Instrumental Techniques for XPS and AES

Abstract

AES and XPS involve respectively the interaction of an electron or X-ray photon beam with the specimen surface to be analysed, followed by detection of Auger elctrons or photoelectrons. If the incident beam is to reach the sample, and the emitted electrons are to be detected, then their mean free paths in the region of the sample must be greater than the physical dimensions of the apparatus involved, otherwise scattering will distort the outcome of the experiment. For physically realistic dimensions, this implies the use of a vacuum. Basic considerations of the kinetic theory of gases lead to the conclusion that, for apparatus with dimensions of the order of a few tens of centimetres, pressures in the 10⁻⁵ to 10⁻⁶ mbar range would be adequate. However, for surface science there is a further and much more stringent requirement to be satisfied, namely that the surface to be examined does not become contaminated by the residual gas in the vacuum chamber during the course of the analysis. This applies even to surfaces which have already been air exposed, as the analysis will often include depth profiling in which the contaminated outer surface layers are removed revealing a fresh surface to be analysed.