lens. The term, "spherical aberration," normally indicates the third-order aberration proportional to α3, and a symbol Cs is used as the spherical aberration coefficient. On the other hand, a symbol C5 is normaly used as the fifth-order spherical aberration coefficient. In recent years, the third-order spherical aberration has been corrected, thus the fifth-order spherical aberration has been needed to be considered. Fortunately, the use of a Cs corrector with the two-stage hexapoles and transfer lenses between them makes possible the value of C5 variable, thus C5 = 0 being available.

  • filler

    A material used for sintering. Yttria, etc., are used as "filler.".

  • finger printing

    "Finger printing" is used for ELNES analysis in EELS. In this technique, the spectra of known chemical bonding states are prepared as fingerprints in advance. Then, an unknown spectrum is identified to a certain bonding state by comparing with the known spectra.

  • first principle calculation

    "First principle calculation" is a method to calculate physical properties directly from basic physical quantities such as the mass and charge, Coulomb force of an electron, etc. based on the principle of quantum mechanics. In other words, the calculation derives physical properties directly from the basic principle without introducing adjusting parameters or modeling which are used to find agreement between theoretical calculations and experimental results. Starting from the Coulomb interaction between electrons, atomic nuclei and electron-atomic nucleus, the properties of materials (mainly electronic properties) are non-empirically calculated. Along with the great improvement of the computer performance, the calculations for systems containing many atoms have become possible with lower cost and high speed, thus the method is increasingly prevailing. The method is indispensable for predicting properties of new materials and of understanding properties of existing materials. In the TEM field, the method is used for the study of fine structures of EELS spectra that reflect the density of states of materials.

  • first zero

    A wave number at which the phase-contrast transfer function at Scherzer focus intersects the phase-zero axis. The reciprocal of this wave number is defined as "resolution" of a structure image.

  • fixed aperture

    An aperture with a certain size fixed at an appropriate position. It includes various apertures: An fixed aperture is placed below the condenser lens to cut unnecessary beams from the electron source. A fixed aperture made of tantalum is inserted into the illumination system to cut X-rays excited in the column for achieving high-accuracy analysis. A fixed aperture is inserted at the entrance of the intermediate lens system of the imaging system to cut reflection electrons in the column. A fixed aperture is inserted below the projector lens to enable differential pumping between the camera chamber and the column part.

  • flashing

    The cold (cathode) field-emission electron gun (CFEG) is operated only under an electric field without heating the emitter, while the Shottky type electron gun is used by heating the emitter. Thus, the emitter surface of the CFEG suffers gas adsorption and ion sputtering, leading to a decrease of the emission current and an unstable emission current. To remove adsorbed gasses and surface roughness due to ion sputtering, the emitter is heated. This procedure is called “flashing.” In the existing CFEG, immediately after flashing, the work function of the emitter becomes large and the emission current decreases due to gas adsorption onto the clean emitter surface. Thus, the CFEG is usually used after the emission current becomes stable with the emitter surface being is covered with a thin absorbed-gas layer, and is used until its emission current is lowered by absorption of many gasses.
    In recent years, a CFEG has been developed, whose pressure of the residual gases in the vicinity of the emitter surface is low. The newly developed CFEG achieves no waiting time after flashing and a stable emission current with less contamination by maintaining the work function small. It provides a high brightness electron beam for a long operation time.

  • fluorescence

    "Fluorescence" means light (electromagnetic wave; infrared ray to ultra violet ray to X-ray) emitted from electronically excited states which are created by absorption of incident X-rays, electrons etc. In EDS, fluorescent X-rays are used for element analysis. In a crystal, an electron in the K-shell of an element is excited to an unoccupied state, and then an electron in the L-shell of the element falls to the K-shell, characteristic K X-rays inherent to the element being emitted.

  • fluorescence excitation effect

    In spectroscopic analysis of characteristic X-rays (EDS), the "fluorescence excitation effect" means that X-rays emitted from non-target elements, whose energy is higher than that of the characteristic X-rays of the target element, are absorbed by the target element and characteristic X-rays of the target element are additionally emitted. Since this effect cannot be neglected in quantitative analysis when a specimen is thicker (~several 10 nm or thicker though depending on target elements), correction for the detected X-ray intensity is required. This effect is, however, smaller than the absorption effect.

  • fluorescent screen

    A tool to visualize TEM images and diffraction patterns. A phosphor on a "fluorescent screen" is excited through electron collision. The emitted visible light produces "light and dark (contrast)" corresponding to the electron intensities to the screen. A fluorescent material is composed of a matrix of zinc sulfide (ZnS) and a dopant of copper (Cu), aluminium (Al) or europium (Eu). A phosphor with high luminous efficiency and with a persistence of ~100 ms is chosen for the fluorescent screen. The fluorescent screen works also as a beam shutter. When acquiring a TEM image, the screen is raised. In recent years, electron microscope images are not observed on the fluorescent screen but on a computer display.

  • fluorescent yield

    A probability of radiation of electromagnetic waves emitted when an excited electron falls to the ground state.

  • focal depth (depth of focus)

    The "focal depth (depth of focus)" is the range of distances for which the object is imaged with an acceptable sharpness on the image plane. The focal depth is proportional to the spatial resolution of a microscope and to the square of magnification, and inversely proportional to the aperture angle. In the case of a TEM, since the aperture angle is smaller than that of an optical microscope, the focal depth is large. For example, if the magnification is 10,000×, the aperture angle is 1×10-3 rad, and the resolution is 1 nm, the focal depth reaches 100 m.

  • focal length

    When an electron passes parallel to the optical axis of a lens, the electron intersects with the optical axis at the back focal plane. The "focal length" is the distance from the center of the lens to the focal plane along the optical axis. The focal length of the objective lens ranges 0.5 to 4 mm depending on the target of observation.

  • focal plane

    A plane that is normal to the optical axis and passes through the focal point.

  • focal step

    The "focal step" means the amount of change of the focal length of the objective lens, when a knob for the objective lens current is varied. The minimum focal step of a recent electron microscope is ~1 nm.

  • focused ion-beam milling

    "Focused Ion Beam (FIB) milling" is a technique of a TEM specimen preparation to mill a bulk specimen with focused gallium (Ga) ions. The target region of the bulk specimen can be selectively thinned down to a desired shape while monitoring and controlling by SEM observation of the milling region. This technique is particularly indispensible for failure analysis of semiconductor devices.
    The FIB milling procedure is as follows: First, the surface of the target region is coated with a platinum- or carbon-protective film in the FIB system to avoid the milling of the target region (Fig. (a)). Next, the target region is milled with a Ga ion beam of a high accelerating voltage of about 30 kV to prepare a section of a thickness of a few μm or less (Fig. (b)). Then, the prepared section is picked up from the bulk specimen and fixed onto a TEM specimen grid (Fig. (c)). Finally, the fixed specimen is milled with a Ga ion beam of a low accelerating voltage of about 5 kV (for decreasing damages to the specimen) to create a thin section of a thickness of 10 to 100 nm (Fig. (d)). If the damaged layers due to the irradiation of Ga ions remain, the layers are removed by another milling with a Ga ion beam of a lower accelerating voltage (about 3 kV or less) or with an argon (Ar) ion beam.

    Focused ion-beam milling

    Fig. (a) Protection layer is formed on the target region by coating of carbon or platinum. (b) Surroundings of the target region are roughly milled with Ga ion beam to prepare a section (thickness: a few μm or less). (c) A section is fixed onto a TEM grid. (d) Thin section (thickness: 10 to 100 nm) is created from the target region by fine milling. (e) The thinned section prepared by FIB is subjected to TEM observation.

  • focusing

    "Focusing" is to focus an electron beam to carry out focusing.

  • forbidden reflection

    Even when a set of lattice planes of a crystal satisfies the Bragg condition, its reflection intensity becomes 0 or vanishes if its crystal structure factor is 0 (zero). Such a reflection is called "forbidden reflection." There are two kinds of forbidden reflections, which occur due to the type of space lattice and the symmetry element of the space group.

  • four-fold astigmatism

    The four-fold astigmatism, one of the third-order axial geometrical aberrations, is a parasitic aberration exhibiting four-fold symmetry. (Note that it is called a fourth-order aberration in terms of the wave aberration.) The four-fold astigmatism appears as a pattern with four-fold symmetry in a Ronchigram. In the case of a TEM equipped with a Cs corrector, prior to microscope observation, aberrations are measured and then the aberrations including the four-fold astigmatism are corrected by the automatic aberration correction function with the use of the deflectors of the Cs corrector.
    It is possible to produce a four-fold astigmatism using magnetic octupole fields. In an octupole Cs corrector, the spherical aberration of the objective lens is corrected using the four-fold astigmatism by utilizing the fact that the four-fold astigmatism has the same order as that of the third-order spherical aberration of the objective lens to be corrected.

  • freeze etching

    Freeze etching is a technique to expose the structure under ice on a freeze-fractured surface of a biological specimen, by sublimating the ice under vacuum.  After the structure is exposed, a replica is made from the fractured surface for TEM observation.  This technique is mainly used to observe the structures of cytoskeletons, organelles, etc. 

  • freeze fracturing

    Freeze fracturing is a technique to cut and expose the inner face of a biological specimen such as cells and tissues.  This technique freezes the specimen with liquid nitrogen, etc., and then fractures the specimen under vacuum by giving an impact on the frozen specimen using a knife.  After the specimen is fractured, a replica of a fractured surface is made for TEM observation.  To observe the fractured surface with a TEM, either of two replication techniques is applied.  One is freeze replication to make a replica of the fractured surface without any processing.  Another is freeze etching by which ice on the fractured surface is sublimated, followed by making a replica of the exposed inner structure without ice. 
    A replica is usually made under vacuum by depositing platinum or platinum palladium onto the specimen and then by depositing carbon onto the already deposited surface.  The specimen is immersed and dissolved in an alkali reagent.  Only the replica is mounted on a mesh (grid) for TEM observation.

  • freeze replication

    Freeze replication is a technique to make a metallic thin-film replica from a fractured surface of a biological specimen.  When a lipid bilayer of the cell membrane is fractured, the extracellular leaflet of the freeze-fractured lipid bilayer is called the E face (or the extracellular face), whereas the cytoplasmic leaflet is called the P face (or the protoplasmic face).  Thus, the technique is used to observe the morphology of membrane proteins.

  • freeze sectioning

    Freeze sectioning is used to prepare a sectioned biological specimen of soft textures. The specimen is frozen under liquid-nitrogen temperature to cut the specimen easier with a cryo-microtome. A cryo-electron microscope is needed for specimen observation.

  • freeze substitution

    Freeze substitution is a technique to replace amorphous ice with an organic solvent (acetone, etc.) (dehydration) in a biological specimen fixed by rapid freeze fixation, where the specimen is subject to rapid freeze fixation (high pressure freezing or metal mirror freezing (slam freezing)).  To prevent destruction of fine structures of the specimen, the substitution is carried out by raising temperature step-by-step from -80 ℃ to 4 °C over a few days.  In the course of the substitution, chemical fixation and electron staining are often performed by adding osmium tetroxide and/or uranium acetate.  Then, the substituted specimen is returned to room temperature and is subject to resin embedding.  A TEM specimen is made by ultrathin sectioning the specimen.

  • full width at half maximum

    A width of a spectrum measured at a half of the height of a spectral peak.

  • gamma curve

    A "gamma curve" is an intensity compression curve for an observed intensity distribution to compress to a visual range when the intensity of a TEM image or a CBED pattern has a large dynamic range.

  • gentle milling

    A surface of a specimen milled by the focused ion beam (FIB) technique or a normal ion milling technique is likely to suffer damage. To remove the damaged layers on the specimen surface, an argon ion beam at a low accelerating voltage of 100 V to 2 kV is used to gently mill the specimen surface. This technique is termed "gentle milling."

  • geometrical aberration

    In geometrical optics where electron trajectories are described as motions of charged particles in an electromagnetic field, the deviation of the real imaging point of an electron from the ideal imaging point with no aberrations is called “geometrical aberration.” Optical properties are expressed by a power-series polynomial of r (distance of an electron beam from the optical axis) and α (angle between the beam and the optical axis), in which a beam emitted from one point on the object plane is mapped to a point on the image plane. If only the first order terms of the expression is taken, the polynomial expresses ideal imaging with no aberrations (Gauss imaging). If the higher order terms than the second order are taken into account, the deviation of the real imaging point from the ideal imaging point appears.
    The order of the geometrical aberration is determined by the sum of the order of α (angle between the beam and the optical axis) and that of r (distance of an electron beam from the optical axis). It is conviniently used for describing the order of the aberration. On the other hand, the wave aberration is useful because the order of the wave aberration is closely related to the symmetry of the aberration. The order of the wave aberration is expressed by adding 1 (one) to the order of the geometrical aberration. For examples, the two-fold astigmatism is a first-order aberration in terms of the geometrical aberration but is the second-order aberration in terms of the wave aberration, and that the three-fold astigmatism is a second order in the geometrical aberration but a third order in the wave aberration.

  • glass knife

    A glass knife is a blade for a microtome. The knife is a fabricated sharp edge line of a glass made by fracturing a thick glass or a glass rod. The glass knife is used to make an exposed specimen surface flat prior to ultrathin sectioning using a diamond knife, or to perform trimming of the resin of a resin embedded specimen.
    Precautions for use; When the knife is applied to hard materials or the same position of the blade is repeatedly used, the blade tends to spill and can damage the surface of a specimen. Thus, the knife needs to be used while changing the position of the blade.

    glass knife
    Glass knife

  • glide plane

    A symmetry element of the crystallographic space group. When a crystal is mirrored with respect to a certain crystal plane (mirror plane) and successively translated parallel to this mirror plane by a length of 1/2 or 1/4 of the crystal unit-cell, the translated crystal can be equivalent to the original crystal. In this case, this plane is called "glide plane." The glide planes are denoted by g, instead of m in the case of mirror planes.

  • goniometer stage

    A stage that has a tilt mechanism. The "goniometer stage" is used for aligning the crystal orientation and for observing a specimen by tilting it with respect to the incident electron beam.

  • grain boundary

    "Grain boundary" separates respective crystalline particles with different orientations in a polycrystalline solid. Grain boundaries are also called crystalline grain boundaries.

    シリコン多結晶の高分解能HAADF-STEM像

    High-resolution HAADF-STEM image of polycrystalline silicon taken at an accelerating voltage of 300 kV. The boundary indicated by red allows is a grain boundary of crystalline silicon. The image reveals that the dumbbells of silicon atoms change their orientations across the boundary. This grain boundary is expressed by Σ3, [110]/{111}.

  • grating

    The grating is an element to obtain a spectrum utilizing diffraction. It is made of a metal and many equally spaced grooves are cut on the surface. (The minimum spacing between the grooves that can be cut is ~0.5 μm.) The spacing must be suitable for the wavelength of the spectrum to be measured. In order to obtain a spectrum from a wide energy range, several gratings with different groove spacings are required. Some gratings are designed to be cut with a non-equal pitch of grooves to reduce aberrations. The grating is used for electronic structure analysis by soft X-ray spectroscopy.

  • grid

    A "grid" is a plate of a metal, etc. with a diameter of 3 mm and a thickness of 20 to 50 μm, which supports a specimen for TEM observation. Various grids such as a square-, a circular-, and a slit-grid, are available. They are selectable depending on observation requirements. The material of the grid is copper (Cu), molybdenum (Mo), gold (Au) or titanium (Ti), etc. In elemental analysis, a grid which does not contain the elements to be analyzed, is used.

    • Reticular grid
      The grid is most commonly used. Fragmented specimens are placed or pasted on the grid. For viruses or small particles, a supporting film is pasted on the grid and then, the specimen is placed on it.
    • Single hole grid
      The grid is used to observe a wide area of specimen without disturbance of the mesh of a reticular grid. Thus, it is suitable for observation at ultra-low magnifications.
    • Reference grid
      The grid is conveniently used at repeated observation of the same field because the marks (letters) to memorize the observation position are prepared on the grid.
    • FIB grid
      The grid is used to attach a thin film specimen prepared by FIB, to the heads of the grid.

    Examples of grid
     Square grid
    1. Square grid
    Single hole grid
    2. Single hole grid
    Reference grid
    3. Reference grid
    FIB grid
    4. FIB grid

  • hairpin filament

    A filament used for a thermoelectron source. A thin tungsten (W) wire is curved to form a hairpin shape and this filament is directly heated at about 2800 K. Its brightness is 5×105 A/cm2・sr at 200 kV. The size of the crossover is ~20 μm. The energy spread of the emitted electrons from the filament is ~3 eV. Nowadays, most electron microscopes are equipped with a LaB6 cathode.

  • hexapole (sextupole)

    A component consisting of six magnetic coils symmetrically placed with respect to the optical axis, which is used for spherical aberration correction of the objective lens.

  • high pressure freezing

    High pressure freezing, one of rapid freeze fixation techniques, is to fix biological specimens such as biological tissues, biological cells and bacteria.  Rapid freezing under an about 2000 atm suppresses formation of ice crystals, which give rise to the destruction of the tissues, by decreasing the melting point of water by about -20 °C and increasing its viscosity.  This technique provides a uniform freezing under amorphous ice with a depth more than one order of magnitude larger (about 200 μm) compared with the freezing depth at the atmospheric pressure.  Specimen preparation for TEM observation of the frozen specimen is carried out by one of the following three procedures.  (1) Applying freeze sectioning to the specimen, (2) Applying resin embedding and ultrathin sectioning to the specimen after the specimen is subject to freeze substitution and is returned to room temperature, and (3) Making a replica of the specimen by freeze fracturing.

  • high-angle annular dark-field scanning transmission electron microscopy

    High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is a STEM method which receives inelastically scattered electrons or thermal diffuse scattering (TDS) at high angles using an annular dark-field (ADF) detector (~50 to sufficiently high angle; e.g. ~200 mrad). A STEM image is acquired by displaying the integrated intensities of the electrons in synchronism with the incident probe position. As the HAADF image intensity is reported to be proportional to 1.4 square to a square of the atomic number, heavy atoms are observed brighter, but light atoms are difficult to be observed. The HAADF image is easily interpreted due mainly to two reasons.
    1) No multiple scattering arises because the scattering cross section of TDS at high angles used for the imaging is small.
    2) The interference effect of electrons does not take place for the imaging (non-interference image).
    The resolution of the HAADF image is almost determined by the incident probe diameter on the specimen. A high-performance STEM instrument provides a resolution better than 0.05 nm. A combined use with EELS, which uses electrons transmitted through the center hole of the ADF detector, enables element analysis column by column. Nowadays, the annular bright-field (ABF) STEM and the low-angle annular dark-field (LAADF) STEM are utilized as STEM methods to effectively visualize light atoms.
    HAADF-STEM
    Fig.(a) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for HAADF-STEM. Typical inner and outer semi-angles of the detector are respectively β1 = ~50 mrad and β2 = ~200 mrad, detecting inelastically scattered electrons at high angles. The value of the convergence semi-angle α is approximately 25 mrad for a 200 kV Cs-corrected TEM. Usually, an ABF detector and a LAADF detector are placed below a HAADF detector.

  • high-contrast polepiece

    When the high-resolution polepiece or the multiuse polepiece is used, the lens excitation is strong. As a result, the focal length of the objective lens is shortened and a diffraction pattern is formed above the objective aperture. The aperture cannot often select one diffraction spot but contain the effect of neighboring diffraction spots. The bright-field and dark-field images are influenced by other diffraction spots. To improve this failure, a polepiece that enables acquisition of one diffraction spot is available, which is called the "high-contrast polepiece." In this polepiece, the magnetic field produced is weakened a little by broadening the polepiece gap so that the diffraction pattern is formed on the objective aperture. For this polepiece, a Cs of 3.3 mm, a Cc of 3.0 mm and a spatial resolution for TEM image of 0.31 nm at an accelerating voltage of 200 kV have been attained. Also, the high-contrast polepiece allows the specimen holder to be tilted to ±(30° to 35°). It is used for bright and dark field microscopy of texture analysis of materials and for the studies of biological specimens.

  • high-definition image

    "The high-definition image" means an image consisting of a small sized pixel, a wide dynamic range and high gray levels. Photo films for imaging have a pixel size of ~3 μm, a dynamic range of 2 digits and a gray scale of <16 bits. CCD has a pixel size of 13.5 μm, a dynamic range of 4 digits and gray scales of 16 bits. The imaging plate has a pixel size of 25 μm, a dynamic range of 5 to 6 digits and gray scales of 20 bits.

  • high-pass filter

    A filter that passes only frequency components higher than a certain frequency.

  • high-resolution electron microscopy

    A technique of transmission electron microscopy to obtain a lattice image or a crystal structure image from a thin specimen by utilizing wave interference between transmitted and diffracted waves using a TEM equipped with a small Cs objective lens. An image which corresponds to the crystal structure is obtained by precisely adjusting the incident beam orientation to a zone axis and by setting the focus of the objective lens to Scherzer focus. The spatial resolution depends on the Cs value of the objective lens and the accelerating voltage of the incident beam. When the specimen is thick (more than 10 nm), the image obtained does not well correspond to the crystal structure due to the dynamical diffraction effect. A combined use with a HAADF-STEM image enhances the reliability of the image interpretation. HREM is used for analysis of crystal structures and lattice detects, including interface structures.

  • high-resolution photography

    For "high-resolution photography," photographic films were used but have been replaced by CCD cameras. Since the pixel size of the film is ~3 μm and that of the CCD camera is ~15 μm, the magnification for the film is 300,000 to 400,000× but that for the CCD camera is 600,000 to 1,000,000×. Photography at a low magnification is recommended from the view point of specimen drift.

  • high-tension (voltage) tank

    A container that houses a high-voltage generator to accelerate electrons emitted from the electron gun.

  • high-tension cable

    A high withstand-voltage cable that connects the high-voltage power supply for accelerating an electron beam with the electron gun in an electron microscope.

  • high-tension wobbler

    A function to align the optical axis to the acceleration voltage center by adding slight voltage fluctuations (~±250 V) to the accelerating voltage of the electron beam.

  • high-voltage generator

  • high-voltage power supply

    A power supply that generates a high negative voltage to accelerate electrons emitted from the electron gun. The stability of the "high-voltage power supply" at present is ~1×10-6.

  • higher-order Laue zone (HOLZ) reflection

    "Laue zones" are defined as the reciprocal lattice planes perpendicular to the direction of the incident beam. A Laue zone containing the point of origin (reciprocal lattice point corresponding to the incidence point) is called the zeroth-order Laue zone (ZOLZ). The Laue zone of the n-th-order counted from the point of origin in the opposite direction of the incident beam is called the n-th-order Laue zone. Laue zones other than ZOLZ are termed "higher-order Laue zones (HOLZ)." The HOLZ reflections appear as narrow ring lines at positions distant from the center of a CBED pattern. The HOLZ reflections provide three-dimensional information on a crystal, whereas, the ZOLZ reflection gives only two-dimensional information on a crystal. The HOLZ reflections have larger diffraction vectors and their positions are sensitive to a slight change of the lattice parameters. Thus, these reflections are efficiently used for high-accuracy analysis of lattice distortions and crystal structures.

  • highly accelerated electron

    An electron that is accelerated at a high voltage (>100 kV) having a high energy. As the accelerating voltage is higher, the wavelength of the electron is shorter.

  • hollow-cone beam illumination

    A beam illumination method by which a cone-shaped electron beam is produced. In the method, the beam is tilted to a certain angle against the optical axis by the first-stage deflection coils and is tilted back by the second-stage deflection coils to illuminate the same position on the specimen, and then the beam is rotated with respect to the optical axis kept the same illumination position. The method is used to wipe out diffraction contrast in the bright- and dark-field images and to observe the symmetries produced only by HOLZ lines without strong intensity due to ZOLZ interaction in the CBED pattern.

  • holography

  • hydrocarbon

    "Hydrocarbon(s)" deposited and adsorbed on the components of a TEM are emitted in the microscope column and are deposited on the surface of a specimen. If this specimen is irradiated with a high-energy electron beam, the hydrocarbons are polymerized, resulting in contamination.

  • hysteresis

    "Hysteresis" is a phenomenon where the state of a system depends on its undergoing progress, and a physical effect occurs at a delayed time against a given physical impact. In the case of TEM, the relevant phenomenon occurs in the lens action. The accuracy of the positional reproducibility of the electron probe on a specimen due to the hysteresis of the condenser lens affects the operability of a TEM. The excitation of the intermediate lens is frequently changed in obtaining different magnifications and at switching between the diffraction mode and image mode. The error of magnifications due to the hysteresis of the intermediate lens ranges from 5 to 10% though depending on the operation mode. For the objective and projector lenses, the hysteresis does not become a problem because the excitations of those lenses are almost fixed.

  • ice embedding

    Ice embedding is one of rapid freeze fixation techniques used for biological specimen preparation in transmission electron microscopy.
    This technique embeds a suspended specimen of biological macromolecules (purified proteins, viruses, etc.) in an amorphous ice film with a few tens of nm to a few hundreds of nm. It enables us to observe a molecular state of the specimen in a liquid solution without staining.
    In an actual experiment, a few μL suspension liquid is dropped onto a micro grid subject to hydrophilic treatment, excessive liquid is removed with a filter paper, then the specimen is rapidly frozen by immersing it into a coolant as quickly as possible. Liquid ethane or liquid propane is used commonly as a coolant because these are easy to sublimate and have a large temperature difference between the melting point and the boiling point.

    Schematic of ice embedding.
    氷包埋法の概略_Schematic of ice embedding.


    Cross sectional view of ice-embedded specimens, which are embedded in amorphous ice.
    氷包埋した試料の断面図_Cross sectional view of ice-embedded specimens.


    Cryo-TEM image of ice-embedded bacteriophage T4.
    氷包埋したT4ファージのクライオTEM像_Cryo-TEM image of ice-embedded bacteriophage T4.
    The morphology of the head and tail parts of the bacteriophage is well preserved.

  • illumination-lens system

    The "illumination-lens system" consists of the condenser lens and condenser mini lens. The pre-magnetic field of the condenser-objective lens (C-O lens) is also included in the illumination-lens system.

  • image rotation

    A phenomenon of an image rotation when the magnification is changed in a TEM, which is inconvenient for image observation. The second and third parts of the intermediate lens system that have opposite excitations to each other are designed to cancel or to minimize the image rotation.

  • image wobbler

    A function to perform focusing of the illumination beam by the use of deflection coils below the condenser lens. The electron beam is repeatedly tilted at slightly positive and negative directions with respect to the optical axis. The excitation of the objective lens or the specimen position is adjusted so that the image does not move while the "image wobbler" is operated. The image wobbler is useful for rough focusing at magnifications up to ~50,000×.

  • imaging lens system

    The "imaging lens system" consists of the objective-, intermediate-, and projector lenses. By adjusting the excitation currents of the respective lenses, various aberrations are suppressed and image rotations are eliminated. Then, images can be taken at a wide range of magnifications, from low magnifications (~50×) to high magnifications (~1,500,000×). In addition, by changing the focal point of the first intermediate lens, a diffraction pattern is obtained.

  • imaging plate

    An integration-type two-dimensional detector that utilizes fluorescence emission generated by a beam of X-rays, electrons, or neutrons. The "imaging plate" is made of a plastic film coated with microcrystlas of photostimulable phosphor (BaFX:Eu2+ (X = Cl, Br, I)). The imaging plate has excellent linear sensitivity. The plate possesses a large recording area of about 80 mm × 100 mm and a large dynamic range of 5 to 6 digits. The exposed imaging plate is irradiated with He-Ne laser light, and then blue light emitted from the plate is converted into electric signals by a photomultiplier tube (PMT). By scanning the laser light on an imaging plate, a microscope image is obtained from the recorded signals. One PMT covers a dynamic range of only 5 digits but a recent reading device which combines a PMT and a semiconductor detector covers a 6-digit dynamic range. Whether a large dynamic range can be effectively used or not is dependent on the performance of a reading device. Also, the positional resolution (pixel size) changes from 15 μm to 50 μm depending on the performance of the reading device. The maximum gray level of the imaging plate is 20 bits. The imaging plate is advantageous for both recording a low-magnification TEM image that needs to cover a large area and a diffraction pattern that has a wide dynamic range. Compared to CCD, the disadvantage of the imaging plate is that its use is limited to offline.

  • immersion lens

    An electrostatic lens that has different potentials at the entrance and exit of the lens. The term of "immersion lens" originates from the immersion lens of light optical lenses, where the refraction index is different at the entrance and exit of the lens. The extraction electrode of the electron gun is an acceleration-type immersion lens. The Wien filter is normally a deceleration-type immersion lens. It is noted that lens action arises when electrons are accelerated or decelerated.

  • immunoelectron microscopy

    Immunoelectron microscopy is a technique to visualize the locations (localization) of specific proteins with a TEM, by utilizing antigen-antibody reactions where antibodies bind specifically to antigenic proteins. First, antibodies (primary antibodies) react with the target proteins. Then, the secondary antibodies labeled with gold colloid, etc., react with the primary antibodies, so that the target proteins can be detected with the TEM. Immunoelectron microscopy is classified into two methods depending on the timing of antibody reactions; pre-embedding (before resin embedding) and post-embedding (after resin embedding).

  • imperfect dislocation

  • impurity

    "Impurities" are defined as substances contained in a pure substance. When a trace amount of impurities are doped into a semiconductor, energy levels characteristic of impurities are created in the band gap, from which electrons are excited to the conduction band or to which electrons are accepted from the valence band.

  • impurity atom

    Foreign atoms mixed in a crystal, which are different kind from constituent atoms of the crystal. An "impurity atom" causes point defect, which is one of lattice defect.

  • impurity level

    "Impurity level" is defined as an energy level created in the band gap in a semiconductor, which is introduced by impurity atoms.

  • in-column type

    An energy filter or an energy analyzer that is installed in the column of a TEM is classified as "in-column type." This type includes an omega filter and an alfa filter.

  • incidence angle

    The angle between the normal of a specimen plane and the incident electron beam. In HREM image observation, it is essential to align the incidence direction of the electron beam with the zone axis of the specimen crystal with high accuracy.

  • inclusion

    Inclusions are impurity particles captured in a solid.

  • incommensurate structure

    "Incommensurate structure" is defined as a structure where the periodicity of a structure arising from phase transformation cannot be expressed by a simple integral ratio with respect to the periodicity of the basic structure before phase transformation.

  • inelastically scattered electron

    Inelastically scattered electrons are defined as the electrons which lose part of their energy (traveling speed becomes slower) through interactions between the electrons and a specimen. The probability of occurring inelastic scattering is small, ~1/10 or less compared with that of elastic scattering. However, as a specimen is thicker (~10 nm or thicker), the inelastically scattered electrons superposed on elastically scattered electrons obscure a TEM image or a diffraction pattern. To remove inelastically scattered electrons, an energy filter is effectively used.

  • information limit

    Information limit indicates the wave number at which phase information carried by the phase-contrast transfer function disappears. That is, it is determined as the wave number at which the envelope function reaches practically zero. The envelope function which gives damping of the phase contrast is determined by an energy spread of the electron beam and stability of the accelerating voltage (these two factors are related to chromatic aberration of the objective lens), stability of the objective lens and the divergence angle of the incident electron beam. The information limit is used as an indication of the resolution of a high resolution image. The resolution of the structure image is given by "first zero" of the phase-contrast transfer function in the ordinary sense. Phase information between the first zero and the information limit can be utilized to obtain a higher resolution of the structure image by computer-processing.

  • inner-shell (core) excitation

    A process where an inner-shell electron is excited to the conduction band by absorption of X-rays or by collision with a high-energy electron or ion, and then an electron vacancy is created in the inner shell. EELS enables us to perform detailed analysis of inner-shell excitation.

  • interband transition

    When the Bragg reflection is treated by the dynamical theory, dispersion surfaces (equi-energy surface), which give allowable wave numbers (equivalent energy planes) near Bragg reflections, are produced. Two dispersion surfaces are produced for each reflection. When the incidence direction of the electron beam onto a specimen crystal (the excitation error of the Bragg reflection) and the surface normal of the specimen are given, the allowed points on the dispersion surfaces are determined. As a result, the wave numbers and amplitudes of the allowed waves in the crystal are determined. If the crystal is perfect, these quantities are kept unchanged. If a defect, for example a stacking fault exists in the crystal, redistribution of the amplitudes occurs on the dispersion surfaces. Wave transfer to a different dispersion surface is called "interband transition." On the other hand, wave transfer within the same dispersion surface is called "intraband transition." The above interband transition in the case of a stacking fault takes place in the scope of elastic scattering. In the case of inelastic scattering, we can consider similar dispersion surfaces which are a little different in their energy from those of elastic scattering. Interband transition occurs at small-angle scattering of thermal diffuse scattering, but intraband transition occurs for plasmon scattering. In the case of core excitation, if interaction that gives rise to the core excitation is small (normally, small), intraband transition occurs. The electrons suffered by the intraband transition produce a similar image to the original image (without transition). However, since the symmetry of the Bloch wave of a dispersion surface is different from that of the other surface, the electrons suffered by interband transition do not form a similar image to the original image. (This term is different from "interband transition" used in solid state physics. In the case of the interband transition in solid state physics, the energy of the electron changes but it does not change in electron diffraction.)

  • interband transition

    A phenomenon in which electrons in a crystal make transitions from the valence band to the conduction band. EELS enables us to obtain the band gap energy from the onset energy of a spectrum that reflects interband transitions.

  • interface

    An "interface" is a boundary surface between two different phases. In the case of crystalline phase, the interface often takes a specific crystalline plane. In materials science, it is important to analyze grain boundaries in metals, and interfaces (boundary surfaces) on multiple layers in semiconductors and ceramics.

  • interference color of ultrathin section

    Interference color of an ultrathin section is produced by the interference of light reflected from the top- and bottom-faces of the ultrathin section when white light is incident onto a section in the almost vertical direction. Since the interference color corresponds to the thickness of the section, the interference color enables us to know a rough thickness of the section. Sections with thicknesses of about 70 nm, about 100 nm and about 200 nm emit interference colors of silver gold, gold and blue, respectively. 

  • interference fringe

    A series of light-and-dark fringe that is produced by interference of electron waves.

  • interference of electrons

    Interference of electrons occurs due to the wave nature of electrons. When the electron waves are superposed, the amplitudes are added when their phases are the same but are canceled when their phases are opposite. When electrons travel a crystalline specimen, they are reflected by various atomic planes in the specimen (Bragg reflections) and diffracted waves are produced in various directions. When these waves meet on the TEM image plane, the waves constructively interfere where their phases are matched, whereas the waves destructively interfere where their phases are opposite. As a result, a fringe or a net image intensity modulation is formed.

  • intermediate lens

    The "intermediate lens" is placed between the objective lens and the projector lens. The intermediate lens works in the following way. The intermediate lens changes focusing position either on a diffraction pattern or a TEM image produced by the objective lens by adjusting its excitation, and forms the magnified pattern or image on the object plane of the projector lens. Normally, the intermediate lens consists of three parts: The first part mainly selects focusing position, the second part magnifies the focused pattern or image, and the third part mainly achieves rotation-free condition. The magnification of the intermediate lens is varied from ~0.5× to ~100×. When the total magnification is 100×, the magnifications for the first, second and third parts are 4× to 5×, ~10×, and 2× to 3×, respectively.

  • intermetallic compound

    A compound that consists of two or more metallic elements.

  • intraband transition

  • inverse Fourier transform

    Fourier transform is a method that transforms a function of certain variables into the function of the variables conjugate to the certain variables. On the other hand, "inverse Fourier transform" is a method that transforms the Fourier-transformed function into a function of the original variable. For example, when a crystal potential as a function of position is Fourier-transformed, crystal structure factors are obtained as a function of wavenumber. Next, when the crystal structure factors are inverse-Fourier-transformed, the crystal potential as the function of position is obtained. In TEM imaging, Fourier transform and inverse Fourier transform of the specimen are automatically executed, so that the diffraction pattern and structure image are obtained at the back focal plane and the image plane, respectively.

  • inversion center

    A symmetry element of the crystallographic point group. An operation that transforms a coordinate (x, y, z) into (-x, -y, -z) (that is, the signs of the coordinates of each point are changed) is called the inversion operation. When this symmetry is operated for a crystal, the crystal can be equivalent to that before the operation. In this case, the point leaving fixed by the operation is called "inversion center."

  • inversion domain boundary

    "Inversion domain boundary" separates two adjacent crystals which coincide with each other by an inversion operation.

  • inverted image

    An image inverted in the vertical and horizontal directions. That is, an "inverted image" is obtained by a 180° rotation of a real image.

  • ion cleaner

    An ion cleaning tool that removes contaminants on a TEM specimen, which are residual substances deposited on a specimen at specimen pre-treatment, and contaminated substances deposited on a specimen in specimen storage. The specimen holder on which a specimen is set is inserted into an "ion cleaner." Then, a glow discharge due to residual gasses in the ion cleaner is generated and the ionized gasses by discharge irradiate the specimen. This chemical-reaction of the ionized gasses removes the contaminants (in particular, polymerized substances of hydrocarbons) deposited on the specimen.

  • ion etching

    "Ion etching" is a technique to selectively remove specific atoms and the atoms at lattice defects. This technique is used for observation of textures on a specimen surface. A technique of uniform and nonselective thinning for specimen preparation of TEM is ion milling. Sometimes, ion etching is confused with ion milling.

  • ion milling

    "Ion milling" is a specimen preparation technique that is used when electrolytic polishing or chemical polishing cannot be used to prepare a specimen. This technique is particularly effective for the specimen preparation when cross-section observations of layered materials are requested. A thin cross sectional film is prepared by milling surface layer atoms with the irradiation of an argon ion beam accelerated at 2 kV to 10 kV with a grazing incidence angle less than 10°. The disadvantage of ion milling is that damage to the specimen is unavoidable. Commercially-available ion-milling instruments are equipped with an optical microscope or a CCD camera with a magnification of several ten times to view the state of the specimen.

  • ion plating

    "Ion plating" is a technique to form a thin metal layer deposited on a surface of a solid by ionizing metal. In this technique, discharge is generated in a low vacuum or a gas environment between the substrate acting as a cathode and a metal evaporation source acting as an anode. Then, metal atoms are evaporated and ionized, and accelerated by an electric field, then finally form a thin layer on the substrate.

  • ion pump (sputter ion pump)

    A dry vacuum pump that does not use oil. In the sputter ion pump, residual gasses are ionized utilizing electric and magnetic fields. The produced ions strike a titanium cathode to sputter titanium atoms. Then, the sputtered titanium atoms form a fresh getter film (film subjected to chemical adsorption). As a result, active gasses (hydrogen, oxygen, carbon monoxide, etc.) are adsorbed to the getter film, and inert gasses (helium, etc.) are ionized and adsorbed to the cathode. The pump is used for evacuation of the electron gun chamber and the column of TEM, which require high vacuum. The working pressure is 10-4 to 10-9 Pa.

  • ion sputtering

    "Ion sputtering" is a phenomenon where atoms are sputtered from a solid surface when ionized and accelerated atoms or molecules hit the solid surface. This phenomenon is utilized for formation of a thin film on a solid surface, specimen coating and ion etching. In the case of specimen coating, discharge is generated in a low vacuum or in an Ar gas environment under a vacuum, the ionized gas is accelerated and hits a target material (Au, Pt, etc.) at the anode. Then, the material is emitted from the target surface and deposit on a specimen at the cathode, specimen being coated.

  • ionization (vacuum) gauge

    A vacuum gage that measures the gas pressure by ionization mechanism. In the ionization (vacuum) gauge, residual gasses are ionized, and produced ions or electrons are captured, then the gas pressure is measured using the produced current. Two types of ionization gauge are available: a thermal-cathode ionization gauge that heats a filament and ionizes gasses using thermoelectrons (ex. B-A gauge) and a cold-cathode ionization gauge that ionizes gasses using plasma (ex. Penning gauge).

  • ionization cross section

    When a neutral atom or molecule loses or receives electrons by collision with other particles, the atom or molecule becomes an ion. This phenomenon is called ionization. "Ionization cross section" is the ionization probability expressed by dimension of area.

  • ionization energy

    An energy required for removing one electron from an atom at the ground state to make a monovalent cation. It is called the first ionization energy.

  • isochromaticity

  • isolation valve

    A valve to isolate a certain part from the other part. Each "isolation valve" is placed between the electron gun chamber and the column, the specimen chamber and the column, and the column and the camera chamber, so that the vacuum in a part is broken without any effect on the vacuum in the other parts.

  • isotropy

    "Isotropy" means that physical properties of a crystalline substance do not change regardless of its crystal orientations.

  • jet polishing

    "Jet polishing" is one technique in electrolytic polishing. In this technique, a jet-like polishing solution is sprayed onto a specimen to make a hole at its center. Spraying the solution makes it possible to keep the solution at the specimen surface fresh, thus a quick and smooth polishing is achieved.

  • k factor

    A factor used for the Cliff-Lorimer method in EDS analysis. For the determination of the k factor, a substance composed of two elements A and B, whose composition is similar to the target substance, is used as a standard specimen. Characteristic X-ray intensities IA and IB of the standard specimen are measured. And then the k factor is determined by equation k = CA/CB・IB /IA using the known compositions CA and CB. The k factor is theoretically given by equation k = (MAQBωBαB)/(MBQAωAαA), where M, Q, ω and α are atomic weight, ionization cross section, fluorescent yield and the ratio of Kα line to the total K lines of a substance, respectively. It is noted that the accuracies of Q and ω are low. In actual cases, the absorption due to the window material is also needed to be taken into account. For the elements up to 3d metals, it is said that the error between the quantitative analysis using experimental k factor and that with the theoretical k factor is about 10%. For a substance composed of elements with greatly different atomic numbers, the accuracy of the analysis is low.

  • kinematical diffraction

    An electron that passes through a crystalline specimen is reflected (diffracted) by lattice planes satisfying a Bragg condition. The kinematical diffraction approximation assumes that the Bragg reflection occurs only one time in the specimen. In this context, the reflection intensity is proportional to the square of the crystal structure factor of the reflection. This approximation holds only when a specimen is thin enough (<3 nm). As a specimen is thicker, the reflection occurs many times. In this case, the dynamical diffraction theory must be applied to the interpretation of reflection intensities and electron microscope images.

  • lanthanum hexaboride

    A material used for the tip of the thermionic-emission electron gun, instead of tungsten used so far. This tip requires a higher vacuum than the tungsten filament, but its brightness is higher than that of the tungsten filament.

  • lanthanum hexaboride single-crystal tip

    A tip used as a thermoelectron source. A lanthanum hexaboride (LaB6) single crystal, which is sharpened to a cone shape, is used. The LaB6 tip is indirectly heated at about 1800 K. Its brightness is 5×106 A/cm2.sr at 200 kV. The size of the crossover is ~10 μm. The energy spread of the emitted electrons from the filament is ~2 eV.

  • large-angle convergent-beam electron diffraction

    In conventional CBED, if the angular diameter of a diffraction disk exceeds the Bragg angle, the disk overlaps with an adjacent diffraction disk. Thus, the angular diameter is limited to an angle smaller than the Bragg angle. The use of the large-angle convergent-beam electron diffraction (LACBED) technique breaks through the angular limitation. Using a condenser-objective (CO) lens, when a specimen is placed on the focus position of the CO lens, the bright-field and dark-field images are formed on the image plane (on the selected-area aperture) but these images overlap with each other. When the specimen is shifted upper or lower from the focused position of the incident beam, the bright-field and dark-field images on the selected-area aperture are separated. If only the bright-field image is chosen with the selected-area aperture and the intermediate lens is set to take a diffraction pattern (the diffraction mode), the bright-field diffraction disk covers an angle of three to four times larger than that for the conventional CBED disk. Since the LACBED pattern contains information on both the image and diffraction pattern, it is effectively utilized for identification of lattice defects and analysis of strains at interfaces.



    (a) In the case of convergence semi-angle to be equal to the Bragg angle or smaller.
    In conventional CBED, the convergence semi-angle a is limited at the maximum to the Bragg angle θ to avoid the overlap of adjacent diffraction disks.

    (b) In the case of the convergence semi-angle to be larger than the Bragg angle.
    When the convergence semi-angle a exceeds the Bragg angle θ, the adjacent diffraction disks overlap, thus making it impossible to extract information on each diffraction disk.



    (c) Ray diagram of conventional CBED where the convergence semi-angle is set to an angle larger than the Bragg angle.
    In conventional CBED, the incident beam is focused on a specimen and the objective lens is focused on the specimen. A diffraction pattern is formed on the back focal plane of the objective lens and an image of the specimen (a spot image in this case) is formed on the selected-area aperture. When the convergence semi-angle of the incident electron beam is set to an angle larger than the Bragg angle, a transmitted wave disk and diffracted wave disks overlap with each other on the back focal plane of the objective lens. The image formed by the transmitted wave and images formed by diffracted waves are superposed on the selected-area aperture to form a spot image.

    (d) Ray diagram of LACBED.
    ①In LACBED, the specimen position is shifted to a higher (lower) position from the focused position of the incident beam without changing the excitation of the objective lens.
    ②The diffraction pattern is formed on the back focal plane of the objective lens as it is in (c). The image of the specimen, which is formed on the selected-area aperture in (c), is shifted to an upper (lower) position, and on the selected-area aperture the (spot) image of the transmitted wave and (spot) images of the diffracted waves are separated.
    ③If only the transmitted spot (beam) is selected using the selected-area aperture in the image observation mode of the intermediate lens system, and then the intermediate lens system is switched to the diffraction mode, a large-angle convergent-beam electron diffraction (LACBED) pattern formed only by the transmitted wave is obtained. The LACBED pattern removes the overlap due to the diffracted wave disks and extends its angular diameter beyond the limitation of the diffraction (Bragg) angle.


    LACBED pattern of Si [111] taken at an accelerating voltage of 200 kV.

  • lattice defect

    Disorder of the atomic arrangement in a crystal. In a perfect crystal, constituent atoms create a regular and an ordered arrangement. However in a real crystal, the order of arrangement is violated and structural disorder exists. "Lattice defect" is classified into planar faults, line defects and point defects, in terms of its form.

  • lattice fringe

    The lattice fringe is a periodic fringe in a TEM image, which is formed by two waves; a transmitted wave exiting from a crystal and a diffracted wave from one lattice plane of the crystal. The spacing of the fringe corresponds to that of this lattice plane.

  • lattice image

    HREM, which allows wave interference between transmitted and diffracted waves to be caused, enables us to obtain a lattice image of a thin crystalline specimen at an appropriate defocus of the objective lens. The lattice image (intensity distribution) does not always correspond to the electrostatic potential in the crystalline specimen projected along the direction of the incident electron beam (projected potential). However, the lattice image displays correctly the periodicity of a crystal. When the image is obtained at the defocus condition, which is determined by the spherical aberration of the objective lens and the accelerating voltage of the incident beam (Scherzer focus), it corresponds fairly well to the electrostatic potential (atomic arrangement) in the crystal. This image is called "crystal structure image."

  • lattice parameter (constant)

    The lattice parameters are the quantities specifying a unit cell or the unit of the periodicity of the atomic arrangement. The lattice parameters (constants) are composed of "a, b, c," lengths of the unit cell in three dimensions, and "α, β, γ," their mutual angles.

  • lattice plane

    In a crystal, equivalent atomic planes are aligned in parallel with an equal distance. Thus, a crystal is regarded as an assembly of planes created by atoms. In this context, a set of parallel planes is named a "(crystal) lattice plane." The distance between the neighboring planes is termed the spacing of the plane. In a crystal, there are many lattice planes with different orientations.

  • lattice vibration

    Constituent atoms in a crystal vibrate around their equilibrium positions due to thermal energy. "Lattice vibration" means that atoms vibrate each other with a specific phase relationship. When the incident electrons on a specimen are scattered with the exciting lattice vibrations, a diffuse intensity distribution is created. This scattering is called thermal diffuse scattering.

  • least-squares method

    "Least-squares method" is a method to determine unknown parameters so that the sum of squares of the residuals between the experimental and calculated values is minimized. It is used for crystal structure analysis and spectral curve fitting. When the sum of the squares of the residuals is minimized by a liner combination of unknown parameters (∑ai・xi + b), the method is called linear least-squares method. When a nonlinear function is used for fitting, it is called nonlinear least-squares method. Nonlinear least-squares method includes cases where fitting of unknown parameters is executed by numerical calculations without assuming a specific nonlinear function. For example, nonlinear least-squares fitting is effectively used to obtain structural parameters (atom positions, Debye-Waller factors) by minimizing the sum of the squares of the residuals between the experimental intensities of CBED patterns and the calculated intensities for crystal structure models. That is, the sum of the squares of the residuals for a set of certain structural parameters is obtained. Then, a set of structural parameters is generated so that the differential of the sum is negative with respect to each parameter. Then the sum of the squares of the residuals is calculated for the set of the generated parameters. Repeating the calculation procedure reaches the minimum sum of the squares of the residuals, determining the unknown parameters.

  • lens action in the magnetic field

    When an electron passes through the magnetic field produced by the polepiece of a lens in the vertical direction (from above to below), the electron traveling at an off-axis position firstly undergoes Lamor rotation due to the horizontal components of the magnetic field. Then, the electron receives a converging force toward the optical axis due to a Lorentz force between the rotational velocity component of the electron and the vertical component of the magnetic field of the lens. The magnetic-field components, which yield the converging force proportional to  (angle between the electron beam and the optical axis), are used as the lens action. The strength of the lens action is controlled by the electric current applied to the coils producing the electro-magnetic-field.

  • line analysis

    In spectroscopic analysis, "line analysis" is to acquire a spectrum from a line scan of an electron beam on a specimen.

  • liner tube

    An aluminum pipe with a diameter of 6 to 7 mm, which is inserted into the illumination and imaging systems in the microscope column, to prevent contamination in the column and specimen chamber due to gasses from lenses and other components.

  • liquid-metal ion source

    A "liquid-metal ion source" uses a metal that is at the liquid state at normal temperature. This ion source, normally liquid gallium, is used for an FIB instrument.

  • liquid-nitrogen trap

    A device to condense gas molecules onto a metallic surface exposed to vacuum by cooling the surface with liquid nitrogen. In a TEM, the "liquid-nitrogen trap" is utilized to maintain high vacuum in the microscope column to prevent contamination.

  • live time

    The live time is an effective measurement time. That is, the live time equals to the net measurement time determined by subtracting the dead time from the total measurement time. This concept is used for EDS.

  • live time scan

    Live time scan is an EDS measurement technique to scan an electron probe over an area by varying the dwell time of the probe so that the effective measurement time (live time) for characteristic X-rays becomes equal at each scanning point.
    If the dwell time is set to a definite time for all the scanning points, the dead time (rate) for the scanning points with a large X-ray generation increases. Thus, the generated X-rays are counted smaller than the true counts. As a result, the concentration of the element concerned is underestimated. To avoid this phenomenon, the dwell time at each scanning point is varied so as to make the live time equal for all the points (live time scan). Therefore, the live time scan enables us to acquire the correct two-dimensional element map of the specimen. 

  • long period structure

    For example, in the phase of CuAuⅠ, five ordered lattices are aligned in a direction followed by another five ordered lattices with an anti-phase shift to the former lattices, resulting into formation of a structure with a unit of 10 times larger than the original lattice. Such a structure is called a "long period structure." The phase CuAuⅠ appears in a narrow temperature range between the ordered phase and the disordered phase. In a diffraction pattern, the reflections from the ordered phase split to diffraction spots appearing at the positions corresponding to a period of 10 times that of the ordered phase. Long period structures appear in intermetallic compounds and SiC.

  • long-range order parameter

    In the case of binary alloys, a degree of order of atom species A and B increases with decreasing temperature after a substance undergoes a transition from a disordered phase to an ordered phase. The "long-range order parameter" means this degree of order. It can be measured from the intensity change of a specific reflection that appears in an ordered phase.

  • loss function

    "Loss function" is given by the negative of the imaginary part of inverse of the dielectric function or Im[‐1/ε(ω)], where ε(ω) is the dielectric function. By removing the effect of multiple scatterings from an EELS spectrum, the EELS spectrum only due to single scattering is obtained. Next, the effect of the incident beam intensity and that of the aperture size from the spectrum are removed. And then, when the intensity of the obtained spectrum is normalized, the absolute value of the electron energy loss or the loss function is obtained. The loss function is the most important quantity obtained from EELS. By using Kramers-Kronig equation, the real part corresponding to the loss function is derived. From the real and imaginary parts, the dielectric function of the specimen is calculated. Various physical properties such as refractive index, optical reflectivity, optical absorption intensity etc. can be calculated from the dielectric function.

  • low energy electron microscope

    A "LEEM (low energy electron microscope)" is an instrument to obtain a surface image of a specimen. The incident electron energy is reduced to several V to several 10 V, and elastically-backscattered electrons from a specimen are accelerated by an electric field just above the specimen. The formed image is enlarged by the imaging lens and observed through a screen or a camera. The spatial resolution of LEEM is 5 to 10 nm. A dark-field image can be obtained by selecting a diffraction spot of LEED (low energy electron diffraction) with an aperture. The specimen area is kept at an ultrahigh vacuum to obtain surface structural information. LEEM is often accompanied with a function of PEEM (photo emission electron microscope) which has the same imaging system as that of LEEM.

  • low-angle annular dark-field scanning transmission electron microscopy

    Low-angle annular dark-field scanning transmission electron microscopy (LAADF-STEM) is a STEM method which receives diffracted or inelastically scattered electrons at low to medium angles (25 to 60 mrad) using an annular dark-field (ADF) detector. A STEM image is acquired by displaying the integrated intensities of the electrons in synchronism with the incident probe position. It is noted that the LAADF image shows diffraction contrast or difference of specimen thickness dependence.
    LAADF-STEM is used in place of HAADF-STEM for a specimen composed only of light elements (molecular crystals, two-dimensional crystals, organic materials like macromolecules, biological materials, etc.). This is because the HAADF image does not have a high signal-to-noise ratio due to weak elastic and inelastic scattering at high angles for such light-element specimens. LAADF-STEM enables us to obtain a high-resolution image with a high signal-to-noise ratio whose intensity depends on the atomic number.

    LAADF-STEM像
    STEM images of the cross-sectional thin film of a semiconductor device (an n-channel MOSFET of CPU). (Accelerating voltage: 200 kV, Convergence semi-angle of the incident electron beam: 11 mrad)

    Fig.(a) HAADF-STEM image taken at an acceptance semi-angle of the detector of 46 to 208 mrad. As is shown by a blue arrow, only a region where heavy atoms (Ta, W, etc.) exist are observed with bright contrast.
    Fig.(b) LAADF-STEM image taken at an acceptance semi-angle of the detector of 14 to 63 mrad.
    SiNx layers in the regions indicated by a red parenthesis are observed brighter than SiO2 regions indicated by a blue parenthesis. Such layer structures are hardly seen in the HAADF image. In addition, the LAADF image also visualizes lattice defects indicated by red allows. 

    Comparison of Ray diagrams of two detectors


    Fig.(a) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for HAADF-STEM. Typical inner and outer semi-angles of the detector are respectively β1 = ~50 mrad and β2 = ~200 mrad, detecting inelastically scattered electrons at high angles. The value of the convergence semi-angle α is approximately 25 mrad for a 200 kV Cs-corrected TEM. Usually, an ABF detector and a LAADF detector are placed below a HAADF detector.
     

    Fig.(c) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for LAADF-STEM. The inner and outer acceptance semi-angles of the detector are respectively taken as that β1 is a little larger than α and β2 is ~60 mrad, where α is the convergence semi-angle. It should be noted that the diffracted waves and inelastically scattered electrons over low to medium angles are used for LAADF-STEM.

  • low-pass filter

    A filter that passes only frequency components lower than a certain frequency.

  • luminous efficiency

    The ratio of the emission energy to the absorbed energy when a fluorescent substance is excited by an incoming radiation and then fluorescence emission is generated.

  • magnetic domain

    A "magnetic domain" means a region where the directions of all the magnetizations of respective atoms are the same.

  • magnetic domain wall

  • magnetic material

    A material that is magnetized when placed in a magnetic field. "Magnetic materials" are classified into ferromagnetic material, paramagnetic material, diamagnetic material and antiferromagnetic material, from the viewpoint of how the material is magnetized.

  • magnetic-field leakage

    "Magnetic-field leakage" is a phenomenon where magnetic fields are leaked in a space other than a necessary local space. The components of a TEM that consist of permanent magnets or electromagnets are designed to generate magnetic fields in a local space but magnetic-field leakage can occur.

  • main pumping

    "Main pumping" means pumping for high-vacuum after rough pumping is completed.

  • many-beam approximation

    Many-beam approximation is an approximation method to interpret the diffraction intensity and TEM image, which takes account of not only a Bragg-reflected beam (diffracted wave) from one lattice plane, but also many reflected beams from other lattice planes.

  • marker method

    In three-dimensional tomography, the "marker method" uses gold particles evaporated on a specimen as markers to perform positional adjustment of each image.

  • maximum entropy method

    "Maximum entropy method (MEM)" is a method, which originates from the information theory, to deduce the most provable solution through maximizing the information entropy when information (measurement) is insufficient. It is well known that it has been successfully applied to the crystal structure analysis by X-ray powder diffraction. For TEM, it is used in the case of EELS spectra, to obtain the inherent spectra by removing the effect of the energy spread of the incident electron beam.

  • mean free path

    The average traveling distance of an electron before a scattering event takes place. As the energy of the electron beam is larger, the "mean free path" for scattering is larger. It becomes smaller as the atomic number of the constituent atoms is larger and as the acceptance angle of the incident electron beam is larger. The mean free path for the total inelastic scattering for an electron beam at 200 kV is about 150 nm. A simple estimation has been proposed that the mean free path (nm) is given by a value of about 80% of the incident electron energy (keV). The mean free path for elastic scattering is about 1/20 that for inelastic scattering.

  • mean inner potential

    When the Coulomb potential at point r from an atom is summed for all the constituent atoms, the total potential is written as V(r). V(r) possesses the crystal lattice periodicity, thus allowing V(r) to be expanded in the Fourier series with the lattice vector. The "mean inner potential" is defined as the zeroth-order term V0 in the Fourier expansion. V0 gives the (average) refractive index of the crystal and normally ranges between 10 and 20 V. The expansion coefficient of each order (Fourier potential) is calculated from the corresponding crystal structure factor.

  • mechanical polishing

    "Mechanical polishing" is a physical polishing technique used for specimen preparation of TEM. The common techniques are as follows: (1) Manual polishing using water-resistant paper (down to ~100 μm thick), (2) Rotational polishing device-based polishing using diamond or corundum particles (down to ~several 10 μm thick), (3) Dimple grinder-based polishing using corundum particles (down to <10 μm thick), and (4) Tripot polisher-based polishing using diamond particles (down to <10 μm thick).

  • metal

    A substance in which ions are bound with each other through free electrons (electrons extending over a crystal). The crystal structure of a "metal" is a face-centered cubic structure, a body-centered cubic structure, or a hexagonal close-packed structure. A metal exhibits high electric conductivity, high thermal conductivity and high light reflectivity, and is highly malleable as well as highly ductile.

  • metal mirror freezing (slam freezing)

    Metal mirror freezing (slam freezing) is one of rapid freeze fixation techniques. This technique punches and rapidly freezes a biological specimen against a metal block cooled by a coolant such as liquid nitrogen.  In many cases, high-thermal-conductivity, high-purity copper with gold plating is used for a metal block.  To increase a temperature drop efficiency of the specimen, the surface of the metal block is prepared into a flat mirror-surface.  This technique can be performed using a relatively inexpensive device, but provides a smaller freezing depth suitable for TEM observation (about 20 µm) than the suitable depth produced by high pressure freezing.  This technique is mainly used to fix tissues.  Specimen preparation for TEM observation of the frozen specimen is carried out by one of the following three procedures.  (1) Applying freeze sectioning to the specimen, (2) Applying resin embedding and ultrathin sectioning to the specimen after the specimen is subject to freeze substitution and is returned to room temperature, and (3) Making a replica of the specimen by freeze fracturing. 

  • micro-channel plate

    A "micro-channel plate (MCP)" is a circular glass plate with a thickness of ~0.5 mm and a diameter of ~10 mm, in which cylindrical electron multipliers with an inner diameter of ~10 μm are arrayed in a honeycomb structure. The front-side surface and the backside surface of the MCP are coated with a metal. The former side acts as the input electrode (cathode) and the latter as the output electrode (anode). When a voltage is applied between the two electrodes, electrons that enter the cathode strike the inner walls of the MCP, emitting multiple secondary electrons. These emitted secondary electrons are accelerated by an electric field in the channels and repeat collision with the (totter) inner walls of the MCP. Finally, the electron flow is received at the anode as an amplified electric signal. Since the MCP has high sensitivity for not only electrons but also ions and X-rays, this plate is used as a detection element for these signals. In a TEM, the MCP is used for a backscattered electron detector attached to the TEM column.

  • microgrid

    A "microgrid" is a supporting film used to support a fine specimen like powder for a TEM. The microgrid is made of cellulose aceto-butyrate and has many holes with the hole diameter of several μm or less.

  • milling

    "Milling" is a technique to mill and fabricate a specimen substance. In specimen preparation for TEM, an ion beam is used for milling.

  • minimum dose system

    "Minimum dose system (MDS)" is a method to photograph a TEM image by reducing damage to the specimen due to electron-beam irradiation. MDS is effectively used to photograph biological specimens. After the field for photographing is searched, focus adjustment of the objective lens and astigmatism correction are performed at a non-photographed position. Then, the MDS sets an electron dose, a magnification and the field for photographing to minimize the electron dose and takes the image.

  • mirror plane

    A symmetry element of the crystallographic point group. When the whole crystal (constituent atoms) is mirrored with respect to a certain plane, the mirrored crystal can be equivalent to the crystal before the mirror operation. In this case, this plane is called "mirror plane."

  • missing cone

    In three-dimensional tomography, "missing cone" means a region where the projection information on the specimen cannot be obtained due to the limitation on the tilt angle of the specimen holder. This gives rise to artifacts. To reconstruct a high-accuracy three-dimensional image, decreasing the angle for the missing cone is more important than increasing the number of tilt-series images. The tilt angle of the ordinary goniometer is limited to ±60°.

  • mixed Gaussian/Lorentzian function

    "Mixed Gaussian-Lorentzian function" is a function composed of a Gaussian and a Lorentzian function. It is used for pre-processing of the background in a spectrum and for fitting of the spectral intensity when the both functions are inappropriate to reproduce an experimental spectrum.

  • mixed crystal

  • modulated crystal

    A "modulated crystal" takes an additional modulation wave of atomic displacement or composition to the fundamental structure, whose wavelength is a non-integral number of the period of the fundamental structure. The modulation provides satellite reflections around the fundamental reflections. The features of the satellite reflections are different depending on the modulation to be a transverse wave type or a longitudinal wave type.

  • molecular beam epitaxy

    "Molecular beam epitaxy (MBE)" is a technique to deposit and grow a crystalline film in an ultra-high vacuum. In MBE, target elements or target materials that will constitute a target crystal are heated and evaporated and then, the target crystalline film is deposited and grown on a heated crystalline substrate. MBE is similar to vacuum evaporation (deposition), but the technique accurately controls the molecular beam to prepare a crystalline specimen controlled at the atomic scale. MBE is used also for specimen preparation for a TEM.

  • monochromator

    A device that can monochromate an electron beam. In a 200 kV TEM, the energy spread of the electron beam is about 2 eV for an LaB6 thermionic-emission electron gun, about 0.7 eV for a Schottky type electron gun and about 0.4 eV for a cold type field-emission electron gun. The use of the "monochromator" dramatically decreases the energy spread, down to less than 0.1 eV. Thus, the monochromator improves the energy resolution of EELS. The use of a monochromator in EELS is crucial for investigation of the electronic structures of solids.

  • movable aperture

    An aperture that allows selection of its hole diameter and adjustment of its position from the outside of vacuum. The "movable aperture" includes the condenser aperture, the objective aperture and the selector aperture.

  • multi-stage acceleration electrode

    The "multi-stage acceleration electrode" consists of a cascade of acceleration electrodes used for accelerating an electron beam, which is emitted from the electron gun, up to a required voltage. In a 200 kV TEM, a six-stage acceleration electrode is adopted.

  • multiple scattering

    When the incident electrons travel a specimen, these electrons strike the constituent atoms in the specimen many times until these electrons exit from the specimen and change their traveling direction. This (scattering) phenomenon is termed "multiple scattering." Among the theories related to multiple scattering, the dynamical diffraction theory, which treats multiple scattering due to elastic scattering (Bragg reflection), provides the intensities of the incident and diffracted waves as a function of specimen thickness and scattering angle.

  • multislice method

    A method for calculating the intensities of transmitted and diffracted waves at the bottom plane of a crystalline specimen when the incident electron beam interacts with the specimen. In the "multislice method," the specimen is assumed to be a stacking of many sufficiently-thin crystalline slices that are parallel to the specimen surface. Incoming electrons are regarded to be scattered by the first slice and undergo a phase change, then propagate down to the next slice. In this manner, the electrons are assumed to repeat scattering and propagation in the subsequent slices and reach the bottom plane of the specimen. Finally, the diffraction amplitudes (intensities) are calculated at this plane.

  • multiuse polepiece

    The "multiuse polepiece" allows the specimen holder to be tilted to more than ±35°. This polepiece makes it possible to attain a Cs of 1.0 mm, a Cc of 1.4 mm and a spatial resolution for TEM image of 0.23 nm at an accelerating voltage of 200 kV. For analytical case, the objective aperture is inserted below the lower pole of the polepiece to avoid unnecessary X-rays from the aperture. This aperture setting causes a problem that the diffraction pattern is not formed on the aperture plane, which leads to a difficulty in acquiring the dark field image from one diffraction spot and the bright field image without the influence of surrounding diffraction spots. It is noted that there is option to insert a special aperture at the polepiece gap to obtain correct bright- and dark-field images.

  • nano-beam diffraction

    A method for qualitative analysis of crystal structures from a diffraction pattern, acquired from a nanometer specimen area. An electron diffraction pattern is first obtained by illuminating the specimen with a parallel electron beam. Then, the electron beam is converged onto a nanometer specimen area using a small condenser aperture, diffraction spots being transformed to disk-like corresponding to the convergence angle of the electron beam. The method enables us to determine the lattice parameter, lattice type and crystal orientation of a specific nano-area. It is useful for crystallographic analysis of fine precipitates and interfaces. It is noted that the selection of a specimen area is made with a selector aperture in the case of SAD, but made with a condenser aperture and the convergence angle of the incident beam in NBD.

  • negative staining

    Negative staining is one of electron staining techniques. This technique leaves heavy metals at the gaps in a specimen and on the supporting film at the surrounding regions of the specimen. Negative staining enables enhancement of the TEM image contrast.
    An aqueous solution of viruses or purified proteins, etc. is dropped onto a supporting film, and then excessive water is removed from the specimen with a filter paper. Immediately after the removal of excessive water, a staining solution containing heavy metals (uranium acetate, phosphotungstic acid, etc.) is dropped onto the specimen. Then, water is removed from the specimen with a filter paper to dry the specimen, leaving the heavy materials at the gaps and on the supporting film at the surrounding regions of the specimen. As a result, these regions appear dark due to strong scattering of incident electrons, and the morphology of the specimen is elucidated. Since the specimen itself is not stained, this technique is termed "negative" staining.

    TEM image of negatively stained bacteriophage T4

  • noise canceller

    In a cold-field emission gun, short-time intensity fluctuations of the emission current (called "emission noise (chip noise)" exist. As a result, light-and-dark horizontal line contrast appears in an image of SEM or STEM. A system to reduce this adverse contrast is called "noise canceller." The noise canceller consists of a detector and an operational circuit. The canceller detects a part of the emission current, feeds back the fluctuation signal to the image signal to remove the effect of the fluctuation, and reduces the horizontal line contrast on-line. In this process, the emission current is detected by a dedicated aperture for current measurement or a condenser aperture fitted with a current-detection mechanism.

  • noise filter

    A filter that removes noise components from various frequency components.

  • nonisochromaticity

    When the incidence direction of an electron beam to an energy filter is tilted from the optical axis of the filter, the energy spectrum undergoes an energy shift ⊿E = ⊿E(α) (α is the incidence angle with respect to the optical axis) due to the second-order aberration of the filter. This energy shift is termed "nonisochromaticity." When a finite-size energy slit is inserted, the energy range selected by the filter changes with the angular position of the slit.

  • nonlinear least-squares method

  • objective aperture

    The "objective aperture" is used for accepting a transmitted wave or one of diffracted waves to obtain a bright-field image or a dark-field image. The objective aperture is inserted into the back focal plane of the objective lens. The diameter of the objective aperture is 5 μm to 100 μm. It is noted that this aperture was inserted when obtaining a lattice image or a structure image, but recently, it has not been used. The reason for this is as follows. If computer processing is applied to the data which has a sharp cut due to the aperture, artifacts can appear in the processed image.

  • objective current center

    When fluctuations are added to the excitation current of the objective lens, a TEM image spirally enlarges and shrinks. The center of this enlargement and shrinkage is called "objective current center." Alignment of the objective current center is carried out to bring the objective current center to the center of the fluorescent screen for viewing the image by the use of a double-deflection coil system. Since the fluctuations of objective-lens current are small, normally the alignment of the objective current center is not performed, but the alignment of the accelerating voltage center is carried out.

  • objective lens

    The "objective lens" is the first-stage lens to form an image using electrons exiting from the specimen. The objective lens is the most important lens in the imaging lens system because the performance of this lens determines the image quality (resolution, contrast, etc). A good objective lens has both a small spherical aberration (Cs) coefficient and a small chromatic aberration (Cc) coefficient. To decrease these coefficients, shortening the distance between the two magnetic poles and decreasing the bore diameter of the polepiece and are required. Since the side-entry-type specimen holder is inserted between the two magnetic poles, there is a limitation on shortening the distance. For the top-entry-type specimen holder, an asymmetric popiece whose upper bore diameter is larger than that of the lower one is used.

  • objective mini lens

    The "objective mini lens" is a weakly excited lens placed below the objective lens. This lens does not have a polepiece to strengthen a magnetic field as the objective lens does. This lens is used to form an image on the selector aperture in Low MAG mode (~50× to 3,000×) by stopping the excitation of the objective lens. The magnification at the objective mini lens is 1× to 2×. This lens is also used to obtain a high quality low magnification image of about 1,000× in the MAG mode. That is, the excitation of the objective lens is kept strong to maintain image quality of the objective-lens, and the image is demagnified (to ~0.5×) by the objective mini lens, thus a wide view image is formed on the object plane of the intermediate lens.

  • occupied state

    In the occupied state in a molecule or crystal, the certain energy level and band are occupied by (valence) electrons. These electrons cannot move freely.

  • octupole (octopole)

    A component consisting of eight magnetic coils symmetrically placed with respect to the optical axis, which is used for spherical aberration correction of the objective lens.

  • off-axial astigmatism and curvature of image field

    When electron beams exiting from a circular object centered on the optical axis do not form a perfect circle but form an ellipse on the image plane, this defect is called "off-axial astigmatism." This astigmatism arises from the fact that the lens action (curvature of the lens) is different for the rays in the two orthogonal planes, that is, for the rays in the plane including the optical axis (tangential plane) and for the rays in the plane normal to the tangential plane but not including the optical axis (sagittal plane). "Curvature of image field" is the aberration that the image plane is deformed from a flat plane to a curved plane. The distance from an off-axis object point to the principal point of the lens (the center of the lens) is longer than the distance from the point on the optical axis at the object plane to the principal point. Thus, the distance from the principal point to the image point for the former ray becomes shorter than that for the latter ray. As a result, the image plane deviates from the Gaussian (flat) plane to a curved plane as the image point goes away from the optical axis. The curvature of image field is different for the two orthogonal directions or for the tangential and sagittal directions due to the off-axial astigmatism.

  • omega filter

    One of the in-column type energy filters, which is installed between the intermediate and projector lenses in a TEM. The filter (spectrometer) is composed of four electromagnets and has a shape of the letter “omega.” Thus, this is called “omega filter.” Its energy dispersion is approximately 1 μm/eV for a 200 kV electron beam. The omega filter is used to date to acquire filtered (zero-loss) images, energy-loss images and zero-loss CBED patterns.

    Ωフィルタの働きと得られるスペクトルの概念図

    Schematic of the action of the omega filter and an acquired spectrum.
    A blue curve shows the trajectory of electrons passing through the optical axis with an energy E. A red curve shows the trajectory of electrons suffered by energy dispersion with an energy loss of ⊿E. When the optical plane at which energy dispersion (S) is created is projected onto the screen, the intensity distribution against energy losses, or an energy spectrum is observed.
    When the achromatic plane (A), at which energy dispersion disappears, is projected onto the screen, an image without energy dispersion is observed. In this case, if the energy slit placed on plane S is inserted to pass no energy-loss electrons, a zero-loss image (so called “filtered image”) is acquired. If the energy slit selects energy-loss electrons, an energy-loss image is obtained.

    Siのエネルギースペクトル

    (a) Energy spectrum of Si acquired at an accelerating voltage of 200 kV. (b) Line profile of the energy spectrum. ZLP is the zero-loss peak. P1 is a peak of a plasmon loss electrons (Ep = 16.7 eV). P2, P3… are peaks produced by multiple scattering of the plasmon. Weak and broad peaks L2,3 are the spectra due to excitation of inner-shell electrons.

    cubic-BN [110]のCBED図形(加速電圧100kV)

    CBED patterns of cubic-BN [110] acquired at an accelerating voltage of 100 kV. The two patterns were taken by projecting the CBED patterns formed at the achromatic plane onto the screen. (a) CBED pattern without using the energy slit (unfiltered pattern). (b) Filtered CBED pattern by selecting zero-loss energy with the energy slit. The graphs below the patterns are line profiles of each line A-B.
    In the unfiltered pattern at the left, the patterns inside the CBED disks are unclear. To the contrary in the filtered pattern at the right, where the lost energies approximately more than 10 eV are cut, the patterns in the disks are clearly seen. The energy filter is indispensable for quantitative analysis of CBED patterns.

    氷包埋したリポソームの像(加速電圧200kV)

    TEM images of ice-embedded liposome acquired at an accelerating voltage of 200 kV, taken by projecting the images formed at the achromatic plane onto the screen. (a) Conventional image without using the energy filter (unfiltered image). (b) Zero-loss image using the energy filter (filtered image).
    It is seen that the image contrast of liposome is enhanced in the filtered image (b).

  • on-site excitation

    In EELS, "on-site excitation" means that the inner-shell excitation occurs spatially only on an excited atom. When excitation of an inner-shell electron to an unoccupied state (band) is considered, since the wavefunction of the inner-shell electron is localized on an atom, if the final state is not overlapped with that wavefunction, the excitation (transition) does not occur. Thus, the ELNES spectrum reveals the anti-bonding state of the excited atom.

  • onset energy

    The onset energy means the energy at which a spectral intensity rises sharply in the EELS valence-loss spectrum or core-loss spectrum. This energy corresponds to the band gap in the valence-loss spectrum and to the bottom of the conduction band in the core-loss spectrum.

  • optical absorption spectrum

    An optical absorption spectrum is acquired by the absorption of specific (wavelength) light in a substance, when the substance is illuminated with visible light.

  • optical diffraction method

    A method used to examine the performance of the objective lens in a TEM. In the "optical diffraction method," an HREM image of a very-thin amorphous specimen is taken and is illuminated with a laser beam to obtain an optical diffraction pattern. Using the obtained optical diffraction pattern, corrections of spherical aberration, coma axis and axial astigmatism of the objective lens are performed. The radius of the diffraction pattern (the most distant visible spot) reveals the maximum spatial frequency or the resolution of the HREM image. If the diffraction pattern is perfectly circular, the astigmatism of the objective lens is fully corrected. Recently, instead of the laser beam, computer processing is used. A computer processed aberration correction is effectively utilized for the alignment of a Cs corrector.

  • optical potential

    A potential to which the imaginary part is added to incorporate the effect of absorption of an incident beam in a specimen. In the explanation of a TEM image using the dynamical theory, the optical potential is used. The absorption effect which gives the imaginary part of the potential includes plasmon excitation, thermal diffuse scattering and single-electron excitation. Plasmon excitation contributes to the average (normal) absorption and thermal diffuse scattering to the anomalous absorption. Single-electron excitation gives little effect to the anomalous absorption comparing to thermal diffuse scattering.

  • optical-axis alignment

    "Optical-axis alignment" is to align the axes of the illumination and imaging systems, ranging from the electron gun to the projector lens, with the optical axis. This alignment enables the electron beam to travel in a straight path.

  • ordered and disordered structure

    For example, in the high-temperature phase of Cu3Au, Cu and Au randomly occupy the lattice points of the face-centered structure. In the low-temperature phase, Au occupies the position of origin and Cu occupies the three face-centered positions. In this manner, the "ordered and disordered structure" means that when the external condition (temperature) changes, the structure of a substance changes where the constituent atoms are arranged orderly or randomly. The order-disorder transformation occurs in not only alloys but also inorganic compounds. If an ordered structure is formed, diffraction spots specific to the structure appear.

  • oscillator strength

    "Oscillator strength" is a classical quantity corresponding to the probability of an electric dipole transition when an electron in the valence band or in the inner shell is excited to the conduction band. In the classical model, this transition can be considered as oscillations of bound electrons (harmonic oscillator) using the Lorentz model. The oscillator strength is a number of the oscillators, and corresponds to the quantum-mechanical probability of the electric dipole transition.

  • outgas

    "Outgas" means gas desorption from an object. In the case of a TEM, outgas means that deposited substances are emitted as gas from surfaces exposed to vacuum in a TEM column.

  • overfocus

    "Overfocus" means that the excitation of the objective lens in a TEM is slightly increased from that at the in-focus (focused on a specimen). At this excitation, the image produced on the selector aperture is the specimen image when the objective lens is focused below the specimen.

  • paraelectric material

    "Paraelectric material" generates dielectric polarizations when an electric field is applied to the material and the material looses the polarizations when the electric field is removed. In "paraelectric materials," three kinds of polarizations exist: (1) electronic polarization, (2) ionic polarization, and (3) orientational polarization. In electronic polarization, electrons are displaced against the atomic nucleus. In ionic polarization, positive ions are displaced against negative ions. In orientational polarization, molecules having permanent dipole moments change their directions under an electric field. The paraelectric material has a small permittivity and a small dielectric loss. A material which is called quantum paraelectrics does not transform to a ferroelectric phase but remains at the paraelectric state even around absolute zero temperature due to zero-point oscillation of phonons.

  • parallel detection

    In the acquisition of an energy-loss spectrum in EELS, the "parallel detection" method uses a one-dimensional detector or a two-dimensional detector to efficiently measure the energy-loss spectrum in parallel.

  • parallel detector

    A detector that can simultaneously read signals obtained with multiple channels. Compared to the serial detector, the detection efficiency improves as much as the multiple of the number of channels.

  • parallel electron energy-loss spectroscopy

    Parallel electron energy-loss spectroscopy (PEELS) is an EELS method which uses a parallel detector on the energy dispersive plane. PEELS has higher detection efficiency than the serial detection method that obtains a spectrum by changing the energy in serial times.

  • paramagnetic material

    A material that is magnetized in the direction of a magnetic field when placed in the magnetic field, and demagnetized when the magnetic field is removed.

  • parasitic aberration

    "Parasitic aberrations" are not inherent aberrations like five Seidel aberrations, but residual aberrations which arise from magnetic non-uniformity of the polepiece material, inaccuracy of machining the polepiece and disagreement of the optical axes between lenses. The parasitic aberrations include axial coma aberration, star aberration, three lobes and axial two-, three-, four-, five- and six-fold astigmatisms.

  • paraxial approximation

    "Paraxial approximation" is an approximation used in ray tracing of an electron beam where the angle between the electron beam and the optical axis is small. In other words, trigonometric functions of the angles appearing in ray optics are approximated by linear functions (ex. sin α → α) and optical surfaces are replaced by portions of a sphere. In the paraxial approximation, five Seidel aberrations do not appear.

  • partial dislocation

    The magnitude of Burgers vector b of a (perfect) dislocation is defined as the distance from a lattice point to the nearest lattice point. There may exist a meta-stable position for an atom given by a vector b1 whose magnitude is smaller than b. The Burgers vector of the perfect dislocation can split to b = b1+b2. The defects having vectors b1 and b2 are called "partial dislocation." Since the dislocation energy is proportional to the square of the Burgers vector, the splitting (extension) of the perfect dislocation is possible. However, a stacking fault is introduced between two partial dislocations. The energy of the stacking fault determines whether or not the extended dislocation occurs and the width of the stacking fault. When the splitting distance of the two partials is short, the use of the weak-beam method is effective for determining this splitting.

  • peak-to-background ratio

    The "peak-to-background ratio" is defined as the ratio of the peak intensity to the background intensity in a spectrum. (1) In an ELNES spectrum of EELS, the ratio is low because of its high-background signal. As the accelerating voltage of the incident electron beam is increased, the peak-to-background ratio is improved, because multiple scattering is decreased and the effective acceptance angle is increased. (2) In an EDS spectrum measured in a TEM, the ratio of the characteristic X-ray peak intensity to the background intensity is very high. When the accelerating voltage is increased, the ratio is further improved because the probability of bremsstrahlung (background) is decreased.

  • penetrating power

    "Penetrating power" is defined as the power (length) of an electron beam transmitted for a substance. An electron beam at 100 kV (accelerating voltage) has a transmissivity of 100 nm. As the accelerating voltage is higher, the transmissivity increases. However, this is saturated due to the relativistic effect; At 1000 kV, the transmissivity is about 3.3 times larger than that at 100 kV. Penetrating power is expressed by the reciprocal of the absorption coefficient. In the case of a TEM, electrons that do not pass through the objective aperture are regarded to be absorbed. Thus, elastic scattering at high angles is regarded as absorption. Among inelastic scattering, plasmon scattering exhibits large energy change (~15 eV) but takes place at small angles less than about 10-3 rad, thus passing through the objective aperture. The Plasmon scattering does not contribute to absorption. On the other hand, thermal diffuse scattering exhibits small energy change (~0.1 eV), but the scattering occurs at high angles; thus this is regarded as absorption. Inner-shell electron excitation exhibits energy changes of larger than 10 eV, but its scattering cross section is small (large mean free path). Thus, its contribution to absorption is small. The mean free path of inelastic scattering is about several 100 nm and this is about 10 times larger than that of elastic scattering.

  • phase contrast

    Contrast produced by changes in phases of scattered waves. The HREM image is formed by the phase contrast. When a specimen is very thin, it is approximated that electron waves hardly suffer absorption but change only their phases (weak phase object approximation). If an aberration-free lens is used to cause interference between transmitted and scattered waves, no contrast appears on the ideal image plane. That is, the phase shift of the scattered waves does not cause the intensity variation in the image. The scattered waves undergo a phase shift of π/2 with respect to the transmitted wave. If the phase of the scattered waves is further shifted by π/2 in such a way that the amount of phase shift due to the spherical aberration of the electron lens is adjusted by the amount of defocus, the resultant phase shift of the diffracted waves becomes πwith respect to the transmitted wave. This phase change of the diffracted waves (π) is converted into the change of amplitude (not complex number but real number), producing contrast in the image due to the interference between the transmitted wave and diffracted waves. When an appropriate defocus amount is selected so that the phase of scattered waves is shifted by π/2 over the wavenumber range as wide as possible and the amplitudes of the waves become as large as possible, the structure image is obtained.

    位相コントラスト:phase contrast
    High-resolution TEM image of a single-layer graphene taken at an accelerating voltage of 80 kV.

    Since the graphene is regarded as a weak-phase object, the image contrast of the graphene reflects the phase change of electrons scattered by carbon atoms. This image is acquired using an electron microscope equipped with a Cs corrector at a slight defocus to create the phase contrast.
    Hexagonal grids of the graphene are seen in most parts. In the area enclosed by white dashed lines at the left, a lattice defect is seen. In the area enclosed by yellow dashed lines at the upper left, the graphene is seen to take an amorphous state.

  • phase plate

    A plate that causes a change in the phase of an electron wave. The phase plate placed at the back focal plane of an electron microscope creates a relative phase change between the transmitted wave and scattered waves from a specimen. By the interference between the transmitted wave and the scattered waves, a phase change due to an object, which is originally difficult to view, can be visualized as an intensity change. Since the phase plate produces the relative phase change effectively at a small spatial-frequency region (corresponding to a long-distance region in the real space), it is effectively used in obtaining high contrast for biological specimen. There are two practically-used phase plates: Phase plate made of a carbon thin film with a controlled thickness having a hole at the center (Zernike phase plate) (Fig. (a)) and Hole-free phase plate (Fig. (b)).

    phase plate

    (a) Zernike phase plate, (b) Hole-free phase plate
    A Zernike phase plate has a hole where the transmitted wave passes and causes a phase shift of π/2 of scattered waves with respect to the transmitted wave.
    A hole-free phase plate does not have any hole, but produces a relative phase change between the transmitted wave and scattered waves. This phase change may be interpreted as follows: The part of the thin film plate which receives the electron beam is electrically charged and suffered by a change in electrostatic potential, and then the phase of the transmitted wave is changed relative to the scattered waves.

  • phase transformation

    A phenomenon where a substance transforms to a different state by a change of the external conditions (temperature, pressure, magnetic field, etc.). "Phase transformation" is classified into the first-order and the second-order phase transformations. The former exhibits a discontinuity in the first derivative of the free energy with respect to temperature, which includes melting of solids and vaporization of liquids. The latter exhibits a discontinuity in the second derivative of the free energy temperature, which includes the transformation of the ferromagnetic state (iron, etc.) to the paramagnetic state at Curie temperature and the transformation of the alloys from a chemical order state to a disorder state (and vice versa).

  • phase-contrast transfer function

    When electrons pass through the objective lens, spherical aberration of the objective lens causes the electron beam to undergo a displacement at the object plane, which is proportional to the cube of the incident angle to the objective lens. For an incident angle, this displacement can be canceled with an opposite displacement proportional to defocus of the objective lens. However, this cancelation is not accomplished for all the incident angles. The cancelation or compensation feature is described by the phase changes of the electron waves passing through the objective lens. That is, the phase changes are given as a function of the incident angle to the lens (or scattering angle from the specimen) and the defocus of the lens for a given Cs coefficient. This function is called “phase-contrast transfer function.”

    phase-contrast transfer function

    Example of the phase-contrast transfer function (PCTF) at an accelerating voltage of 200 kV for (a) Cs: 0.5 mm and for (b) Cs: 5 mm. The horizontal axis stands for spatial frequency and the vertical axis for the amount of information on the crystal structure of a specimen transferred to a TEM image. The negative region of PCTF contributes to form dark image for the atom sites and the positive region contributes to form bright image. It is noted that only the image of a very thin specimen, to which the weak phase object approximation can be applied, is interpreted using PCTF.
    It is desirable that the value of PCTF is constant (ideally –1 (or +1)) for all spatial frequencies. However, the real TEM accompanies aberrations such as spherical aberration, thus the absolute value of PCTF becomes less than 1 and is not constant. In particular at the high-frequency side, the value fluctuates greatly over the positive and negative values. The structural information on a specimen is transferred without contrast reversal till the frequency at which PCTF firstly crosses the horizontal axis (called First Zero). That is, the inverse of the frequency at First Zero means the resolution of the structural image. When PCTFs (a) without Cs correction and (b) with Cs correction are compared, it is clearly seen that the negative region of PCTF extends to a higher spatial frequency in (b). This indicates that the structural information on a specimen is much better transferred in the case of (b) with Cs correction.

  • phonon

    "Phonon" is the quantum form of lattice vibrations which cause thermal diffuse scattering. Since the phonon energy is very small (0.1 eV or less), EELS with the present performance cannot resolve phonons. An existing energy filter cannot remove inelastically scattered electrons due to phonons.

  • photo emission electron microscope

    A "PEEM (photo emission electron microscope)" is a microscope to form a surface image of a specimen using photoelectrons generated from a specimen illuminated with ultraviolet rays or vacuum ultraviolet rays. The generated photoelectrons are accelerated by an acceleration electric field (acting as an immersion lens) just above the specimen. This electric field also acts as an objective lens. The imaging lens system is placed after the objective-lens field. An enlarged image is observed with a screen or a CCD camera. Since the intensity of the photo-electron image depends on the work function and the wavelength of the excitation light, selecting an appropriate excitation wavelength enables us to acquire an image sensitive to electronic structures of atoms on the specimen surface, and also to view a contrast formed by surface structures. To obtain information on a clean surface, the specimen area is kept at ultrahigh vacuum. PEEM can be further equipped with an imaging filter for energy selection of photo electrons to obtain detailed information on electronic structures. The function of PEEM is often added to LEEM (low energy electron microscope) which has the same imaging system as that of PEEM.

  • photo film

    A film that records a TEM image or a diffraction pattern. The "photo film" is directly exposed to electrons in the camera chamber of a TEM. The sensitivity characteristic of the photo film to the electron is similar to that to visual light. Since the photo film has a small dynamic range of 2 digits and insufficient linearity, it is not suitable for quantitative measurement. However, since its positional resolution is as high as about 3 μm, the photo film has been used for the photography of high-definition images. Its gray level is <16 bits. Now, the imaging plate and CCD are available as recording devices that have higher sensitivity than the photo film.

  • photodiode array

    A detector on which photodiodes (photo-sensitive elements converting light signals into electric signals by optical absorption) are arrayed.

  • photomultiplier tube

    "Photo multiplier tube (PMT)" consists of a phototube that incorporates an electron multiplier. PMT detects weak light, and converts the light signal into an electric signal, then amplifies the electric signal (×105 to 106). PMT is used for a detector, such as a secondary-electron detector in SEM or HAADF detector for scattered electrons in TEM.

  • pixelated STEM detector

    A STEM detector that records a diffraction (CBED) pattern formed on the detector as a two-dimensional (2D) image with a high-speed frame rate in synchronism with the scan of the incident electron probe. To take a two dimensional CBED pattern during the stay time of the STEM probe at one pixel, a direct electron detector of CCD or CMOS, which has a very high-speed frame rate of a few thousands of fps or more, is required. At present, using such a direct electron detector, a synchronous STEM scan with taking a 256 x 256 pixel CBED pattern is achieved.
    A conventional STEM detector and a pixelated STEM detector are compared in Fig. 1.

    pixelated STEM detector
    Fig. 1 Conventional single-channel STEM detector and Pixelated STEM detector
    (a) Conventional single-channel STEM detector. The electron beam is converted to light using the scintillator and the light is guided to PMT. Then, the light creates electrons in the PMT. Finally they are measured as the output voltage signal. It should be noted that the signal is obtained as an integrated value from a CBED pattern area determined by the shape of the scintillator.
    (b) Pixelated STEM detector. The electron beam enters the direct electron detector (CCD or CMOS) and converted to electric signals. The detector records a CBED pattern as a 2D image unlike the conventional single-channel STEM detector.
    In the conventional STEM detector, the electron beam is converted to light with a scintillator and the intensity of the obtained light is measured as the output voltage of a photomultiplier tube (PMT) (Fig. 1(a)). All or part of the light signals from a CBED pattern formed on the detector plane are detected with a single-channel PMT, and then a 2D STEM image is acquired by displaying the output voltage as the function of the incident probe position. By selecting the acquisition region of the scattered electrons with the use of a circular or annular scintillator (detector), BF-, ABF-, or HAADF-STEM image can be obtained. It is noted that angle-resolved information is not available because the detector is single-channeled, thus the intensity of the CBED pattern is integrated.
    In the pixelated STEM detector, the CBED pattern is recorded as a 2D image (Fig. 1(b)). Furthermore, the electron probe is two-dimensionally scanned, and then the acquired final data becomes 4D data. It is emphasized that, the angle-resolved information about the diffraction intensity, which is lost in the conventional STEM detector, can be effectively used. For example, a variety of STEM images are created by flexibly changing the integration area of the CBED pattern.
    Fig. 2 shows five sort of STEM images created from the final 4D data of SrTiO3 [100] taken at an accelerating voltage of 200 kV. It is seen that different information can be obtained by changing the integration area of the CBED pattern.
    The other applications of the pixelated STEM detector include creation of an electric-magnetic field map utilizing positional shift of the CBED pattern, and reconstruction of a phase image of a specimen by utilizing ptychography image processing.

    pixelated STEM detector

    Fig. 2 Various STEM images of SrTiO3 [100] taken at an accelerating voltage of 200 kV, by changing the integration area of the CBED pattern. Those STEM images are created from the final 4D data.
    The integration area of each CBED pattern is shown in insets (lower right of each STEM image), indicated by translucent red and blue colors. Acquisition conditions are as follows: Number of pixels of the STEM image: 256 x 256 pixels, Frame rate of the pixelated STEM detector: 4,000 fps (dwell time 250 μs), Data acquisition time: approx. 16 s.
    (a) STEM image created by integrating the whole area of the CBED pattern. Sr columns and Ti+O columns are observed as dark. O columns are not seen.
    (b) BF-STEM image. Sr columns are observed as dark, but O columns as bright.
    (c) ABF-STEM image. All of Sr-, O-, and Ti+O columns are observed as dark. Compared with the BF image, the ABF image enables intuitive interpretation of the atomic columns.
    (d) e-ABF (enhanced ABF) image. The image is created by subtracting the intensity of the area B from that of C. O columns appear more clearly compared with those of the ABF-STEM image.
    (e) LAADF-STEM image created by integrating the area except for the transmitted wave disk. Sr columns and Ti+O columns are observed as bright. O columns are not seen.
    (f) Conventional HAADF-STEM image acquired using a circular single-channel STEM detector.

  • plasma cleaning

    "Plasma cleaning" is to irradiate a specimen with plasma ions to remove contaminants (polymerized substances of hydrocarbons) on the specimen. In actual plasma cleaning, the specimen is placed between the two electrodes, and then a voltage is applied between the electrodes under a low vacuum so that residual gasses transform to the plasma state. The contaminants are removed by the strong reaction of the plasma ions. In the plasma state, atoms or molecules become ions and radicals due to strong collision with each other, which are at high energy states and are very reactive with other molecules.

  • plasma oscillation

    Plasma oscillations generally mean various oscillations of electrons and ions in a plasma state. The plasma oscillations detected and analyzed by EELS are collective oscillations of free electrons (volume plasmon) and oscillation modes of electrons on a solid surface (surface plasmon). The oscillation frequency of the volume plasmon is proportional to the square root of the electron density. The oscillation frequency of the surface plasmon is (1/√2) times that of the volume plasmon when the solid surface is exposed to vacuum.

  • plasmon

    "Plasmon" expresses the quantum form of plasma oscillation. Since the plasmon (volume plasmon) is longitudinal wave oscillation, it cannot be observed by the optical method but can be directly observed as a plasmon excitation spectrum by EELS.

  • plating

  • plating

    "Plating" is a technique to form thin metal layers deposited on a surface of a solid.

  • pneumatic damper

    A mount that absorbs vibrations using pneumatic springs. The "pneumatic damper" is used for preventing vibrations of a TEM unit.

  • point analysis

    In spectroscopic analysis, "point analysis" is to acquire a spectrum from a point on a specimen by stopping an electron beam at the point.

  • point defect

    "Point defect" is defined as atomic-size defect. In particular, when no atom exists at a lattice point, this defect is called "vacancy type point defect." On the other hand, when an atom is introduced into a position other than a lattice point, the defect is termed "interstitial type point defect." Vacancies can exist in a crystal at the thermal equilibrium state and play an important role for diffusion of atoms. Interstitial atoms are normally impurity atoms, but they can be the same atoms of the crystal when the atoms at the lattice points are ejected by irradiating the crystal with high-energy particles. In the latter case, since the interstitial atoms are easy to move, the atoms are likely to disappear by recombination with vacancies.

  • point resolution

    The minimum distance between two points at which these points appear to be separated. The "point resolution" of a TEM image becomes higher as the accelerating voltage is increased and the spherical aberration of the objective lens is decreased.

  • point spread function

    When light transferred from incident electrons by a thin fluorescent material is detected with CCD, the size of the emitted light is larger than that of the incident electrons due to scattering of electrons by the fluorescent particles. The size of the light can be larger than one pixel size of the CCD (~15 μm). The function that expresses the spread is called "point spread function." As a guide example, the intensity at the nearest pixel is about 1/3 of that of central pixel.

  • polepiece

    "Polepiece" is a magnetic pole made of a soft magnetic material (pure iron, permendur), which concentrates the magnetic flux produced by the electromagnet and guided by the yoke to produce a strong magnetic field in a narrow gap (space) in the polepiece. Rotationally symmetric strong magnetic fields produced between the upper and lower poles enable electron beams to be focused. Several types of polepieces are available for the objective lens. They include the ultra-high-resolution polepiece that gives a strong magnetic field for high-resolution image observation, the multiuse polepiece that allows a specimen to be tilted at large angles, and the high-contrast polepiece that enhances image contrast.

  • polishing

    "Polishing" is a mechanical or chemical treatment to prepare a smooth and uniform surface without selecting crystal orientations unlike etching and cleaving.

  • polycrystal

    A crystal composed of crystalline particles with different crystal orientations.

  • post-column type

    An energy filter or an energy analyzer that is installed behind the column of a TEM is classified as "post-column type." This type includes a GIF and a Tridiem.

  • post-embedding

    Post-embedding is a technique of embedding in the course of immunoelectron microscopy for the primary antibodies to react with the target antigenic proteins after ultrathin sectioning. The name of “post-embedding” originates from the fact that the immune-reaction is performed after ultrathin sectioning (subsequent to resin embedding). Post-embedding has two major advantages. One is that the locations (localization) of the target proteins are precisely elucidated because the primary antibodies react with the proteins exposed on an ultra-thin section. Another is that the structures of tissues, organelles, etc., are preserved better compared with the case of pre-embedding because post-embedding requires a few steps until embedding. However, post-embedding has a major disadvantage that antigens can be lost by dehydration or can be denatured by resin embedding, thus degrading the staining efficiency.

  • post-magnetic field

    In a modern electron microscope, a specimen is placed in the objective lens. The magnetic field produced below the specimen (at the intermediate lens side) is called "post-magnetic field." Since the spherical aberration and chromatic aberration of the post-magnetic field determine the image resolution, this field is the most important part for image formation.

  • postfixation

    Postfixation is the second-step fixation technique of chemical fixation for biological specimens observed with a TEM.  This technique aims at preservation of lipids or enhancement of a TEM image contrast.  In the preparation of a biological specimen for TEM observation, dehydration and resin embedding are applied.  In these procedures, fixation only with aldehyde (glutaraldehyde, paraformaldehyde, etc.) cannot fix lipids in the specimen.  Thus, the membrane structures that consist of lipids flow out of the specimen when performing dehydration with ethanol or an organic solvent. In addition, aldehyde bounded to the specimen does not contribute to the enhancement of the image contrast because it consists of only light elements.  These problems are solved by postfixation with heavy metal oxide reagents (osmium tetroxide, etc.) which is a subsequent process of prefixation with aldehyde.  Osmium tetroxide fixes lipids by binding it to a double-binding region of unsaturated fatty acid and by constructing a cross-link for preventing outflow and deformation of the lipids.  In addition, osmium is a heavy metal, thus enhancing the image contrast of the binding region.  Osmium tetroxide is an effective reagent, but it has disadvantages that include slow permeation into the specimen and destruction of proteins in the specimen.  To prevent these disadvantageous phenomena, fixation (prefixation) of proteins with aldehyde is required before applying fixation with osmium tetroxide.

  • potential barrier

    "Potential barrier" means a barrier expressed in the potential dimension that stops the passing of particles through certain regions.

  • pre-embedding

    Pre-embedding is a technique of embedding in the course of immunoelectron microscopy for the primary antibodies to react with the target antigenic proteins immediately after fixation of block tissues or cells. The name of “pre-embedding” originates from the fact that the immune-reaction is performed prior to resin embedding. The advantage of pre-embedding is that antibodies can react with the antigenic proteins in advance of the occurrence of denaturation or outflow of the antigenic proteins. However, pre-embedding has two major disadvantages. One is that antibodies do not stain uniformly the whole block tissue because staining is made for the block tissue prior to resin embedding. Another is that a large amount of expensive antibodies is needed for pre-embedding. In general, pre-embedding is a simpler technique than post-embedding.

  • pre-evacuation

    "Pre-evacuation" is to evacuate the pre-evacuation chamber for the specimen holder from atmospheric pressure to a certain low pressure. In the pre-evacuation process, first the specimen pre-evacuation chamber is vented and a specimen is loaded on the specimen holder, and next the chamber is pre-evacuated, and finally the isolation valve is opened and the specimen is transferred to the specimen chamber.

  • pre-magnetic field

    In a modern electron microscope, a specimen is placed in the objective lens. The magnetic field produced above the specimen (at the condenser lens side) is called "pre-magnetic field."

  • precession electron diffraction

    Precession electron diffraction is an electron diffraction method to obtain an electron diffraction pattern with a less dynamical diffraction effect by precessing the incident electron beam under a certain tilt angle with respect to the optical axis. A two-stage deflector coil in the illumination lens system is used to precess the tilted beam (up to approximately 5°) and to illuminate the beam onto a certain point on a specimen. Then, a two-stage deflector coil (below the specimen) in the image-forming lens system is used to compensate the displacement of the incident electron beam from the optical axis (de-scan), so that the incident beam subjected to the precession stays at the center of the screen. Illumination of such a precessed electron beam suppresses enhancement of the intensity of the reflections due to “double reflection” (an additional excitation via the other strongly-excited reflections). As a result, the observed diffraction intensities become close to those expected from the kinematical diffraction theory. Owing to the precession of the incident electron beam, the number of reciprocal lattice points crossed by the Ewald sphere increases and thus, high-order diffraction spots appear.
    The obtained diffraction intensities are utilized for crystal structural analysis by applying the analysis techniques (direct method, etc.) developed for X-ray crystal structure analysis. The electron diffraction method is effectively used for structural analysis of inorganic crystals with complicated structures (e.g. zeolite) and organic crystals, which are often crystallized only with a size up to a micrometer order.

    precession electron diffraction

    (a) Ray diagram of precession electron diffraction. A precession electron diffraction pattern is obtained by precessing the incident electron beam under a certain tilt angle with respect to the optical axis using a two-stage deflector coil in the illumination lens system, and then by compensating the displacement of the incident electron beam from the optical axis using a two-stage deflector coil in the image-forming lens system.
    (b) An ordinary electron diffraction pattern and (c) a precession electron diffraction pattern obtained from garnet at [111] incidence. In Fig. (c), the precession of the incident electron beam enables us to obtain a diffraction pattern that has diffraction intensities being close to those expected from the kinematical diffraction theory. Furthermore, more high-order reflections appear compared with the number of reflections in the ordinary diffraction pattern (b). 

  • precipitate

    "Precipitate" is a solid formed as a new stable phase separated from the matrix crystal by atoms excessively dissolved in a supersaturated solid solution. When the degree of supersaturation of the dissolved atoms is low, they do not form a phase but are collected on grain boundaries or dislocations.

  • prefixation

    Prefixation is the first-step fixation technique of chemical fixation for biological specimens observed with a TEM.  This technique aims at inactivation and preservation of proteins.  In the preparation of a biological specimen for TEM observation, dehydration and resin embedding are applied.  Under these procedures, a robust fixation of the fine structures of biological tissues is required.  For such a fixation, it is effective to use strong oxidizing reagents (osmium tetroxide, potassium permanganate, etc.).  However, these reagents have disadvantages that include slow permeation into the specimen and destruction of proteins in the specimen.  To avoid these disadvantageous phenomena, the specimen is pre-fixed with paraformaldehyde or glutaraldehyde, which permeate quickly the specimen and cross-links the proteins to preserve the original morphologies.

  • probe diameter

    The diameter of the incident electron beam on the specimen. The minimum probe size (diameter) at present is ~0.2 nm for the field-emission electron gun, whereas ~1 nm for the LaB6 thermionic-emission electron gun. When a Cs corrector is used, the probe diameter less than 0.1 nm is achieved.

  • probe-current detector

    A detector that monitors the illumination beam current onto the specimen using a Faraday cup in EELS or EDS analysis.

  • process time

    The process time is a time index to select whether to use the EDS analysis focusing on its energy resolution or focusing on its analysis time (throughput). By changing the process time, an EDS analysis suitable for analysis purpose can be carried out. Some EDS manufacturers name it as "time constant."
    In the element analysis by EDS, characteristic X-rays generated from a specimen are subjected to dispersion according to their energy. In the process of EDS, averaging of the noise components of X-ray signals which are detected with a semiconductor detector is performed to obtain accurate energy values of the X-rays. By changing the process time, the averaging time is changed. If the process time (averaging time) is longer, a high-energy resolution EDS spectrum is acquired because the measurement error of the X-ray energy values decreases. However, the X-rays entering the detector in the averaging time cannot be measured, and then the dead time increases. If the process time (averaging time) is shorter, the dead time decreases and the time required for analysis is shortened. However, since the error of the X-ray energy values increases because of a shorter averaging time, the energy resolution is sacrificed.

  • projected potential

    The projected potential is the electrostatic potential of a crystal projected along a direction (normally along a low-index zone axis). When a crystalline specimen is very thin and the weak phase object approximation holds, an image taken at Scherzer focus exhibits fairly well a projected potential (projected atomic arrangement). Thus, this image is called a crystal structure image.

  • projector (projection) lens

    The "projector (projection) lens" is the final lens in the imaging lens system. The projector lens further magnifies the image magnified by the intermediate lens, and then forms the final image on the fluorescent screen or the detector. The magnification of this lens is fixed at a value of ~150×.

  • pulse-height analyzer

    An instrument used for analyzing energy spectra of radial rays by counting voltage pulses output from a radiation detector. The "pulse-height analyzer" counts only the number of pulses falling within certain voltage ranges (channels), where the upper and lower limits of the pulse heights are arbitrarily specified. This analyzer is used to measure characteristic X-rays by EDS.

  • quadrupole

    A component consisting of four magnetic coils symmetrically placed with respect to the optical axis, which is used for correction of axial astigmatism of the objective lens by varying the focal lengths in the two orthogonal directions.

  • quantitative compositional analysis

    "Quantitative compositional analysis" is to analyze not only the composition (kinds of constituent elements) but also the element abundance ratio (concentrations) of a substance. The composition is analyzed from the energy-peak positions in a spectrum and the element abundance ratio is quantitatively analyzed from the peak intensities.

  • quasicrystal

    "Quasicrystal" takes a new ordered structure which has no periodicity but a long-range order with bond orientational order between the constituent atoms. Thus, the quasicrystal is totally different from amorphous. Quasicrystals have been found in many alloys composed of aluminum and two transition metals exhibiting ferromagnetic properties.

    準結晶:quasicrystal
    HAADF-STEM image of an Al-Mn-Pd two-dimensional quasicrystal taken
    with a JEM-ARM200F at an accelerating voltage of 200 kV.

    In the image, decagonal (D) structure units, star-shaped pentagonal (P) units and hexagonal (H) units are clearly seen. All the D units are joined by edge-sharing linkage, and the gaps between those D units are completely filled with P units and H units. (cf. JEOL News Vol. 50, p25 (2015))

    Courtesy of the image:
    Professor Emeritus K. Hiraga, Tohoku University

  • radial distribution function

    A diffraction pattern (halo pattern) is taken from an isotropic amorphous substance. From the intensity distribution curve of the pattern, the smooth background passing through the center of the wavy curve is subtracted to remove only the interference term between different atoms. The "radial distribution function" is obtained by the Fourier transform of the interference term. The average interatomic distance (or the distribution of the interatomic distance), the coordination number and the density of the substance can be examined from the function.

  • radiation damage

    "Radiation damage" is defined as structure deterioration of a specimen due to the irradiation of an electron beam. The primary radiation damage includes the knock-on damage and the ionization damage. In the knock-on damage process, an atom that suffers the collision by an incident electron is ejected from its lattice site, forming an interstitial atom and an atomic vacancy at the corresponding lattice site. In the ionization damage process, outer-shell electrons of an atom are ejected and the atom is ionized. In a subsequent relaxation process after the primary radiation damage process, particular lattice defects or amorphous structures are created. This process is called the secondary radiation damage process.

  • rapid freeze fixation

    Rapid freeze fixation is a technique to physically fix the morphology of biological tissues, biological cells, bacteria and suspended specimens composed of viruses, purified proteins, etc., by rapidly freezing the specimens. Chemical fixation, which is used in ordinary specimen preparation of biological tissues for TEM, may cause outflow of mineral salts, etc., in the tissues or deformation of the membrane structures of the tissues. On the other hand, rapid freeze fixation enables us to fix a specimen while preserving the morphology of the tissues. Rapid freeze fixation techniques include high pressure freezing, metal mirror freezing (slam freezing) and ice embedding.

  • real lattice

    "Real lattice" is a set of points which have an infinitely regular arrangement in the real space and are arranged to be equivalent when viewed from every point. When a structure unit is given to each lattice point, a crystal is formed.

  • real space

    The space in which we exist. In a TEM, the "real space" includes the space of an object to be observed and the space where an image of the object is formed (image space).

  • reciprocal lattice

    "Reciprocal lattice" is a set of points which have distances inversely proportional to the spacings of lattice planes of a real lattice in the directions perpendicular to the original lattice planes. The reciprocal lattice is generated automatically from the real lattice through a mathematical relation.

  • reciprocal space

    A space constructed by reciprocal lattices. In a TEM, the "reciprocal space" appears in the back focal plane of the objective lens on which a diffraction pattern is formed.

  • reference wave

    In electron holography, the "reference wave" is an electron wave directly coming from the electron source, which interferes with the transmitted wave suffered by a phase shift through an object. The two waves are superposed by an electron bi-prism to obtain an electron hologram.

  • relativistic correction

    Relativistic correction is a correction to take in the relativistic effect arising when the velocity of an electron becomes high to an extent which cannot be neglected compared with the velocity of light. In the field of electron microscopy, the relativistic effect due to increase of the mass or decrease of de Broglie wavelength is taken into account by replacing the accelerating voltage with the corrected accelerating voltage. That is, the accelerating voltage after the relativistic correction E* [V] is calculated by the following equation for a given (before relativistic correction) accelerating voltage E [V], here, m0 is the rest mass of an electron, e the elementary charge of an electron, and c the velocity of light.
    相対論補正 relativistic correction

  • relaxation time

    When the external condition of a system changes and then the system reaches its equilibrium state or its steady state under the new condition, the process is called "relaxation." The "relaxation time" means the time that characterizes the velocity of the process from an initial non-equilibrium state to the final equilibrium state.

  • replica

    A "replica" is a reproduction of a topographic shape of the surface of a substance (specimen). To make a replica, a thin layer of a plastic material or an inorganic material is formed on the surface of the substance, and then, this thin layer is removed, the topographic shape of the surface being copied. The replica technique is used for a reproduction of a grating or observation of the topographic shape of the specimen.

  • residence time

    "Residence time" is a time when a particular compound or element spends in a system, or a time when a reactant spends in a process vessel or contacts a catalyst.

  • retarding

    "Retarding" means the retardation of an electron beam in a TEM. In the optical system of the electron microscope, a certain voltage is applied to an additional electrode, a lens or a specimen stage to lower (retard) the velocity of the electron beam. Retarding of the electron beam improves the energy resolution of EELS spectra. As an attempt for multi-functionality of TEM, a retarding voltage is applied to the specimen stage for the observation of a low-accelerating-voltage TEM image while the accelerating voltage of the TEM itself is kept unchanged. In addition, changing the retarding potential (voltage) enables us to measure the energy distribution of backscattered electrons and to take a reflection diffraction pattern only from elastically scattered electrons.

  • rocking curve

    A "rocking curve" means the intensity distribution with the change of diffraction condition (with deviation from the Bragg angle). The rocking curve for a very thin specimen corresponds to the intensity distribution due to kinematical diffraction. That for a thick specimen corresponds to the intensity distribution due to dynamical diffraction and can be seen in a CBED pattern.

  • rotary-inversion axis

    A symmetry element of the crystallographic point group. When the whole crystal (constituent atoms) is rotated by an angle of 90° with respect to a line, and successively an inversion is operated for the rotated crystal, the resultant crystal can be equivalent to the crystal before the operations. In this case, the combination of the two symmetries is called "rotary-inversion axis." Only the 4-fold rotary-inversion axis is a symmetry of the crystallographic point group.

  • rotation axis

    A symmetry element of the crystallographic point group. When the whole crystal (constituent atoms) is rotated by a certain angle (180°, 120°, 90° or 60°) with respect to a line, the rotated crystal can be equivalent to the crystal before rotation. In this case, this line is called "rotation axis." Corresponding to the respective rotation angles, there exist 2-fold, 3-fold, 4-fold and 6-fold rotation axes.

  • rotation-free image

    "Rotation-free image" means an image that does not rotate even when the magnification is changed. To prevent both inconvenience caused by the change of an observation position and disappearance of the orientation relationship between the diffraction pattern (magnification: zero) and the corresponding image, an image rotation created by the first two-parts of the intermediate lens are corrected by the final-part of the intermediate lens, so that the image rotation is eliminated or minimized. When the excitation of the third- (final-) part is changed, the magnification is also changed. Thus, it is necessary to change the excitation of the second part of the lens. Combinations of appropriate excitation of each part in obtaining a rotation-free image are available in tables.

  • rotationally symmetric lens

    A lens whose symmetry does not change even when rotated with respect to the lens axis. Conventionally-used lenses (objective lens, condenser lens, etc.) are "rotationally symmetric lenses." On the other hand, recently-developed lenses for analytical purposes, such as the Wien filter, the omega filter, the alfa filter and the Cs corrector, are non-rotationally symmetric lenses.

  • rough pumping

    "Rough pumping" means pumping from atmospheric pressure to the pressure for switching to the main high-vacuum pumping. In the rotary-diffusion pump system, the switching pressure is between 1 Pa and 0.1 Pa.

  • satellite reflection

    A modulation which has a longer period than that of the fundamental lattice takes place in certain crystals. In such cases, weak diffraction spots due to the modulation appear around the fundamental diffraction spots from the basic lattice. The weak diffraction spots are called "satellite reflections."

  • scanning coil

    Electromagnetic coil(s) used to scan a target area with an electron beam.

  • scanning electron microscope

    An electron microscope in which a small electron probe is scanned over the surface of a bulk specimen, and secondary or backscattered electrons emitted from the surface are collected by a detector, and finally, the intensity of the detected signal is displayed on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe. An image formed by secondary electrons enables us to observe the fine structure or morphology of the surface of the bulk specimen, whereas an image formed by backscattered electrons provides the difference in the composition of the specimen. The "scanning electron microscope (SEM)" is used by adding various analytical functions (tools) such as EDS and WDS. An important factor that determines the SEM resolution is the size of the incident electron probe (beam) on the specimen. To decrease the probe size, the most essential point is to decrease the size of the electron (emission) source. The cold cathode FEG has the smallest source size, followed by the Schottky FEG, the LaB6 (tip) electron gun, and the tungsten (filament) electron gun. The second most essential point that determines the probe size is the selection of the objective lens. Three types of objective lenses are available; (1) out-lens type, (2) snorkel lens type, and (3) in-lens type. In the out-lens type, a specimen is placed below the objective lens allowing tilting a large specimen. In this lens, limitation on specimen size is not severe but the production of a small beam is difficult because the focal length of the lens becomes large. In the in-lens type, a specimen is inserted into the objective lens in the same manner as the TEM. In this lens, a small probe can be produced because the focal length can be made small. However, the specimen size is limited to several mm. The snorkel lens has a compromise design between the out-lens and in-lens. (The term, "snorkel," originates from the fact that the lens geometry resembles a snorkel used by divers.) A specimen is placed below (not far from) the objective lens. Thus, a relatively small probe can be produced and also a relatively large specimen can be treated. The SEM resolution is degraded at low accelerating voltages due to the chromatic aberration effect. Normally, the resolution is defined at 20 to 30 kV. An ultra-high resolution SEM achieves a resolution of about 1 nm, whereas a general-purpose SEM provides a resolution of about 10 nm. The use of a Cs corrector or a Cc corrector enables a further decrease of the incident-beam size, but causes a disadvantage of a short focal depth because the acceptance angle becomes larger. As the accelerating voltage of the incident electrons is decreased, the penetration depth of the electrons becomes smaller. This reduces a spread of secondary electrons generated by backscattered electrons in the specimen, thus enhancing the image contrast. Advantages of low accelerating voltage include the reduction of the background in the image, the decrease of charging, and the decrease of damage to the specimen. In SEM, charging degrades the image quality. If the current of the incident electrons exceeds the current of electrons flowing out of the specimen, charging takes place. The charging disturbs the image, sometime forming nothing like the true image. When a nonconductive specimen is used, to prevent charging, the specimen is coated with a conductive material, such as noble metal, Al and C. In a low-vacuum SEM, a nonconductive specimen may be observed without coating.

  • scanning low energy electron microscope

    A "SLEEM (scanning low energy electron microscope)" is an instrument to form a surface image, in which the SLEEM image is obtained as follows. An incident electron beam with an energy of several 10 V to several 100 V is used. A small-sized focused electron probe is scanned over the surface of the specimen. Scattered electrons and secondary electrons from the specimen are detected by a SEM detector. To obtain surface information, the specimen area is kept at ultra high vacuum. The SLEEM instrument is usually made by modifying a SEM instrument, thus the beam scan system and secondary electron detector for the SEM instrument are used for the SLEEM instrument. Applying a negative voltage to the specimen gives rise to the formation of a cathode lens or a retarding electric field near the specimen. By varying the value of the negative voltage, the energy of the incident electron beam is varied. A high image resolution is attained at a low incident energy. The present SLEEM achieves a resolution of 10 nm at an incident energy of 10 eV.

  • scanning transmission electron microscope (STEM) image

    "Scanning transmission electron microscope (STEM) image" is obtained as follows. A small-sized, focused electron probe is scanned over a thin specimen using the double-deflection system. The intensity of the transmitted wave (or the diffracted wave) exiting from a point on the specimen is detected with an annular detector. Then, the intensities are displayed on a computer monitor as a series of bright spots in synchronism with the scanning electron-probe. The resolution of the STEM image is determined by the probe diameter. The STEM method has two observation modes; bright-field mode and dark-field mode.

    STEM image,  scanning transmission electron microscope (STEM) image
    Fig. Bright-field (left) and annular dark-field (right) STEM images of Pt catalyst particles on a graphite support.
    In a bright-field (BF) image, Pt particles appear dark because the incident electrons are scattered at high-angles. In an annular dark-field (AF) image, Pt particles appear bright or show reversed contrast to the BF image because the scattered electrons at high angles are received by the ADF detector.

  • scattering angle

    "Scattering angle" means the angle of an electron scattered by an atom in a specimen. It decreases with the scattering angle. The scattering amplitude is larger for the atom with a larger atomic number.

  • scattering contrast

    Incident electrons are scattered by constituent atoms in a specimen. When scattered electrons are stopped by the objective aperture, those scattered electrons act as if there arises electron absorption in the specimen. This is expressed as scattering absorption. The scattering cross section of the electron becomes larger as the mass of the atom is larger, and then image contrast produced by differences in scattering amount is termed “scattering contrast.” Since the scattering cross section for electrons is large for the atom with a large mass, the scattering contrast is sometimes called “mass contrast.” The contrast of a TEM image taken from a non-crystalline specimen is explained by the scattering contrast. In the case of a crystalline specimen, elastically scattered waves behave as diffracted waves. Thus, this image contrast is interpreted by the behavior of the diffracted waves.

    散乱コントラスト:scattering contrast
    TEM image of kidney tubules of a mouse taken at an accelerating voltage of 120 kV.

    The specimen was chemically fixed using gultaraldehyde and osmium tetroxide and then, subjected to electron staining by uranium acetate and lead citrate. The parts containing heavy elements (osmium, uranium and lead) scatter more electrons at large angles (than the areas containing light elements), and such electrons are intercepted by the objective aperture. As a result, the parts are observed with low intensity (as dark).

  • scattering cross section

    "Scattering cross section" is a probability of scattering of an electron by an atom, expressed by unit of area. It is large at low scattering angles and becomes small at high scattering angles because the atom has a finite size.

  • scintillator (fluorescent substance)

    The scintillator is a substance that emits fluorescence by absorbing the energy of an electron beam or electromagnetic wave. A fluorescent substance for the "scintillator" is selected by taking account of the fluorescence wavelength (color) and its persistence time according to the sensitivity characteristic of a photoelectric converter placed behind the scintillator. The scintillator is used for various purposes: a fluorescent screen (yellow-green light, persistence light: ~100 ms) to visualize a TEM image, a secondary-electron detector (blue light, persistence light: ~μs) to measure the beam intensity of secondary electrons, and a TEM image detector (YAG) (blue white light, persistence light: ~μs) placed before a CCD.

  • screw axis

    A symmetry element of the crystallographic space group. When the whole crystal is rotated by a certain angle(180°, 120°, 90°, 60°) with respect to an axis and successively translated along the axis by a length of 1/2, 1/3, 1/4 or 1/6 of the crystal unit-cell, the translated crystal can be equivalent to the original crystal. In this case, this axis is called "screw axis." The screw axes are denoted by 21,31,32,41,42,43,61,62,63,64,65.

  • scroll pump

    A dry vacuum pump that does not use oil. By rotating the scroll blade of the scroll pump, residual gasses are carried to the central part of the pump, are compressed and evacuated. Since oil is not used except a grease for the scroll shaft, the pump is oil-free and oil mist is not exhausted, which occurs for the rotary pump. The pump features low vibration and no noise. The working pressure is from 105 Pa to 1 Pa. Recently, requirements for an oil-free pump evacuation system are increasing even for the camera chamber. For such needs, a combination of the turbo-molecular pump and the scroll pump is adopted, instead of the diffusion pump.

  • secondary electron

    When incident electrons travel a specimen, these electrons lose their energy while repeating collision with constituent atoms in the specimen (inelastic scattering). In this process, outer-shell electrons of the constituent atoms are ejected, and then part of them overcome the binding energy and are emitted from the specimen surface. These emitted electrons are called "secondary electron(s)." Since the energy of secondary electrons is small (normally, several 10 eV), only those generated near the top surface of the specimen (depth: 10 nm or less) are emitted from the specimen. The secondary electron yield becomes larger as the incidence angle of the electron beam with the specimen is smaller (at a grazing incidence). The difference of the secondary electron yields in a secondary electron image reveals surface morphology of the specimen.

  • secondary-electron detector

    A detector that is used for detecting secondary electrons emitted from the specimen surface by electron-beam illumination. The main elements of the detector are a scintillator and a photomultiplier tube (PMT). To efficiently collect low-energy secondary electrons (normally, several 10 eV), a positive potential (voltage) of about 10 kV is applied to the scintillator against the specimen. Accelerated secondary electrons are converted into visible light by the scintillator, and the light is guided to the PMT through a light pipe. Then, the light signal is converted into an electric signal and the electric signal is amplified. A secondary-electron image is obtained by displaying the intensity of the detected secondary electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe.

  • sector analyzer

    One of the post-column type energy filters, which is installed behind the column of a TEM. Since the magnet of the analyzer is sector-shaped, this analyzer is called "sector analyzer." Its energy dispersion is 4 to 5 μm/eV for a 200 kV electron beam.

  • segregation

    A phenomenon where the impurities or constituent elements in a metal or an alloy become unevenly distributed. "Segregation" often occurs when coagulation of alloy occurs.

  • selected-area aperture (intermediate-lens aperture)

    An aperture for selecting a specimen area in the selected area diffraction (SAD) mode. The aperture is inserted into the image plane of the objective lens (or object plane of the intermediate lens). The aperture diameter ranges normally from 10 μm to 100 μm.

  • selected-area diffraction

    A method for qualitative analysis of crystal structures from a spot diffraction pattern, acquired by illumination of a parallel electron beam on a specimen. By inserting a selector (selected-area) aperture into the image plane of the objective lens, a diffraction pattern is obtained from a specimen area of a several 100 nm diameter. The method enables us to determine the lattice parameters, lattice type and crystallographic orientation of the selected area.

    制限視野回折:selected-area_diffraction
    Overview of the standard optical ray diagram of the imaging lens system which is composed of the objective lens (OL) and the four-stage imaging lens system (intermediate lenses (IL1, IL2, IL3) and projector lens (PL)).

    (a)Image observation mode, in which the magnified image of a specimen is observed on the screen by focusing the imaging lens to the image formed by the objective lens.
    In this mode, the selected-area aperture (SA) is inserted into the image plane of the objective lens so that an observation area (field of view) is selected.

    (b)Diffraction pattern observation mode, in which the diffraction pattern of a specimen is observed on the screen by focusing the imaging lens to the back focal plane of the objective lens. By switching from the imaging mode to the diffraction mode, the diffraction pattern formed only from the selected area in step (a) is obtained.


     

  • selection rule

    In EELS, when interband transitions due to Coulomb interactions are considered, if only small-angle scattering is taken into account (small-angle scattering approximation), interband transitions are limited only to the dipole transition (dipole approximation). That is, only the transitions with the change of an angular orbital momentum being ⊿l = ±1 is allowed. The rule which clarifies the allowed and forbidden transitions is called "selection rule." Thus, the transitions from the 1s shell to the unoccupied p states (2p, 3p, etc.) occur. In ELNES, partial density of unoccupied states, instead of total density of states, is obtained. In the low energy-loss region corresponding to valence-electron excitations, scattering at large angles is allowed and this can give rise to transitions in which the selection rule does not hold.

  • semiconductor detector (solid-state detector)

    An X-ray detector utilizing a semiconductor silicon (Si) or germanium (Ge). The detector is used for an EDS system. The Si-Li detector has long time been used, but in recent years, it is being replaced by the silicon drift detector (SDD).
    For the window of the detector, three types are available: beryllium window, ultra-thin window (UTW) and windowless.

  • serial detection

    In the acquisition of an energy-loss spectrum in EELS, the "serial detection" method uses a single channel detector (zero-dimensional detector) and measures the energy-loss spectrum in serial times along the energy axis. Recently, a detection method for EELS has changed to parallel detection.

  • shadowing

    Shadowing is a technique to obliquely deposit metals (platinum, etc.) of a high scattering power with a low angle to the specimen surface of a biological specimen composed of proteins, DNA, viruses, etc., under vacuum. By shadowing, the TEM image contrast due to roughness of the surface of those specimens is enhanced. Biological molecules, such as proteins, DNA and viruses, are composed of light elements of carbon, nitrogen, oxygen or hydrogen. When a specimen of these molecules is observed with a TEM, most incident electrons pass through the specimen without suffering scattering. As a result, an obtained image hardly shows contrast. To solve this problem, metals (platinum, etc.) are vacuum-deposited onto the molecule particles. This deposition makes it possible to enhance the contrast of the surface roughness of fine structures. As the deposition angle is lower, the finer structures can be observed. When the deposition angle is higher, the fine structures are more embedded in the deposited metal. The technique is called "shadowing" or "shadow casting" because the metal-deposited part in the TEM image appears as a shadow.

  • short-range order parameter

    In the case of a binary alloy, "short-range order parameter" means a probability of existence of atom B around atom A. It can be measured from the intensity of diffuse scattering that appears around a reflection specific to the ordered phase.

  • side-entry stage

    A specimen stage into which the specimen is inserted from the side of the polepiece of the objective lens. Compared with the top-entry stage, the "side-entry stage" has disadvantages of instability to vibrations and heat. However, in the case of this stage, a space above the specimen stage can be effectively used to add analytical functions (tools) and to achieve large tilt angles of the specimen. Thus, the side-entry stage is very useful in applications to materials science and tomography. Furthermore, the advantages of this stage include: a nano-sized beam is produced on the specimen with the use of the C-O lens, and a large take-off angle of an EDS detector is achieved. Therefore, the side-entry stage is suitable for analytical electron microscopy.

  • side-entry type EDS detector

    An EDS detector which is placed on the side of the objective lens with a viewing angle of 30° or less to a specimen set horizontally. Since the "side-entry type EDS detector" can be set near the objective lens, its detection solid-angle of emitted X-rays is large or its X-ray detection efficiency is high. To improve the accuracy of quantitative analysis, the diffusion length of the emitted X-rays in the specimen is required to decrease. For this purpose, the specimen is tilted as needed against the detector so that X-rays coming to the detector take a large angle against the specimen surface. Since the X-ray intensity is sensitive to the local roughness of the specimen, the accuracy of quantitative analysis is not high enough. However, this type detector is broadly used these days as it is suitable for the analysis of a smaller specimen area because it can be combined with a high resolution objective pole-piece which has a small gap and a small bore.

  • silicon drift detector

    One of the energy-dispersive X-ray detector used for EDS. The principle, in which characteristic X-rays entering the detector element are converted to electron-hole pairs, is the same as for the Si(Li) detector. But unlike the Si(Li)detector, electrons generated from the detector element by the incident characteristic X-rays are efficiently guided to a small anode at the center of the element by a concentric electrode structure with a potential gradient. This unique electrode structure reduces capacitance, thus enabling high-speed signal response. Thus compared with the Si(Li) detector, voltage pulses are collected with higher-speed, higher-signal-to-noise ratio. As a result, SDD may not suffer the influence from dark current due to thermal noise, and works at approximately –15 °C by Peltier cooling. SDD provides an energy resolution comparable to the Si(Li) detector and also performs X-ray analysis with high count rate (>1 x 105 cps), which is more than one order higher than the Si(Li) detector. Since it is unnecessary to use liquid nitrogen, the detector can be compactly designed. Thus SDD has more been used in place of the Si(Li) detector.

  • silicon-intensifier-target (camera) tube

    A (camera) tube for light detection using a silicon-based electron multiplication target. The "silicon-intensifier-target (camera) tube" has functions of electron-multiplication and charge-accumulation, and then operates even with a low intensity of light. To use the tube for electron microscopes, a fluorescent screen is placed at the front of the tube for converting incident electrons to light. The photoelectrons generated on the reflexible surface are accelerated to a several kV and imaged on the silicon-based target. Then, many electron-hole pairs are created, leading to a high amplification. Readout is done by sweeping an electron beam from behind the target. The advantages of the tube are high sensitivity and wide dynamic range. A disadvantage is that the surrounding part of the tube becomes dark through the lens action. Recently, the tube has been replaced with a compact and inexpensive CCD, which has unfortunately low sensitivity and narrow dynamic range but has a low noise level and does not have dark surrounding parts. Thus, the tube is not used currently for a TEM.

  • single crystal

    The single crystal is a crystal in which the constituent unit cells are aligned in the same orientation. A large single crystal is obtained under a specific growth condition.

  • single particle analysis

    An electron microscopy method to obtain the three-dimensional (3D) structure of biological macromolecules, such as proteins and nucleic acids, by analyzing electron microscope images of the macromolecules (proteins, etc.) recorded as dispersed particles (single particles). A solution containing a protein is made into a thin film on a holey carbon film attached on an EM grid and is then rapidly frozen so that the particles are embedded in an amorphous ice film. TEM images are taken at Liq.N2 temperature, and the 3D structure is reconstructed by image processing and analysis.
    A number of projection images of the 3D structures of protein particles in various orientations are recorded in the acquired Cryo-TEM images. Each particle image is rotated and translated in the image plane to collect the images having the same external shape and the same density distribution to sort them into groups of different 3D orientations. Then, the images in the same groups are added and averaged to improve the signal-to-noise ratio of the images. Next, the Euler angles of the projection orientations (angles of the particles against the image plane in the present case) are estimated. Finally, the 3D structure of the protein is reconstructed by back projection of these particle images along the estimated Euler angles.
    Single particle analysis has been dramatically improved in its achievable resolution and practical application by the advent of a CMOS direct electron detector, together with innovations in specimen preparation techniques and analysis software programs. The method is applicable to an extremely small amount of sample solution (protein concentration: several mg/mL, solution volume: several µL/grid), and is especially effective for structural analysis of proteins that are difficult to crystallize. As of 2018, the highest resolution achieved with the method is 0.16 nm.

    Specimen preparation and observation method

    A highly purified protein solution is prepared in which protein particles with a homogeneous structure are dispersed. The protein particles are ice-embedded by rapid freezing (at –180 °C or less) so as to preserve their structures. It is important that the ice film embedding the protein particles must be prepared as thin as possible to produce high contrast TEM images because proteins are composed of light elements (C, H, O, N, S, etc.) and produce low contrast against the ice film by a small difference between their densities.
    The specimen grid is inserted into the microscope column with its frozen state kept. TEM images of the specimen are recorded at Liq.N2 temperature. To suppress the damage to the specimen due to electron-beam irradiation, the electron dose onto the specimen must be set low. The specimen is observed using phase contrast because protein particles consisting of light elements are difficult to observe using scattering contrast. For observation of protein particles, a large amount of defocus of the objective lens (–0.5 to several µm) is used to enhance the image contrast, particularly in a low spatial frequency range to allow the detection and orientation determination of the particles.

    Image acquisition

    Images of protein particles recorded at a low electron dose have a very low-signal-to-noise ratio (S/N). To obtain the particle images with a high S/N, hundreds to thousands of Cryo-TEM images are collected, from which several tens of thousands to several hundreds of thousands of particle images are extracted and added and averaged after image classification and alignment. In recent years, several thousands of Cryo-TEM images are automatically acquired over several days using an automatic image acquisition system.
    It is emphasized that the electron-beam-induced drift of ice-embedded protein particles is the main cause of degrading the resolution of the TEM image in the single particle analysis method. Extremely high frame rate of direct electron detectors (scintillator not used) that allow single electron counting and movie-mode image recording has improved the resolution of the TEM image. Recording a TEM image in the movie mode at a frame rate of 5 to 10 frames/sec allows the particle image drift to be greatly reduced by post image processing, namely motion correction of the particle images between the frames enables acquisition of high-resolution protein particle images.

    Image analysis

    From several thousands of the Cryo-TEM images, several tens of thousands to several hundreds of thousands of the protein particle images are automatically extracted using a particle detection software program. Since the particle images are projected in various orientations, the extracted images are rotated and translated so as to sort them out into many classes of the projection images in the same orientations. Then, the images sorted in each class are aligned in their positions and orientations and are averaged. As a result, 100 to 200 protein particle images with a high S/N are obtained. In this step, the 3D orientations of the particle images (Euler angles or the angles of the particles against the image plane in the present case) are not yet determined.
    To obtain the Euler angles, first an initial particle model with arbitrary orientations is given. Secondary, the projection images of the model are produced. The Euler angle of each particle image is estimated by a comparison of the projection images and the TEM image. The particle TEM images with the estimated Euler angles are subjected to back projection to produce an initial 3D structure of the particle. Then, this 3D structure is subjected to projection again to create the projection images. The re-projected images and the particle TEM images are compared to estimate the Euler angles again. These particle images are back-projected to obtain a better 3D structure in the next step. These steps are repeated until the obtained 3D structure is converged. In each step of calculations, the results are evaluated by statistical approaches. Finally, the most probable 3D structure is obtained.

    Comparison with other analytical methods

    Structural analysis methods using electron microscopy include Electron Crystallography, Electron Tomography and Single Particle Analysis. In Electron Crystallography, a 2D or thin 3D crystal is prepared and analyzed using techniques similar to those of X-ray Crystallography. Electron Crystallography provides a resolution better than 0.2 nm. In Electron Tomography, one specimen is placed in the specimen holder in the microscope column, and is subjected to serial tilt-series imaging at various angles. Then, back projection is applied to the acquired projection images for obtaining the 3D structure. In the method, it is not possible to improve the signal-to-noise ratio and the resolution of the images by averaging. For biological applications, the method is used for analysis of functional structures at the cell level, its resolution being on the order of several nm to 10 nm. But when the tomogram contains many particles of the same proteins, the use of the sub-tomogram averaging method improves the resolution close to 0.3 nm.
    Conventionally-used structural analysis methods of proteins are X-ray Crystallography and Nuclear Magnetic Resonance (NMR) spectroscopy. The former method requires a crystal with a size of 10 µm or more. Thus, the method cannot be applied to proteins that are unable to crystallize. The latter method can be used only for proteins with a molecular weight of ~50,000 or less. However, Single Particle Analysis does not have such restrictions and its resolution has been becoming as high as that of X-ray Crystallography, although at present it is still difficult to apply this method to proteins with molecular weight less than 100,000 because of low S/N of their TEM images. In the future, the advancement and the more use of Single Particle Analysis are highly expected.

    (Proofread by Specially Appointed Professor Keichi Namba and Specially Appointed Associate Professor Takayuki Kato, Osaka University)

    single particle analysis
    Data courtesy: Specially Appointed Professor Keichi Namba and Specially Appointed Associate Professor Takayuki Kato, Osaka University

  • single scattering

    When the incident electrons that pass through a specimen are scattered only one time until these electrons exit from the specimen, this scattering phenomenon is termed "single scattering."

  • single-tilt cooling holder

    A specimen holder used to cool the specimen with liquid nitrogen or liquid helium. Tilt of the specimen is for only one axis (X axis). Some liquid-helium cooling holder can be cooled down to 5 K. Although the holder cannot adjust specimen orientation perfectly, it can be easily operated and is inexpensive than a double-tilt cooling holder.

  • single-tilt heating holder

    A specimen holder used to heat the specimen. The maximum temperature that can be reached is ~800 ℃. Tilt of the specimen is for only one axis (X axis) and the achievable tilt angle is ~±20°. Some holder can reach ~1000 ℃. Although the holder cannot adjust specimen orientation perfectly, it can be easily operated and is inexpensive than a double-tilt heating holder.

  • sintering

    "Sintering" is to heat a powder specimen until each powder in the specimen adheres to each other without melting.

  • site occupation (occupancy)

    "Site occupation (occupancy)" means that atoms occupy specific sites. This term is particularly used in ALCHEMI to determine what impurity atoms or doped atoms occupy what sites.

  • six-fold astigmatism

    "Six-fold astigmatism" is one of the fifth-order parasitic aberrations having six-fold symmetry. ("Two-fold astigmatism," which is conventionally called "astigmatism" has two-fold symmetry.) In the case of a current Cs corrector of the two-stage three-fold-field type, a six-fold astigmatism is generated as a combination of the aberrations of the three-fold fields in the two stages. The six-fold astigmatism is the largest aberration when the third-order spherical aberration, parasitic aberrations up to the fourth order and the fifth-order spherical aberration are successfully corrected. Thus, the aberration-corrected range is restricted by the six-fold astigmatism. A Ronchigram clearly shows a hexagonal shape pattern at its peripheral part, when the Cs corrector is incorporated in the illumination system. Two aberration correctors that can correct the six-fold astigmatism have been developed. One is an aberration corrector of a three-stage three-fold-field type composed of dodecapoles. This corrects the six-fold astigmatism by making the vector sum of the astigmatism produced in each three-fold field to be zero. Another is an aberration corrector of a two-stage three-fold-field type composed of hexapoles. This corrects the six-fold astigmatism using that of the opposite sign produced by the combination of the effect of the transfer lens and the three-fold field through optimizing the length of the hexapoles.

  • slow-scan CCD camera

    A CCD camera, whose scan speed is slower than that of a normal CCD TV camera to acquire a higher-quality image with a higher signal-to-noise ratio. A normal TV scan rate is 1/30 s per frame but the scan rate of the "slow-scan CCD camera" is 1 to 2 s per frame in the photography mode and ~0.1 s per frame in the observation mode.

  • smoothing

    "Smoothing" is to perform an averaging of data and to obtain a smoothly connected data set (smooth curve) by eliminating singular points and noises from a series of data with the help of the data at the neighboring points. For a simple example, a data at a certain point is replaced by the average between the data at the point and the data of the nearest neighbor points. Smoothing is frequently used for image processing and processing of spectroscopic data.

  • solid angle

    "Solid angle" for detection of EDS signals is a three-dimensional angle formed by the cross-sectional detection area (sphere) of the detector with respect to the incidence point of the electron beam on the specimen from which characteristic X-rays are emitted. The solid angle can be made larger for a larger detection area and a smaller distance between the specimen and the detector.

  • solid solution

    When two or more kinds of substances are mixed and have a uniform structure (particularly in the case of ion crystals and semiconductors), the mixed crystal is called a solid solution. Different atoms are randomly arranged on equivalent lattice sites in the solid solution. Below a certain temperature, the different atoms can take an ordered arrangement.

  • space lattice

    "Space lattice" means a periodic array of a unit lattice (a parallelepiped) in a space. The space lattice is classified into 7 crystal systems according to lattice parameters (a, b, c, α, β, γ). Unit lattices of several crystal systems can locate within them. According to the location of the lattice points, face-centered lattice, body-centered lattice and base-centered lattice are created. As a result, it amounts to 14 type lattices, which are termed Bravais lattices. Lattices other than primitive lattice cause forbidden reflections to occur.

  • spatial frequency

    When considering an electron microscope image as a superposition of electron waves, spatial frequency is expressed as the reciprocal of wavelengths of these waves that are components of the image. The intensity distribution with respect to the spatial frequency is called "power spectrum."

  • specimen drift

    "Specimen drift" means the amount of specimen movements that arise from thermal or mechanical instability of the goiometer, specimen holder, etc. Specimen drift is an important factor that degrades high-resolution image and micro (nano) area analysis. Normally, the drift is suppressed to ~1 nm/min.

  • specimen environment

    An environment when a specimen is observed or analyzed in TEM. Various specimen environments are used, which include a high-temperature environment, a low-temperature environment, an atmospheric pressure environment, and an environment with tensile force, electric fields or magnetic fields.

  • specimen preparation

    "Specimen preparation" is to prepare a suitable specimen in observation and analysis of TEM.

  • specimen rotating holder

    A specimen holder which can rotate the specimen with respect to the direction of the incident electron beam. The range of rotation angles is ±180° and the range of tilt angle is about ±25°.

  • spherical aberration

    When an electron beam exiting obliquely from the point at the optical axis on the object plane passes through the objective lens, it does not come to the optical axis on the ideal Gaussian image plane, but intersects the optical axis at a point slightly shifted from the image plane (in the TEM, slightly before the image plane (lens side)). As a result, a circular blurred image is produced on the image plane. This defect is called "spherical aberration." The amount of blur is given by Csα3 on the object plane, where Cs is a spherical aberration coefficient and α is an angle between the electron beam and the optical axis. The spherical aberration is the most important aberration of the objective lens among the aberrations. For a rotationally symmetric lens with respect to the optical axis, the value of Cs is always positive. In TEM, the third-order spherical aberration has nowadays been successfully corrected. Cs correctors using combination of hexapoles and transfer lenses have become main stream.

  • spin orbit coupling

    The magnitude of "spin orbit coupling" is different depending on whether the electron spin is parallel or anti-parallel to the orbital shell spin l, thus the two states possess different energies. In 2p electrons, L2 (total angular momentum j = l - s = 1/2) and L3 (total angular momentum j = l + s = 3/2) are formed. In an EELS spectrum of transition from the 2p occupied state to the 3d unoccupied state (band), L3 and L2 peaks sequentially appear in the energy-loss order of low to high. It should be noted that those peaks provide information on the energy splitting of the occupied state (several eV to 20 eV) but does not provide information on the unoccupied state expected by EELS. However, the intensity ratio of L3 to L2 varies from 2:1 due to the effect of the expected chemical-bonding state in the unoccupied state. When the experimental intensity profile is compared with theoretical calculations, information on the valence of 3d electrons is obtained, where the calculations must take account of the effect of core-hole interaction, correlation between 3d electrons and valences. The intensity ratio of L3 to L2 tends to be large at the high spin state whereas small at the low spin state.

  • spin polarized electron

    An electron exhibits "up" state or "down" state corresponding to the quantum numbers ±1/2. Recently, a new electron gun technology using a photocathode made of a strained super lattice semiconductor enables the emission of electrons with a spin polarization of more than 90 % at room temperature. Originally, the electron source was developed for collision experiments in the field of elementary particle physics. However, due to the increase of its brightness, the source has recently been applied to photoelectron surface microscopes and transmission electron microscopes (TEM). Then, surface images and TEM images have been obtained. A TEM equipped with the gun allows us to observe the interaction between the incident electron spin and the spins and magnetic moments of the atoms in a specimen.

  • sputter ion pump

    A dry vacuum pump that does not use oil. In the sputter ion pump, residual gasses are ionized utilizing electric and magnetic fields. The produced ions strike a titanium cathode to sputter titanium atoms. Then, the sputtered titanium atoms form a fresh getter film (film subjected to chemical adsorption). As a result, active gasses (hydrogen, oxygen, carbon monoxide, etc.) are adsorbed to the getter film, and inert gasses (helium, etc.) are ionized and adsorbed to the cathode. The pump is used for evacuation of the electron gun chamber and the column of TEM, which require high vacuum. The working pressure is 10-4 to 10-9 Pa.

  • stacking fault

    A kind of planar lattice defect (planar fault). When considering a perfect crystal to be produced by periodic stacking of (different kinds of) atomic planes, "stacking fault" means a fault where the order is violated.

  • standardless correction

  • standardless quantitative analysis

    A system by which quantitative analysis of target elements in EDS can be conducted without using the standard specimen in each analysis. In a recent TEM, many k-factors of the Cliff-Lorimer method, which have been measured separately, are stored and tabulated in the memory of the computer on the TEM. Quantitative analysis of the elements can be performed by referring the table without measuring the standard specimen. The system or method is called "standardless quantitative analysis." In TEM, when a specimen thickness is 10 nm or less, the Cliff-Lorimer method (thin film approximation) is generally used because the absorption effect and the fluorescence excitation effect can be neglected. For example, when a specimen is composed of two elements A and B, the mass concentration ratio of element A to element B is well assumed to be proportional to the measured characteristic X-ray intensity ratio of element A to element B (proportionality factor: k). The k factor, which depends on ionization cross section, fluorescent yield, X-ray absorption by material of detector window, etc, is actually obtained from characteristic X-ray measurements of a standard specimen. It is noted that the k factor is not a constant when a specimen is thick.

  • standing wave

    "Standing wave" means that the ratio between the amplitude of a wave at a certain point and that at the other point is constant regardless of time. When an electron wave exactly satisfies a Bragg reflection in a crystal, the incident wave and the diffracted wave form a standing wave.

  • star aberration

    The star aberration, one of the third-order axial geometrical aberrations, is a parasitic aberration exhibiting two-fold symmetry. (Note that it is called a fourth-order aberration in terms of the wave aberration.) The star aberration cannot usually be detected due to the existence of a large spherical aberration of the objective lens, but is detected after correcting the spherical aberration of the objective lens. The star aberration appears as a pattern with a two-fold symmetry in high angle areas of a Ronchigram. In the case of a TEM equipped with a Cs corrector, prior to microscope observation, aberrations are measured and then the aberrations including the star aberration are corrected by the automatic aberration correction function with the use of the deflectors of the Cs corrector.

  • starting pressure

    When the evacuation pump can start without damage and also can perform a normal evacuation action at a certain pressure, the pressure at the start of the pump is called the "starting pressure."

  • stereo microscopy

    A method that obtains stereoscopic images of a specimen. In "stereo microscopy," two electron micrographs are taken from two specimen orientations which differ in a tilt angle by 5° to 10°, and they are viewed independently with right and left eyes. When a complicated dislocation configuration is viewed, diffraction conditions for the two micrographs should be set very similar to reveal the correct three-dimensional structure.

  • stigmatic focus

    "Stigmatic focus" means image formation without off-axial astigmatism. For example, in the original Wien filter, focusing action occurs only in the electric field direction but does not occur in the magnetic field direction. Thus, even when an incident beam to the filter is circular, a liner beam is obtained on the image plane. To accomplish astigmatism-free imaging in the Wien filter, application of curved electric and magnetic fields produces magnetic components in the original electric field direction and thus attains the focusing action in the original magnetic field direction.

  • stigmator

  • stigmator

    An instrument inserted below the objective lens, which is used to correct the axial astigmatism. This astigmatism can be corrected by a set of four-pole electromagnetic coils (quadrupole), but a real "stigmator" uses two pairs of quadrupoles for operation convenience.

  • stopping power

    "Stopping power" means the average amount of energy loss per unit distance when a charged particle passes through a substance. In the case of TEM, the energy loss occurs mainly due to ionization and excitation of electrons. The "stopping power" is used for calculation of the temperature increase of specimen and the degree of specimen damage.

  • stray magnetic field

    "Stray magnetic field(s)" are adverse magnetic fields that exist around the instrument. The stray magnetic fields are classified into static magnetic fields and alternating magnetic fields. The former includes non-periodic magnetic fields generated by cars and trains, and geomagnetism. The latter includes periodic magnetic fields generated by power supplies, fluorescent lamps and external devices.

  • sublimation pump

    In the sublimation pump, titanium is heated and sublimated under a high vacuum. A fresh titanium-deposited film is formed on the inner wall of the pump. The getter action (chemical adsorption of gas molecules) of the fresh film surface is efficient for pumping in the high-vacuum region. The working pressure is 10-5 Pa or better. Since the pump needs heating, its long-time use is inadequate to maintain a high vacuum. To overcome this problem, the sublimation pump is combined with the ion pump, where the ion pump is constantly operated and the sublimation pump is operated when needed. The pump is not used so much to date.

  • substitution

    "Substitution" means that an atom or a group of atoms in a crystal or a molecule is substituted with other atom or other group of atoms.

  • substitutional atom

    When the other kind of atom (solute atom) occupies an atom position instead of an original atom, the solute atom is termed a "substitutional atom."

  • sum peak

    In EDS analysis, characteristic X-rays emitted from a specimen are detected with a semiconductor detector. Pulse voltages that are proportional to the energies of the detected characteristic X-rays are generated, and then these voltages are measured with a multi-channel pulse-height analyzer. When two different characteristic X-rays almost simultaneously enter the detector, these X-rays cannot be recognized as separate pulses. Thus, an EDS spectrum exhibits a spectral peak at an energy position of the sum of energies of the two characteristic X-rays, together with spectral peaks due to characteristic X-rays of the specimen. The peak is called a "sum peak" and therefore, care must be taken for spectral analysis.

  • superlattice

    "Superlattice" is formed, for example, when a structural modulation occurs on a crystal structure to have a longer period (normally an integral multiple of the original period) than that of the original crystal structure. Such a modulation can be formed in a low-temperature phase due to phase transformation. An artificial superlattice is produced as a long-period multi-layer structure which is composed of more than two kinds of layer crystals deposited by molecular beam epitaxy (MBE). The superlattice structures are used for quantum-well laser diodes, high-temperature superconductive materials, and magnetic materials.

  • supporting film

    A "supporting film" is a thin carbon, polyvinyl formal, or collodion film with a thickness of several 10 nm or less. It is pasted on a grid to support a specimen for a TEM on the specimen holder.

  • surface plasmon

    When a metal surface or an interface between a metal and a dielectric material (with vacuum) is irradiated by a charged-particle or a light wave, electric charges are induced at the surface or the interface for generating an electric field. With this electric field as a driving force, the charges on the surface or the interface induce longitudinal oscillations. This phenomenon is called "surface plasmon".

    [Creation of surface plasmon]

    The electric field generated by the surface charges possesses the components parallel and vertical to the surface (Fig. 1(a)). The electric-field component parallel to the surface (Ex) acts as a driving force to induce longitudinal oscillations. On the other hand, the change in time of the electric-field component vertical to the surface (Ez) induces electromagnetic waves along the surface. The longitudinal wave that matches the periodicity of these electromagnetic waves is enhanced and propagates. As a result, a surface plasmon is created. The surface plasmon is abruptly attenuated as the depth from the surface is large. Unlike the volume plasmon, the surface plasmon is observed with not only an electron wave but also a light wave. This is due to the fact that, the electric field vertical to the surface corresponds to the transverse wave component against the surface plasmon, and the external electromagnetic waves interact with the transverse wave component.

    [Equations of surface plasmon]

    For the interface between a metal and a dielectric material, the electric-field components vertical to the metal side and the dielectric material side are expressed as Ez=0- 、Ez=0+, respectively. Since the electric flux densities of the metal side and the dielectric material side are equal, εMEz=0-=εdEz=0+ is given. Here, εM and εd are the dielectric constant for metal and that for dielectric material, respectively. As shown in Fig, 1(b), -Ez=0-=Ez=0+, and thus

    ε n + ε d =  ・・・(1)

    is obtained. Then, the dielectric constant for metal εM is replaced by the dielectric function εM (ω)=1-(ω_P/ω)2 based on the free electron model. Here, ω p = 4 π e 2 N / m is the angular frequency of the surface plasmon. When the equation (1) is solved for ω, the surface plasmon energy ES is given by the following equation.

    E s = ω = ω p 1 + ε d = E p 1 + ε  ・・・(2)

    Here, EPωP corresponds to the volume plasmon energy. The surface plasmon energy of the interface between a metal and a vacuum is E s = E p / 2 because the dielectric constant of the vacuum is εd=1.

    surface plasmon
    Fig. 1 (a) Schematic of surface plasmon for metal. Due to an external charged-particle or a light wave, the positive (+) and the negative (–) charges are induced. With an electric field generated by these charges (Ex (z=0±) in Fig. 1) as a driving force, a longitudinal oscillation wave of free electrons is created. (b) Illustration of the electric-field component vertical to the interface between a metal and a dielectric material (with vacuum) Ez=0± and the electric-field component parallel to this interface Ex (z=±0). Both of the continuity for the vertical component of the electric flux density εMEz=0-εdEz=0+ and the continuity of the parallel component of the electric field are satisfied.

    [Experimentally-obtained surface plasmon]

    Fig. 2 shows a spectrum of aluminum (Al) acquired by electron-energy loss spectroscopy (EELS). In general, the surface of Al is coated by an oxide film. A spectral peak at 15 eV is due to the volume plasmon. A peak at 7 eV created by the surface plasmon is also seen. Since the dielectric constant of an Al oxide film (Al2O3) is εd=3.7, the surface plasmon energy is calculated to be ES=6.9 eV, which shows a good agreement with an experimental value.

    surface plasmon
    Fig. 2 EELS spectrum of Al. A peak at 15 eV is due to the volume plasmon. A peak at 7 eV is created by the surface plasmon which originates from the interface between Al and an Al oxide.

    In metals containing d electrons (Ag, Au, etc.), the interband transitions of d electrons takes place at an energy position which is very close to that of the expected surface plasmon energy, and the surface plasmon strongly interacts with the interband transition. Thus, the aforementioned equation (2) is not applicable and the surface plasmon is seen at a lower energy side compared to the value calculated in the equation (2). In semiconductors like silicon (Si) and GaAs, the surface plasmon is observed at the energy position that is the same as the energy calculated in the equation (2). However, the surface plasmon loses its energy due to the interband transition, leading to the oscillation attenuation and a resultant broad spectral peak. In insulators (diamond, ion crystal, etc.) where valence electrons are strongly bounded, the spectral peak due to the interband transition dominates strongly in an EELS spectrum and thus, the surface plamson appears as a weak background for making its observation difficult.

    It should be noted that, a surface plasmon is excited at the surface of a very small metallic particle or a metallic wire with its size being the nanometer-order. In this case, a compressional wave of the surface charge is induced as a characteristic oscillation mode (standing wave) which depends on the shape of the very small particle or the wire. Since this wave is confined in the very small particle, this phenomenon is called "localized plasmon".

    (By Associate Professor Yohei Sato, Tohoku University)

  • systematic reflection

    A series of reflections g, 2g, 3g... aligned in a certain direction (for example, 100, 200, 300...) in an electron diffraction pattern. Reflections other than "systematic reflection(s)" are called "accidental reflection(s)."

  • take-off angle

    In the case of EDS analysis, "take-off angle" means that the angle at which characteristic X-rays emitted from the specimen are received with a detector placed above the specimen. This angle is defined by the angle of the line connecting the specimen center and the center of the detector against the normal plane to the optical axis. By setting this angle larger, signals cut by the specimen and specimen holder are reduced and the diffusion distance of the emitted X-rays in the specimen can be made shorter, thus the accuracy of quantitative analysis is improved. Previously, the take-off angle was set at 60°to 70°to reduce the above effects, which is called "High-Angle EDS" or "Top Take-off method." Recently, the take-off angle is however set at a low angle of ~20°by attaching the detector at the side of the objective polepiece because the detector is requested to place as near as possible to the specimen to increase the detection efficiency of the emitted X-rays or to increase the solid angle of the detector against the specimen, and the bore of the polepiece is requested to make small to obtain a high spatial resolution. This way of signal acquisition is called "Side Take-off method."

  • thermal (thermally assisted) field-emission electron gun

    The thermal (thermally assisted) field-emission electron gun (TFEG) emits electrons from a tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is heated at ~1600 K in a strong electric field. Compared with the cold cathode type, its emission current is very stable for a long time because the emitter does not adsorb residual gases due to constant heating. Thus the electron gun is more advantageous for micro-area or nano-area analysis than the cold cathode type. The energy spread of the emitted electrons from the TFEG is ~0.7 eV. Its brightness is as high as <8×108 A/cm2.sr at 200 kV. The size of the virtual source produced is >10 nm. This type of gun is not available commercially but has been replaced by a Schottky type gun.

  • thermal diffuse scattering

    "Thermal diffuse scattering" is inelastic scattering where the incident electrons on the specimen excite thermal vibrations (lattice vibrations) of atoms in the specimen and scattered in a diffuse manner. Since this scattering loses only a small energy (0.1 eV or less), this is often called quasi-elastic scattering.

  • thermal noise

    Noise that occurs in a conductor or a semiconductor material due to random thermal motions of electrons in this material. As the temperature is raised, "thermal noise" is increased. Since this noise does not have frequency dependence but shows a flat spectrum, it is also called "white noise." To suppress this thermal noise, a semiconductor detector in an EDS or a YAG single-crystal scintillator in a slow-scan CCD camera is cooled.

  • thermionic-emission electron gun

    An electron gun, which emits thermoelectrons from the tip of the cathode by heating a tungsten filament or a lanthanum hexaboride (LaB6) tip.

  • thermoelectron

    Electron(s) emitted by heating a substance. For example, electrons are emitted from a heated tungsten filament or an LaB6 tip.

  • thin-film approximation method

  • thinning

    "Thinning" is to fabricate a specimen to a thin film for a TEM.

  • three-fold astigmatism

    The three-fold astigmatism, one of the second-order axial geometrical aberrations, is a parasitic aberration exhibiting three-fold symmetry. (Note that it is called a third-order aberration in terms of the wave aberration.) The aberration is corrected using magnetic fields produced by hexapoles.
    A widely used two-stage hexapole Cs corrector corrects the spherical aberration of the objective lens using a combination effect in the three-fold astigmatism field. In the Cs correction system, the first hexapole simultaneously produces a large three-fold astigmatism and a negative spherical aberration. Then, the second hexapole placed below the first hexapole produces a three-fold astigmatism having an opposite sense to the former and a negative spherical aberration. The three-fold astigmatism produced in the first stage is canceled by that with an opposite sense produced in the second stage. The positive spherical aberration of the objective lens is corrected by the negative spherical aberration produced in the two stages.

  • three-lobe aberration

    The three-lobe aberration, one of the fourth-order axial geometrical aberrations, is a parasitic aberration exhibiting three-fold symmetry. (Note that it is called a fifth-order aberration in terms of the wave aberration.) In addition to the three-lobe aberration, the fourth-order axial geometrical aberrations include the fourth-order coma aberration and the five-fold astigmatism. In the aberrations exhibiting three-fold symmetry, there is the three-fold astigmatism which is the second-order axial geometrical aberration and a low order aberration compared to the three-lobe aberration. In a two-stage hexapole Cs corrector, even when the three-fold astigmatism is corrected, a Ronchigram sometimes exhibits a three-fold symmetry pattern at its high angle areas. This pattern can be due to the residual three-lobe aberration.

  • three-window method

    A method used for quantitative element (elemental) mapping by EELS. The process of the "three-window method" is as follows. Two background intensities before the inner-shell excitation of a certain element are acquired. The background intensity at the inner-shell excitation of the element is obtained by extrapolation using the two background intensities. Then, the obtained background intensity at the inner-shell excitation is subtracted from the spectral intensity of the inner-shell excitation. This method enables us to perform quantitative element mapping.

  • threshold value

    The minimum physical quantity (normally the minimum energy) that is required to cause reactions or other phenomenon.

  • through-focus method

    The "through-focus method" is an imaging method used in observation of lattice images or crystal structure images. It acquires the images through gradual change of the focus so that an optimum image can be obtained.

  • tomography

    A reconstruction method of three-dimensional internal structures through computer image processing of many projection images, which are acquired from sequential tilt-series images of a specimen. "Tomography" utilizes the principle of X-ray CT (computerized tomography) and MRI (magnetic resonance imaging) for a TEM image, which are broadly used in the medical field. When the analytical polepiece is used, 121 images sequentially acquired at angles from ±60° to +60° in steps of 1° are used for three-dimensional reconstruction. Various techniques for positional adjustment of each image have been devised by several TEM manufacturers. To avoid artifacts due to the missing cone, a specimen holder that allows image acquisition at tilt angles from -80° to +80°, and a specimen holder that enables image acquisition from all directions, has been developed. Furthermore, a cold stage that can cool biological, high polymer and organic substances with liquid helium is available. In tomography using STEM, focal shifts do not occur with the specimen position (occur for TEM), and also the use of the HAADF method enables us to remove diffraction contrast in a crystalline specimen. However, the disadvantages of STEM tomography are a long image-acquisition time, and unavoidable radiation damage and contamination.

    Sperm

    Sperm
    Fig. 1 TEM images of sperms.
    Left) A head, a flagellum and mitochondria of a sperm are indicated by yellow lines.
    Right) The mid-piece of a sperm where mitochondria stood in a line (inside a yellow frame). Tilt-series images were taken from this mid-piece, and then, 3D reconstruction was performed.
    Instrument: JEM-1000EES (at Research Center for Ultra-High Voltage Electron Microscopy, Osaka University) Accelerating voltage: 1000 kV

    Orthogonal views of 3D reconstruction image

    Orthogonal views of 3D reconstruction image
    Fig. 2 3D-reconstructed cross-section image of the mid-piece of a flagellum. Mitochondria are seen to stand on the flagellum.

    3D image (Volume Rendering) by Segmentation

    3D image (Volume Rendering) by Segmentation
    Fig. 3 3D image of the mid-piece of the flagellum, where each structural part was color-segmented (Left: Vertical cross section. Right: Lateral cross section).

    MOVIE

    Different colors were given for each mitochondrion. Each mitochondrion is seen to be well separated.

    ◆Click the "replay" button in the box above, and the movie will start (for 18 seconds)◆

  • top surface

    In a solid surface, the "top surface" is defined as an extremely thin surface (~0.5 to several nm from the surface). This term is used in the following manner. (1) The top surface of a specimen is fabricated. (2) Auger electrons are emitted from the top surface.

  • top-entry stage

    A specimen stage into which the specimen is inserted from above the polepiece of the objective lens. The "top-entry stage" has a construction where the specimen holder is rotationally symmetric with respect to the optical axis because it is inserted in the objective lens from the top. Since this stage is stable to vibrations and heating, it is advantageous for high-resolution image observation in an ultra-high voltage electron microscope, etc. However, due to the construction of the top-entry stage, large tilt angles of a specimen cannot be achieved, and analytical functions (tools) are difficult to be added. Due to these disadvantages, the side-entry stage is used for an analytical electron microscope.

  • top-entry type EDS detector

    An EDS detector which is placed above the objective lens with a viewing angle of about 70° to a specimen set horizontally. Since X-rays coming to the detector take a large angle against the specimen surface, the diffusion length of the emitted X-rays in the specimen is kept small without any specimen tilt, leading to a high accuracy of quantitative analysis. However, since the detector is distant from the specimen, the solid angle of the detector against the specimen is small or its detection efficiency is not high. Since the detector takes X-ray signals from the top of the objective lens, it requires a large bore polepiece for the lens. The use of such a polepiece is unfavorable to obtain a high resolution image and a small probe size on the specimen. Due to these disadvantages, this type detector has not been used recently.

  • topography

    A graphic method that shows three-dimensional configurational data. A backscattered electron image providing a topographic map of a specimen surface, which is obtained with the use of a two-segment detector is called a TOPO image. The term, "topograph," is used as X-ray topograph. Since a lens does not exist for X-rays in a usual sense, the two-dimensional distributions of lattice defects, lattice distortions, impurities, domains, etc., are obtained by taking one-to-one correspondence between the specimen position and diffraction intensity from the position. The obtained image is called an X-ray topograph. (In a TEM, STEM corresponds to this method.) The bright-field/dark-field image can be regarded as a kind of topograph.

  • transition radiation

    When a relativistic charged particle passes from a substance to another with different dielectric permittivities (from vacuum to a substance), electric dipoles are induced on the interface of the substances. At this event, radiation is emitted because the magnitude of the dipole changes with time. This phenomenon is called "transition radiation" and is observed strongly for metals and semiconductors. The emission intensity of the radiation (light) is approximately proportional to the square of the electronic polarizability of the substance.

  • transmission electron microscope (TEM) image

    An image formed by electrons transmitted through a specimen (transmitted electrons). The TEM image is classified into two kinds: The bright-field image and the dark-field image used for tissue observation at low-to-medium magnifications, and the structure image for atomic-scale structure observation at high magnifications. These images are produced by elastically scattered electrons. As a specimen is thicker (~10 nm or thicker), inelastically scattered electrons are superposed on elastically scattered electrons, thus obscuring these images. When inelastically scattered electrons are removed with an energy filter, a clear image is obtained.

  • transmitted wave

    A wave that passes through a specimen and exits (from the specimen) in the same direction as the incident electron wave. In the two-beam dynamical theory, the intensity of the transmitted wave periodically changes with specimen thickness.

  • tripot polisher

    "Tripot polisher" is a mechanical polishing device that consists of three legs with micrometers and a leg for supporting a specimen. First, a bulk specimen is set on the leg for the specimen, and the polishing angle is finely adjusted by the micrometers, the tripot polisher is mounted on a rotational polishing device, and then the specimen is polished while the tripot polisher is supported with human fingers. A thinned specimen with a thickness of ~10 μm can be obtained from a bulk specimen (~3 mm × ~3 mm × ~2 mm thick). As a polishing material, lapping film coated with diamond of 0.1 μm to 30 μm is used. In the finishing process, the polished specimen with a mirror surface is fabricated using colloidal silica. For TEM observation, the specimen is usually polished to wedge-shaped. Normally, the specimen is finished by ion milling. Since the use of the tripot polisher enables preparation of a wedge-shaped specimen over a wide area, the device is particularly suitable for cross-sectional specimen preparation.

  • turbo-molecular pump

    A mechanical, dry vacuum pump that does not use oil. In the turbo-molecular pump, a high-speed rotor provides momenta to residual gas molecules in the pumping direction and the gas molecules are pumped. A rotary pump or the other pump is required to evacuate the backside of the turbo-molecular pump. The pump operates at low to high vacuum. The working pressure is 10-1 to 10-8 Pa. Recently, problems of sound noises and mechanical vibrations have been solved. Any gas can be pumped by this pump. Unlike the diffusion pump, the pump is suitable for pumping water H2O. Thus, the pump is sometimes used for a biological TEM. The price of the pump is higher than the ion pump. Care has to be paid for mechanical failure.

  • twins

    When two adjacent crystals are symmetric with each other about a specific plane or a specific axis, these two crystals are called "twins."

  • two-beam approximation

    "Two-beam approximation" is an approximation method to interpret the diffraction intensity and TEM image, which assumes the existence of only two waves in a crystal: a primary beam (transmitted wave) traveling in the incident-beam direction and a Bragg-reflected beam (diffracted wave) from one lattice plane.

  • two-window method

    A method used for qualitative element (elemental) mapping by EELS. In the "two-window method," a background intensity immediately before the inner-shell excitation (I1) and the peak intensity at the inner-shell excitation (I2) of a certain element are acquired. The ratio I = I2/ I1 is calculated. This method enables us to easily perform qualitative element mapping though it does not have a sufficient quantitative accuracy.

  • ultimate pressure

    In a vacuum pump system, the lowest pressure that can be reached within a realistic evacuation time is called the "ultimate pressure."

  • ultra-high vacuum

    A high vacuum of 10-5 Pa or better. To achieve an "ultra-high vacuum," high-end technology is required, which includes selection of appropriate materials of pumps, containers, etc.

  • ultra-high voltage electron microscope

    "Ultra-high voltage electron microscope (UHV-EM)" is an electron microscope which accelerates electrons with a high voltage of 1000 kV or higher. The wavelength of an electron at 1000 kV is shortened down to 0.00087 nm. Utilizing the short wavelength, a resolution of about 0.1 nm has been obtained, and light elements such as carbons have been clearly observed. However, recent high resolution imaging has been achieved by 300 kV microscopes equipped with a spherical aberration corrector. The advantage of UHV-EM is the capability of observing thick specimens using its high transmission power, of investigating radiation damage of specimens using the high energy electron beam and of investigating chemical reaction at controlled conditions utilizing an enough space of a specimen chamber.

    超高圧電子顕微鏡:UHV-EM
    The ultra-high voltage electron microscope (UHV-EM) is an electron microscope which accelerates incident electrons with a high voltage of 1000 kV or higher. The wavelength of an electron at 1000 kV is shortened down to 0.00087 nm. Utilizing the short wavelength, a spatial resolution of the microscope image with about 0.1 nm has been attained. The other advantages of UHV-EM include observation of thick specimens using its high transmission power, investigation of radiation damage of specimens using its high-energy electron beam, and study of chemical reactions under controlled environments by utilizing a large space of the specimen chamber.
     

  • ultra-high-resolution polepiece

    The "ultra-high-resolution polepiece" makes it possible to obtain a Cs of 0.5 mm, a Cc of 1.0 mm and a spatial resolution for a TEM image of 0.19 nm at an accelerating voltage of 200 kV, in which the tilt angle of a specimen holder is limited to ±(20°to 25°). It is used for high spatial resolution microscopy and for analytical microscopy from nm diameter areas. This polepiece gives rise to a drawback which cannot be overlooked. That is, the correct bright and dark field images cannot be often obtained. The reason is as follows: The objective aperture is normally inserted into the lower part of the polepiece to prevent the objective aperture from contacting the specimen holder when it is tilted. In this case, the diffraction pattern is formed above the aperture plane. As a result, the aperture cannot often select one diffraction spot but takes influence of neighbor spots. Thus, the bright and dark field images are influenced by the effect of other diffraction. It is noted that there is option to insert a special aperture at the polepiece gap to obtain correct bright- and dark-field images.

  • ultra-thin window (UTW) EDS detector

    An EDS detector that uses an organic thin film of 0.3 to 0.5 mm thickness as a window material to prevent contaminants onto a detector element.
    Aluminum is evaporated on the surface of the organic thin film so as to avoid charging on the window. Compared with the beryllium window EDS detector, the ultra-thin window (UTW) EDS detector better suppresses absorption of low energy X-rays. Thus, the detector can analyze elements lighter than sodium (Na).

  • ultramicrotomy

    A specimen preparation method that is used for thinning biological specimens, high-polymer specimens and composite materials. “Ultramicrotomy” is also used to prepare thin films of soft metals. The use of a diamond knife enables a specimen to be sequentially cut with a thickness of several 10 nm to 100 nm.

  • ultrasonic cleaning

    "Ultrasonic cleaning" is a technique to remove contaminants on a specimen and parts of a TEM. In "ultrasonic cleaning," a cleaning solution is vibrated by an ultrasonic transducer and hits the object to clean. Since the cleaning solution can enter the fine gaps of the object, ultrasonic cleaning is suitable for precision components. Acetone is used as a cleaning solution in many cases.

  • ultrasonic knife

    An ultrasonic knife is a diamond knife that thins a specimen while vibrating the blade head of the knife using ultrasonic wave. In ordinary thinning with a diamond knife, a pressure can be applied to the cutting direction of the specimen. This may cause “compression” (deformation of the specimen due to compression in the up-and-down direction) or falling of granules with different hardnesses such as lipid droplets. The ultrasonic knife which vibrates its blade head right-and-left in cutting, makes it possible to disperse the pressure to the specimen. Owing to this feature, the ultrasonic knife prevents “compression” or the falling of the granules.

  • ultrathin section

    An ultrathin section means a section sliced thin enough to transmit an electron beam. In many cases, the ultrathin section has a thickness of 100 nm or less.

  • underfocus

    "Underfocus" means that the excitation of the objective lens in a TEM is slightly decreased from that at the in-focus (focused on a specimen). At this excitation, the image produced on the selector aperture is the specimen image when the objective lens is focused on a position above the specimen. To enhance contrast in a bright (or dark) field image (to make the outline of the image clear), the image is often taken at a slight underfocus, instead of at the in-focus.

  • unit cell

    The minimum unit of the periodicity of a crystal. The size and form of the unit cell is defined by six lattice parameters or constants (a, b, c, α, β, γ).

  • unoccupied state

    In the unoccupied state in a molecule or crystal, the certain energy level and band are not occupied by (valence) electrons.

  • vacuum evaporation (deposition)

    "Vacuum evaporation (deposition)" is a technique to form a thin substance layer on a surface of a substrate by evaporating the substance in high vacuum. For a TEM, this technique is used for specimen preparation of a metal and an alloy with uniform thickness.

  • vacuum evaporator

    A "vacuum evaporator" is an instrument to form a thin substance layer on a surface of a substrate by vacuum evaporation (deposition). The vacuum evaporator consists of a heating element, a case and a substrate in a vacuum chamber. A substance (in most cases, a metal) that is placed in the case is evaporated by melting at high temperature, and a thin film (layer) is formed on the substrate. In specimen preparation for a TEM, a heating element, such as a tungsten (W) wire or a tantalum (Ta) plate, is resistively-heated and the substance is evaporated (deposited).

  • valence-electron excitation

    A phenomenon in which valence electrons are excited to the conduction band. EELS enables us to obtain the dielectric function of a substance through the valence-loss spectrum (low-loss spectrum).

  • valence-loss spectrum (low-loss spectrum)

    A spectrum in the low energy region (below ~50 eV) in an EELS spectrum is called "valence-loss spectrum (low-loss spectrum)." This spectrum enables the band gap energy (0 to 10 eV) and the plasmon energy (10 to 50 eV) to be obtained. Dielectric and optical properties of solids can be investigated from the spectrum.

  • venting

    "Venting" is to make the pressure in the vacuum chamber to atmospheric pressure. In venting, air or nitrogen gas is used. (The latter case is used to prevent the entering of moistures in the chamber.)

  • virtual source

    In the case of the filed-emission electron gun, the action of the electrostatic lens is weak and the crossover is not produced in front of the tip of the electron gun. The "virtual source" is a point behind the tip, where all trajectories of the emitted electrons virtually meet (at one point) by the extrapolation of the electron trajectory.

  • volume plasmon

    Collective oscillations of free electrons with a longitudinal wave or compressional-wave, which are induced in metals by an electron beam or a charged-particle, are called "volume plasmon". The volume plasmon is excited also in semiconductors and insulators. Its oscillation energy (plasmon energy) is proportional to the square root of the free electron density (valence electron density for semiconductors and insulators). The volume plasmon is directly observed as a peak in an electron energy-loss spectrum. It should be noted that the volume plasmon cannot be excited nor can be observed with a light wave or a transverse wave.

    When a high-speed electron beam is incident on a solid metal, the Coulomb force formed by the beam induces a density change (compression) in the homogeneously distributed free electrons in the metal. With the induced Coulomb force as a driving force, a longitudinal oscillation wave of the free electrons with a specific frequency is created (see Fig.1). Quantization of this collective motion of the free electrons is called volume plasmon. The energy of the volume plasmon, EP, is expressed by the following equation.

    Here, ωp is the angular frequency of the plasma oscillation. ħ= h/2π is Planck’s constant. e, m, and N respectively express the elementary charge of an electron, the mass of an electron, and the density of the free electrons. The plasmon energy is proportional to the square root of the free electron density in a metal.
    Although the volume plasmon is originally considered for free electrons, it is excited also in semiconductors and insulators as a collective oscillation of the whole valence electrons. The valence electrons vibrate collectively against positive ion cores. The energy or frequency of the plasmon is calculated by substituting the density of the valence electrons into the above equation.

    Fig.1
    Fig. 1(a) Scematic of plasmon induced by free electrons. Metal is electrically nutral because free electrons and positive ions consisting of an atomic nuclei and inner shell electrons are homogenously distributed. When a high energy electron beam is incident on a metal from outside, the Coulomb force of the incident beam creates a density change in the free electrons, or positive and negative charged regions (indicated by (+) and (-) in Fig. 1(a)). With the electric field (indicated by blue arrows) caused by the density change as a driving force, the free electrons collectively starts a characteristic oscillation (plasma oscillation). (b) Schematic of plasmons of bounded electrons. Red dots and blue circles respectively show positive ions and valence electrons. The bounded valence electrons are displaced by the Coulomb force of the incident electron beam, density changes of the valence electrons being created. Then, the valence electrons collectively oscillate with plasma frequency. Black arrows at the top of Fig. 1(b) represent polarization caused by the displacements of the valence electrons. Plasma vibration of the bounded electrons is a longitudinal wave vibration caused by polarization of the atoms.

    The charge density of Al is N = 1.8 × 1023 e/cm3. The plasmon energy is calculated to be 15.7 eV, showing a good agreement with an experimental value of 15.0 eV. For monovalent metals (Li, Na, etc.), the calculated energies agree well with the energies experimentally obtained.
    For diamond (insulator), the plasmon energy is calculated to be 31 eV using the valence electron density (N = 7.0 × 1023 e/cm3), showing a rather good agreement with an experimental energy of 34 eV. In the cases of ionic crystals such as LiF, NaCl etc., their plasmon energies calculated using the valence electron densities well reproduce the energies experimentally obtained. Table 1 shows the plasmon energies for various materials.
    However, for a material in which interband transitions strongly occur close to the expected plasmon energy, the experimental plasmon energy can be largely deviated from the expected energy. For example, in the case of Ag, the plasmon energy expected from the valence electron density of N=0.59×1023 e/cm3 is 9.0 eV but the experimental plasmon energy is observed at 3.9 eV due to a strong interband transition of the d orbital electrons at 4.0 eV.

    Table1. Plasmon energies for several materials

    Materials Valence electron density [e/cm3] Ep [eV] (Calculation) Ep [eV] (Experiment)
    Li 0.47 × 1023 8.0 7.1 [1]
    Na 0.27 × 1023 6.1 5.7 [1]
    Al 1.8 × 1023 15.7 15.0
    Ag 0.59 × 1023 9.0 3.9
    Si 2.0 × 1023 16.5 16.7
    GaAs 1.8 × 1023 15.7 15.9
    Diamond 7.0 × 1023 31 34
    LiF 4.9 × 1023 25.9 25.3 [1]
    NaCl 1.8 × 1023 15.7 15.5 [1]

    [1] H. Raether (1980) "Excitation of Plasmons and Interband Transitions by Electrons" Springer Tracts in Modern Physics, Vol. 88, Springer-Verlag. New York.

    Plasmon is experimentally observed as a spectral peak by electron energy-loss spectroscopy (EELS). Fig. 2 shows plasmon-loss spectra of Al, Si and diamond acquired by EELS. Their peak values show good agreements with those on Table 1. In the spectra of Al and Si, weak peaks are observed at the positions of integer multiples of their plasmon energies (highest peaks). These weak peaks are the energy-loss spectra caused by successive plasmon excitations (twice, three times and more). The plasmon peaks of Si and diamond are broader than that of Al. This is due to a large attenuation of plasma oscillations of Si and diamond, in other words due to the collapse of the plasma oscillations caused by interband transitions of the valence electrons.
    (By Associate Professor Yohei Sato, Tohoku University)

    Fig.2
    Fig. 2. Plasmon peaks of Al, Si and Diamond in EELS spectra

  • wave aberration

    The difference between the wavefront W of ideal imaging with no aberrations (Gauss imaging) and the wavefront S in actual imaging with various aberrations is called “wave aberration.” It is defined as the optical path difference between wavefronts W and S measured along an electron trajectory given by geometrical optics. The wave aberrations neccessary for the interpretation of a high-resolution image are axial aberrations (spherical aberration and parasitic aberrations).

  • wavelength of electron

    wavelength of an electron is calculated for a given energy (accelerating voltage) by using the de Broglie relation between the momentum p and the wavelength λ of an electron (λ=h/p, h is Planck constant). As a result, the wavelength of an electron λ is expressed by the following first equation, where m0 is the rest mass of an electron, e is the elementary charge of an electron, E [V] is the accelerating voltage before the relativistic correction, and E* [V] is the accelerating voltage after the relativistic correction. The velocity v [m/s] of an electron under the accelerating voltage E [V] is expressed by the following second equation.
    Table 1 shows a comparison list between the accelerating voltage E, the accelerating voltage after the relativistic correction E*, the wavelength of an electron λ, the velocity of an electron v, and the ratio of the velocity of an electron to the velocity of light β=v/c.
     
    wavelength of electron
     
    Table 1
    Accelerating voltage
    E[kV]
    Relativistically corrected
    accelerating voltage  E*[kV]
    Wavelength of electron
    λ[pm]
    Velocity of electron
    v[m/s]
    Ratio to the speed of light c
    β = v/c

    1

    1.0010

    38.764

    1.8728E+07

    0.06247

    10

    10.098

    12.205

    5.8455E+07

    0.19499

    20

    20.391

    8.5885

    8.1503E+07

    0.27187

    30

    30.881

    6.9791

    9.8445E+07

    0.32838

    40

    41.566

    6.0155

    1.1214E+08

    0.37406

    60

    63.523

    4.8661

    1.3377E+08

    0.44622

    80

    86.262

    4.1757

    1.5062E+08

    0.50240

    100

    109.78

    3.7014

    1.6435E+08

    0.54822

    120

    134.09

    3.3492

    1.7588E+08

    0.58667

    160

    185.05

    2.8510

    1.9430E+08

    0.64811

    200

    239.14

    2.5079

    2.0845E+08

    0.69531

    300

    388.06

    1.9687

    2.3280E+08

    0.77653

    400

    556.56

    1.6439

    2.4819E+08

    0.82787

    500

    744.62

    1.4213

    2.5868E+08

    0.86286

    1000

    1978.5

    0.87192

    2.8213E+08

    0.94108

    1250

    2778.9

    0.73571

    2.8689E+08

    0.95697

    2000

    5913.9

    0.50432

    2.9352E+08

    0.97907

    3000

    11806

    0.35693

    2.9660E+08

    0.98935

  • wavelength-dispersive X-ray spectroscopy

    Wavelength-dispersive X-ray spectroscopy (WDS) is an element analysis method. Characteristic X-rays generated from a specimen are measured using Bragg reflections of X-rays with analyzing crystals, based on the diffraction angles of the reflected X-rays caused by the analyzing crystals. The analyzing power of light elements surpasses EDS. That is, WDS can analyze elements from boron (B) on down. However, the detection efficiency of WDS is lower than EDS. Thus, the illumination current of the electron beam for WDS needs to be set larger than that for EDS (several nA to several 100 nA). As a result, care must be taken for the beam damage to the specimen to suppress the damage. Normally, the resolution of WDS is about 10 eV. Its quantification accuracy is 0.1 to 0.2%. Recently, a high energy-resolution (exceeding 1 eV) analyzer that uses a grating has been developed. Due to its superbly high resolution, this analyzer can be used for analyzing the density of states of the valence band. "WDX" is also used as the abbreviation of wavelength-dispersive X-ray spectroscopy.

  • weak phase object approximation

    An approximation that is used for interpreting an HREM image. A specimen is regarded as an object that does not change the amplitude of the incident electron wave but slightly changes the phase of the wave. This approximation holds well for a very thin specimen composed of light atoms. In this case, a structure image obtained at Scherzer focus exhibits the projected potential of a crystal. When the specimen is thick, image interpretation is necessary by applying dynamical diffraction that takes account of multiple scattering.

  • weak-beam method

    The "weak-beam method" is a technique to take a dark-field image of a weakly excited reflection (for example, the 1st order reflection) at the exact Bragg setting of a high-order reflection (for example, the 3rd order reflection) under a systematic reflection condition. The observation of a dislocation by this method enables us to elucidate only a highly strained part of the dislocation with bright contrast against dark background. Thus, the dislocation is more sharply imaged and the dislocation position is more accurately determined. Accurate analysis of a narrowly extended dislocation can be performed.

  • windowless EDS detector

    An EDS detector that does not use a window material for preventing contaminants onto its detector element. Since the windowless EDS detector suffers no X-ray absorption by the window material, the detector can analyze light elements such as beryllium (Be) and boron (B). Owing to a very high vacuum of the today’s microscope column and less severe deposition condition at Peltier cooling temperature than at liquid nitrogen temperature, the window material has become unnecessary. As a result, the windowless EDS detector has more been used in recent years.

  • work function

    "Work function" means the energy required for removing an electron from a solid.

  • yoke

    A "yoke" is made of ferromagnetic iron, encloses the excitation coil of a lens and efficiently guides the magnetic flux produced by the coil to the magnetic polepiece.

  • yttrium aluminum garnet

    A "YAG (yttrium aluminum garnet)" is a substance that emits fluorescence (scintillator) by absorbing the energy of an electron beam or electromagnetic wave. A YAG is placed before a CCD for acquisition of a TEM image.

  • zero-loss peak

    A sharp peak with an energy loss of 0 (zero) appearing in an EELS spectrum. The "zero-loss peak" is composed of no-scattered electrons and elastically scattered electrons. In a real spectrum, the peak shows an energy broadening (less than 0.7 eV) due to the energy spread of the incident beam. In the analysis of low-loss EELS spectra to obtain a dielectric function, it is important to precisely subtract the tail of the zero-loss peak.

  • zeroth-order Laue zone (ZOLZ) reflection

    "Laue zones" are defined as the reciprocal lattice planes perpendicular to the direction of the incident beam. A Laue zone containing the point of origin (reciprocal lattice point corresponding to the incidence point) is called the "zeroth-order Laue zone (ZOLZ)." The ZOLZ reflections appear around the transmitted beam in a CBED pattern, which have symmetry characteristic of a specimen crystal and show loose angular change. The ZOLZ reflections give two-dimensional information on a crystal projected along the direction of the incident beam.

  • zone axis

    In a crystal, a group of planes parallel to a certain direction is called a crystal zone, and this direction is termed a "(crystal) zone axis."

  • α fringe

    "α fringe" means a fringe contrast, which is observed in bright- and dark-field images under a two-beam approximation condition in the case where the upper and lower crystals are shifted to each other (for example, stacking faults) at an interface oblique to the surface of a crystalline specimen. The end fringes, which appear at the intersection between the interface and the upper and lower surfaces, exhibit symmetric contrast in the bright-field and anti-symmetric contrast in the dark-field with respect to the center of the fringe (specimen).

  • δ fringe

    "δ fringe" means a fringe contrast, which is observed in bright- and dark-field images under a two-beam approximation condition in the case where the orientations of the upper and lower crystals are different to each other (for example, twin boundary) at an interface oblique to the surface of a crystalline specimen. The fringe contrast, which appears at two positions (edges) where the interface intersects the upper and lower surfaces, exhibits symmetric contrast in the dark-field and anti-symmetric contrast in the bright-field with respect to the center of the fringe (specimen).

Term that contains the "" in the description