扫描透射电子显微镜模式分析
A general introduction to STEM detector
1. BF detector
It is placed at the same site as the aperture in BF-TEM and detects the intensity in the direct beam from a point on the specimen.
2. ADF detector
The annular dark field (ADF) detector is a disk with a hole in its center where the BF detector is installed. The ADF detector uses scattered electrons for image formation, similar to the DF mode in TEM.The measured contrast mainly results from electrons diffracted in crystalline areas but is superimposed by incoherent Rutherford scattering.
3. HAADF detector
The high-angle annular dark field detector is also a disk with a hole, but the disk diameter and the hole are much larger than in the ADF detector. Thus, it detects electrons that are scattered to higher angles and almost only incoherent Rutherford scattering contributes to the image. Thereby, Z contrast is achieved.
In addition, there is the option to install a Secondary Electron Detector above the sample like in a SEM and thereby to obtain additional morphological information.
4. The central DF and BF mode in normal TEM
In normal TEM, we also mentioned the DF and BR mode. Actually, they are different with ADF and BF mentioned here completely. In normal TEM, the direct transmitted beam is blocked by the aperture and the diffracted beam is allowed passing. When the diffracted beam hits the screen and forms a image which is called as DF. Otherwise, diffracted beam is blocked with aperture and transmitted beam forms BR image.
However, in STEM mode, they are totally different because the beam on sample is cone-shaped beam rather than parallel light.
STEM模式和TEM模式的对比图
Z-contrast actually uses an annular detector which can only collect the scattered electrons outside the beam cone. That is to say, the transmitted electrons inside beam cone are involved. This is good thing for other imaging mode because diffraction contrast was avoided and the imaging only has dependence on atomic structure. This imaging mode is called as STEM-HAADF. Here, two issues are not addressed yet. One is whether STEM-BF and STEM-ADF imaging involves diffraction contrast. Personally, I think STEM-ADF should involve phase contrast and Z contrast. For STEM-BR, phase contrast is greatly involved. However, in case of HAADF, only Z-contrast works.
Camera Length affects the Inner Collection angel. In our TEM, when camera length is 11 cm, the inner collection angel is 60 mrad. Once the camera length reducing to 5 cm, the inner collection angel is 110 mrad. Therefore, the collection angle is controlled by the camera length. Who affects the camera length? According to equation (L=f*Mp*M l) where f is the focal distance of objective lens (f is related with the excited current, acceleration voltage, the number of coil), Mp and M l are the magnification of projective lens and intermediate lens respectively. Experimentally, the f
can be changed significantly through adjusting the excited current. Besides, in order to change the camera length, you
also can change the excited current of projective lens or intermediate lens. When inner collection angle is bigger than 100 mrad, the Rutherford scattering dominates the imaging.
The inner angles for HAADF detectors are at least three times the angle of electron-probe-forming aperture. In many cases of HAADF imaging, the annular detection angle was practically set to be 60 - 170 mrad due to the limitation of combinations between aperture sizes and camera lengths. Above image shows the positions of the detectors which can be installed in a STEM system. Depending on the scattering angle of the transmitted electrons, various signals can be detected as a function of the position of
the scanning probe: BF (bright-field)-STEM, DF (dark-field)-STEM or HAADF (high angle annular dark field)-STEM.
The DF detectors are annularly shaped to maximize the collection efficiency and the range of the collected scattering angles can be adjusted through the magnification of the intermediate lenses.
FEI HAADF detector