On using the Compton and Rayleigh scattered radiations in pseudo-monochromatic TXRF thickness analysis of SiO2 film on a GaAs substrate

Abstract

The thickness analysis of a thin film on top of a substrate by TXRF (Total-Reflection X-Ray Fluorescence) is based on the phenomenon of total external reflection of x-rays which occur at a very small angle of incidence. An x-ray beam impinges on a film-substrate sample and the x-ray scattered or emitted by the film and substrate are measured for different glancing angles. The x-ray incident to and reflected by the substrate interfere coherently to establish a standing wave. The film is in the region of the standing wave, hence the interaction of the film and incident x-ray is, among other factors, proportional to the standing wave. Normally the fluorescent x-ray emitted by the photo-electric interaction is what is measured since its energy is a unique property of the emitting element. For a sufficiently thin film, the fluorescent intensity of an element in the film vs angle has a peak structure near θ_c, the critical angle for total reflection of the film. The width and the shape of the high-angle side of the peak depend on the thickness of the film.
Usually the incident beam comes from an x-ray tube whose output is polychromatic — a strong characteristic line superimposed on a broad bremsstrahlung. A simple mirror can be used, as in the case of the PNRI TXRF spectrometer, to eliminate the bremsstrahlung above the strongest tube characteristic line and produce a pseudo-monochromatic beam. When the photo-electric edge of the element of interest is very near the characteristic tube line, a pseudo-monochromatic begun is a good approximation of a truly monochromatic excitation. Otherwise, the contribution from the bremsstrahlung broadens the peak structure towards the high angle because of the inverse relation between energy and the critical angle.
This report will demonstrate that in samples where the pseudo-monochromatic nature of the incident x-ray leads to excessive broadening of the angular dependence of the film's fluorescent x-ray, the Compton and Rayleigh scattering of the tube characteristic line may be used instead of the fluorescent x-ray. The thickness analysis will not be affected by the bremsstrahlung contribution. The use of the Compton and Rayleigh scattered x-rays will be illustrated for a SiO₂ film on top of a GaAs substrate.