Scattering Phenomena in Electron Microscope Image Formation

Abstract

Equations are derived for the relative intensities in electron microscope images formed without a limiting aperture in the objective lens. Three components of intensity at the image plane can be distinguished: I_T, a transmitted intensity of very small relative aperture; I_K, an intensity scattered within the effective aperture of the objective lens; and I_B, a background intensity which does not contribute to the recognizable image. The equations derived involve two scattering cross sections, both of which are readily measurable under normal operating conditions. The form of the equations is verified experimentally; and cross sections are measured for Be, SiO, Cr, Ge, Pd, Pt, and U. The effect of altering the beam potential is discussed in the light of the results and the minimum detectable increment in specimen thickness due to scattering is estimated.