Effects of elastic-electron scattering on measurements of silicon dioxide film thicknesses by X-ray photoelectron spectroscopy

Abstract

Abstract It is now customary for the effects of elastic-electron scattering to be ignored in measurements of the thicknesses of overlayer films by X-ray photoelectron spectroscopy (XPS). It is known, however, that elastic scattering can cause the effective attenuation length (EAL), needed for the thickness measurement, to be different from the corresponding inelastic mean free path (IMFP). We have investigated the effects of elastic-electron scattering in measurements of thicknesses of SiO 2 films on Si from XPS measurements with Al and Mg Kα X-rays. Calculations were made of substrate and oxide Si 2p photoelectron currents for different oxide thicknesses and emission angles using an algorithm based on the transport approximation. This algorithm accounts for the occurrence of elastic scattering along electron trajectories in the solid. The calculations simulated an angle-resolved XPS experiment in which the angle ψ between the X-ray source and the analyzer axis was 40°, 54°, or 70°. For each SiO 2 thickness and set of measurement conditions, an average EAL was determined from the substrate currents with and without the oxide overlayer. The ratio of the average EAL to the IMFP varied with SiO 2 thickness, emission angle α , the angle ψ , and the X-ray energy. For α ≤65° and for oxide thicknesses such that the substrate current was reduced to not less than 10% of its original value, the mean EAL for this range of thicknesses was between 0.912 and 0.926 of the corresponding IMFP for Mg Kα X-rays (for the three ψ values) and between 0.922 and 0.935 for Al Kα X-rays. For larger emission angles, the ratio of the mean EAL to the IMFP varied considerably with α and ψ . An EAL value appropriate for the measurement conditions should be chosen for measurements of SiO 2 thicknesses by XPS.