Atomic Contrast (Z-contrast) in SEM
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SE (secondary electron) yield depending on the incident angle of primary beam can be given by,

          SE yield depending on the incident angle of primary beam------------------------ [4571]

where n is about 0.8 for heavy elements while is about 1.2 for light elements and thus, this difference gives the SEM contrast difference between heavy and light elements.

At short working distance, the major contribution to the signal collected using the lateral detector is that of SE3 electrons, which are backscattered by the pole piece as shown in Figure 4571. The corresponding contrast is a Z contrast.

SE3 can be enhanced with a shield to increase atomic number contrast. In this case, BEs that strike the shield can he converted into SEs when the shield is positively biased or covered with a material that emits more secondary electrons. Refer to discussions provided by Reimer [1] and Peters [2].

Source of Secondary Electrons in SEM

Figure 4571. Source of Secondary Electrons in SEM. BS1 and BS2 are
backscattering electrons; SE1 - SE4 are secondary electrons.

[1] Reimer, L., Scanning Electron Microscopy, Springer Verlag, New York, 1985.
[2] Peters, K. R., "Conditions Required for High Quality High Magnification Images in Secondary Electron-I Scanning Electron Microscopy," SEM 1982, Vol. IV, SEM, Inc., Chicago, 1982, pp. 1359-1372.