• ピクセル型STEM検出器

    pixelated STEM detector



    pixelated STEM detector⇒図1
    図1: 通常のシングルチャンネルのSTEM検出器とピクセル型STEM検出器。



    pixelated STEM detector⇒図2
    図2:CBEDパターン中の積分領域を変更して作成したSrTiO3[100] のSTEM像。加速電圧:200 kV。四次元データを元に作成したSTEM像。
    CBEDパターン中の積分領域を像中の右下に、半透明赤もしくは青で示す。STEM像のピクセル数:256×256ピクセル、ピクセル型STEM検出器のフレームレート:4,000 fps(ドゥエルタイム250 us)。データ取得時間:約16秒。
    (d)e-ABF(enhanced ABF)像。(b)、(c)での積分領域をそれぞれB、Cとしたとき、C–Bとして作成されたSTEM像。(c)のABF像と比較してOコラムの像コントラストがよりはっきり現れている。

    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.