Contrast and resolution in REM, SEM and SAM

Reflection electron microscopy (REM) in fixed beam and scanning modes, secondary electron microscopy (SEM) and scanning Auger electron microscopy (SAM) are related in that they provide nanometer-scale resolution of surfaces in moderate or ultra-high vacuum using incident electron beams having energies of the order of 100 keV. They may, in many cases, be used in conjunction or in parallel to provide complementary information on crystal structure, morphology and compositional variations with sensitivity to single surface-layers of atoms. Experiments have shown in each case that the achievable resolution is considerably better than had previously been thought possible. Recent theoretical developments and exploratory investigations have provided at least a partial explanation of why sub-nanometer resolutions can be achieved. In the case of REM, the main limitations to resolution and contrast of the images arise from the inelastic scattering processes. Experiments with a new energy-filtering electron microscope have confirmed that images are considerably improved if formed by using only elastically scattered electrons.