Compositional depth profiling by sputtering

Abstract

A summary of the principles of quantitative depth profiling by sputtering in combination with surface analysis methods such as AES, XPS, SIMS and ISS is presented with emphasis on recent advances in improving the depth resolution towards its physical limits. In addition to nonlinearities in both the conversion of sputtering time to depth and of analysis signal intensity to concentration, a large number of effects lead to characteristic deviations between the measured profile and the original shape of the in-depth distribution of composition. Degradation of the depth resolution is due to changes in surface topography and composition of the sample during sputter profiling. These alterations are caused by instrumental factors, by sample characteristics and by the fundamental effects of the ion beam - sample interaction. An outline of current model descriptions of the relevant phenomena is given, particularly with respect to recent progress in understanding the influence of atomic mixing, sample roughness, crystalline structure, ion beam energy and incidence angle on depth resolution. Optimized conditions for high resolution depth profiling are derived, leading to the conclusion that sample rotation is the most favourable method to achieve a depth resolution in the low nanometer region even for metallic samples and large sputtered depths. Crater edge profiling with high spatial resolution scanning or imaging techniques is an alternative method to attain high depth resolution. Based on the knowledge of the depth resolution function, deconvolution of measured profiles can be carried out and the true in-depth distribution of composition is obtained with limiting uncertainties in the atomic momolayer regime.