Modeling Electron Beam-Specimen Interactions

Abstract

In order to interpret features in SEM images or to develop microan-alyticàl procedures, it is often necessary to formulate a model of the interaction of the electron beam with the specimen. Such models can provide a quantitative description of one or more of the measurable products of the beam-specimen interaction: backscattered electrons, secondary electrons, characteristic and bremsstrahlung x-rays, absorbed current, and so on. There are at least four distinct approaches to modeling these interactions: (1) simple analytic equations based on scattering cross sections; (2) Monte Carlo electron trajectory simulation; (3) electron transport calculations; and (4) dynamical diffraction theory. A detailed description of the various approaches to modeling could easily fill an entire volume. In our discussion, consideration will be limited to a description of analytic forms and Monte Carlo techniques, which are the most flexible of the modeling techniques and the most easily understood. Readers interested in electron transport methods should refer to the detailed review by Fathers and Rez (1979); dynamical diffraction theory, which is applicable to crystalline materials, particularly in the form of thin foils imaged in the transmission electron microscope (TEM) or scanning transmission electron microscope (STEM), is described in detail by Hirsch et al. (1965).