WEB Advanced transmission diffraction and microscopy in SEM
Diffraction techniques are ubiquitous in electron microscopy. In TEM fast switching between diffraction and imaging mode enables efficient use of an objective aperture to increase contrast and form dark field images. SEM also offers several diffraction techniques, such as EBSD and TKD. However, the exploration of transmission diffraction is still in its infancy. Several developments have been made to take advantage of the unique features of SEM [1-3] . However, further developments to also include an aperture system have not been made so far. Here we present recent results on diffraction in SEM using our Low Energy Nano Diffraction (LEND) setup. We demonstrate the diffraction mode and showcase the use of an aperture for true dark field imaging. The latter is compared with dark field imaging in TEM and 4D-STEM in SEM.
The LEND setup is based on the combination of a fluorescent screen positioned below the sample with a dedicated CMOS camera (Fig. 1a). The technique has been implemented and successfully tested for several material systems with a focus on low voltage applications . To acquire single diffraction patterns the electron beam is set on a single spot or scanned over a dedicated sample area while recording with the camera. This results in TEM-like diffraction patterns, containing rich information on the scattering behavior and crystal structure of the specimen (Fig. 1c).
Based on this technique we developed a new approach for bringing true dark-field imaging to SEM. An aperture is inserted within the transmitted beam path in such a way, that all electrons are blocked except one diffracted beam, which passes to the STEM detector (Fig. 1b). Upon scanning a dark field image is generated, which is directly comparable to dark field images in TEM. The apertures are prepared with a laser cutting device enabling a rational design of aperture shapes for specific applications (Figure 1d). The navigation of the aperture is performed with a piezo-controlled micromanipulator that offers precise movement in all dimensions. To showcase the power of this approach a comparison of dark-field images of dislocations in bilayer graphene acquired in SEM (20kV) and TEM (80kV) is shown (Fig. 2). Both image sets are very comparable, however SEM has the advantage of much shorter acquisition times due to the increased contrast at low voltages. We also compare true dark-field images with virtual dark-field images obtained by 4D-STEM in SEM.
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