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

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 SiO2 films on Si from XPS measurements with Al and Mg K alpha 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 psi between the X-ray source and the analyzer axis was 40 degrees, 54 degrees, or 70 degrees. For each SiO2 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 SiO2 thickness, emission angle alpha, the angle psi; and the X-ray energy. For alpha less than or equal to 65 degrees 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 K alpha X-rays (for the three psi values) and between 0.922 and 0.935 for Al K alpha X-rays. For larger emission angles, the ratio of the mean EAL to the IMFP varied considerably with alpha and psi An EAL value appropriate for the measurement conditions should be chosen for measurements of SiO, thicknesses by XPS. (C) 2001 Elsevier Science B.V. All rights reserved.