The use of proton-excited X-rays for thin-film profiling

Abstract

Measurement of characteristic X-rays excited at the surface of a solid by proton bombardment provides quantitative compositional analyses that, when combined with low-energy argon ion sputtering, can be used to provide depth profiles of thin films. Several examples of the use of this technique will be presented to demonstrate its broad applicability. Proton-excited X-ray (PEX) analyses are obtained with 100–200 keV proton bombardment and energy-dispersive X-ray analyses using both lithium drifted silicon solid-state detectors and gas-flow proportional counters. Using the appropriate X-ray production cross sections and geometrical and X-ray absorption factors, the absolute number of atoms/cm 2 of a particular element is obtained for the thin film. As successive sections of the film are removed using 1 keV argon ion sputtering, the number of atoms/cm 2 of a particular element that remains is accurately monitored thereby generating an integral profile that can, subsequently, be differentiated to produce the depth profile. From measured values of the sputtering yield, the depth scale can be reported in angstroms. The PEX profiling technique has been routinely used in the characterization of corrosion products, electrodeposited coatings, surface alloys, and metal/oxide interfaces as well as in the determination of absolute sputtering yields. Specific examples to be described come from extensive studies of corrosion films formed in geothermal brines, molten-salt electrodeposition of platinum group alloys, surface alloys fabricated by ion implantation, and studies of the metal/oxide interfaces resulting from low-temperature oxidation of iron.