色収差

chromatic aberration

入射電子線や試料を透過した電子線のエネルギー(波長)が様々な要因で拡がり(幅)を持つと、レンズでの屈折角度が波長によって異なるため、結像の際、像がぼけること。電子線のエネルギーの拡がりは、加速電圧の変動、電子銃からの放出電子の初速度の拡がり、ベルシュ効果や、レンズコイルの励磁電流の変動による焦点距離の変動などで起きる。また、試料が厚くなると(～10nm以上)、非弾性散乱による電子エネルギーの損失(波長の変化)も色収差を引き起こす。

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.