Monte Carlo simulations of electron transport in solids: applications to electron backscattering from surfaces

We report results of Monte Carlo simulations to investigate the effects of backscattered electrons in scanning Auger microscopy (SAM) on the radial distributions of emitted Auger electrons. We considered the emission of copper M3VV and L3VVAuger electrons from a thin Cu overlayer on a substrate of silicon or gold for primary electrons with energies of 5 and 10 keV that were normally incident on the sample. The Cu layer was assumed to be sufficiently thin that there were no changes in the angular and energy distributions of primary and backscattered electrons passing through the overlayer. We report values of the information radius, rP a , from which a selected percentage P of the emitted Auger electron intensity originates. Values of rP a found here range from 119 A ° (Cu L3M45M45 Auger transition, E0 = 5 keV, Au substrate, P = 80) to 6757 A ° (Cu M3VVAuger transition, E0 = 10 keV, Si substrate, P = 95). For the same substrate, primary energy, and chosen value of P, values of rP a are larger for Auger electrons from the CuM3VVAuger transition than for the Cu L3M45M45 Auger transition. In addition, values of rP a increase with primary energy and are larger for the Si substrate than the Au substrate. The values of rP a are generally much larger than the radius of the primary beam (assumed to be 50 A° here) on account of inner-shell ionizations by backscattered electrons. We also report values of the mean escape radius, hri, that range from 82.5 A ° (Cu L3M45M45 Auger transition, E0 = 5 keV, Au substrate) to 1169 A ° (Cu M3VVAuger transition, E0 = 10 keV, Si substrate). Knowledge of rP a and hri is important in the analysis of fine features in SAM because appreciable Auger signal can be collected from the nearby region as well as from the feature of interest. Finally, we report Monte Carlo simulations of Auger line scans across the edge of a thin Cu overlayer on a Si or Au substrate. The shapes of the line scans depended only weakly on the Cu Auger transition, although the differences were more pronounced for the Si than the Au substrate. On account of backscattered electrons, the lateral distance corresponding tosignal variations of 25% and 75% of the maximum intensity in a line scan varied from 53.6 A ° (Cu L3M45M45 transition, E0 = 5 keV, Si substrate) to 75.1 A ° (Cu M3VV transition, E0 = 10 keV, Au substrate).