Atomic-Resolution Secondary Electron Imaging

AC-STEM with Secondary Electron (SE) imaging provides the unique capability of evaluating specimen surface morphology with atomic-resolution.

Aberration Corrected Scanning Transmission Electron Microscopy (AC-STEM) is now widely used in material science research, product process control, and device failure analysis for a wide variety of different materials.  The aberration-corrected electron probe scanned across a thin specimen provides atomic-resolution imaging not only from transmitted electrons (TE – HAADF, BF, ABF…), but also from secondary electrons (SE) emitted from the surface of the specimen.  AC-STEM SE imaging provides a unique method of characterizing material structure including important depth information.

When surface information of a thin specimen is desired, STEM SE imaging can be utilized. This includes tasks such as particle morphology, CD measurements, and defect inspection to name a few.  In the case of defects, it is often important to understand their location within the thickness of a (S)TEM specimen.  Investigations based solely on transmitted electron information can often be inconclusive especially in the case of 3-D structures since TE images only provide a projection of the entire thickness of the sample.

Atomic-Resolution Secondary Electron Imaging
Ultra-high-resolution images at Si twin boundaries

In Figure 1, the BF-STEM (TE mode) image on the left, shows a brighter region above a threading dislocation induced V-defect at the ITO/GaN EPI interface. This contrast is indicative of a region having lower average Z, but it is impossible to distinguish whether the feature is simply filled with lighter material such as Carbon or is a void.  However, SE imaging provides the conclusive answer.

BF-STEM and SE images of voiding metal/EPI interface above a threading dislocation
Figure 1: BF-STEM and SE images of voiding metal/EPI interface above a threading dislocation. Left: BF-STEM in TE mode; right: STEM-SE mode

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