Surface analysis of rf plasma oxidized In and PbInAu films using ESCA

Abstract

Variable-angle x-ray photoemission spectroscopy (XPS or ESCA) has been used to quantitatively determine the composition of the upper portion of oxide layers on the surface of In and PbInAu alloy films. The oxide layers were formed using a combination of thermal oxidation and rf oxidation, a process in which the films are mounted on a Pb-coated electrode which is coupled to an rf generator to produce a glow discharge in oxygen. In addition to indium oxide, lead oxide was detected in all films. The concentration of the lead oxide was essentially independent of the composition of the underlying metal film (0?cPb?86 at. %), but was strongly related to the rf-oxidation conditions. This indicates that backscattering of material sputtered from the Pb-coated rf electrode during rf oxidation as the primary factor in determining the concentration of the lead oxide within the 20-Å XPS sampling depth. Furthermore, by varying the detection angle, and consequently the effective sampling depth, we obtained results which indicate that this lead oxide was concentrated within the top 10 Å of the oxide. Films which were oxidized at 2.7 Pa O2 for 30 min were thus found to have the equivalent of 2.5–4.0 Å of lead oxide distributed within the top 6 Å of their oxides, independent of their Pb concentration. Increasing the pressure during rf oxidation to 6.7 Pa and extending the oxidation time to 120 min resulted in the backscattering of the equivalent of 6–8 Å of lead oxide distributed within the top 10 Å of the oxide. Electron microprobe measurements of the Pb content of similarly oxidized indium samples showed good agreement regarding the relative amounts of Pb in the surface oxide region but indicated a Pb level about twice that determined by XPS.