When the angle θ between the incident electron and the normal to a surface changes, the yield of secondary electrons Y varies approximately as secθ. The topographic contrast thus produced renders secondary electrons useful for surface studies. On the other hand, as the atomic number Z increases, the backscattering coefficient η increases more rapidly than Y. Therefore, backscattered electrons should be collected as signal when atomic number contrast is desired. Figs. 1 and 2 exemplify the increase of atomic number contrast as one switches from secondary to backscattered electron mode.Backscattering is not a localized process, since both single and plural/ multiple scattering are involved. In Everhart's model, incident electrons are retarded by the inelastic scattering and scattered backwards by large angle Rutherford scattering.
Discussion of Electron Induced Atomic Number Contrast
