Influence of detector geometry on image properties of the STEM for thick objects.

Plural electron scattering within thick objects broadens and smoothes the intensity distribution in the detector plane of a scanning transmission electron microscope. Detector arrangements have been determined which give maximum contrast and optimum S/N when the object details are large compared to the scanning spot. Asymptotic expressions for the optimum detector angles, specimen resolution, and S/N were obtained which are valid for objects thicker than approximately four elastic mean free path lengths. Exact calculations of the changes in contrast and S/N with thickness fluctuations in amorphous carbon foils were performed for atbitrary foil thicknesses. Elastic and inelastic electron scattering was taken into account.