Determination of Surface Impurity Concentration Profiles by Nuclear Backscattering

An analysis has been made to extract the concentration profiles of impurities in the upper two microns of a substrate surface. The technique used is nuclear backscattering. Since the stopping power of materials is nonlinear with energy, detailed calculations are necessary to unfold the impurity distributions. The calculations were tested by analyzing arsenic impurities in silicon. Four MeV helium was used as the projectile. Because the reaction 28Si(α, α)28Si at 2–4 MeV occurs in a region of many energy levels in 32S, a fitting technique was used to extract cross sections from an optical-model calculation of the reaction cross section. The experimental data were corrected by the technique of deconvolution for detector and electronic noise to obtain a final depth resolution of about 200 Å. The extracted impurity profile is shown to be as accurate as those obtained from neutron-activation profiling, differential Hall measurements, and differential resistivity measurements.