pixel array STEM detector (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 pixel array STEM detector (pixelated STEM detector) are compared in Fig. 1.

Fig. 1 Conventional single-channel STEM detector and Pixel array STEM detector (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) Pixel array STEM detector (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 pixel array STEM detector (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 pixel array STEM detector (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.