Contrast Dependence on Features (Edge Effect) in SEM
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.



Secondary electrons (SEs) detection in SEM imaging includes several processes. Two important processes are generation and transmission of SEs. The generation process is controlled by yield of SEs, while the transmission process from materials to vacuum is controlled by the mean free path of the SEs. If the adjacent materials (e.g. region B in Figure 4810) has a smaller mean free path (MFP), this small MFP will weaken the intensity of SE transmission through the adjacent materials in that direction as shown by II in the top image. Here the lower MFP means more SE will be lost during transmission process. This intensity weakening causes non-symmetric distribution of emitted SEs, different from the symmetric case in I. Note that the lengths of the arrows represent the intensities in the relevant direction. This weakening of SEs induces lower contrast at the feature edge of region A as shown in the bottom image. On the other hand, if the adjacent materials has a longer MFP, the adjacent materials will enhance the intensity of SE transmission, improving the contrast as indicated at the edge of region II shown in the bottom image in Figure 4810.

Contrast Dependence on Features (Edge Effect)

Figure 4810. Schematic shows contrast dependence on features (edge effect). Top image: electron beam scans
at different locations; bottom image: SEM image due to edge effect.




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