Origins of topographic contrast in the scanning electron microscope (SEM) are different at different resolution levels. At low resolution, tilt contrast of large features dominates; at medium resolution, diffusion contrast of features smaller than an interaction volume of primary electrons dominates.
The secondary electron (SE) signal, commonly used in the SEM, does not give a good tilt contrast; better contrast can be obtained with backscattered electron (BSE) signal of a converter and a sector-shaped ring detector. For obtaining topographic images from signals containing topographic and material contrast, signals of detector systems containing two or more detectors are mixed. Detector systems containing BSE detectors give more reproducible signals with a more uniform dependence on tilt angles than systems containing SE detectors. Tilt contrast of specimens coated with thin layers of heavy metals is similar to the contrast of uncoated specimens in the case of an SE detector, and better tilt contrast can be obtained with a sector-shaped ring BSE detector.
Diffusion contrast dominates at medium resolution. Contrast obtained with three selected detectors: SE detector, sector-shaped ring BSE detector, annular top BSE detector, is also discussed. The contrast is lowest for the top BSE detector and highest in the case of SE detection, especially for materials of low density. In the case of coated specimens, the SE detector and the sector-shaped ring BSE detector give good contrast and both are suitable for medium resolution studies.
The discussion in the paper concerns untilted or slightly tilted specimens.
"Detection of Topographic Contrast in the Scanning Electron Microscope at Low and Medium Resolution by Different Detectors and Detector Systems,"
Scanning Microscopy: Vol. 8
, Article 1.
Available at: https://digitalcommons.usu.edu/microscopy/vol8/iss2/1