Study of copper surface oxidation by grazing angle X-ray excitation

Abstract

This work reports measurements of copper surface oxidation by XRF analysis under grazing angle excitation. The mathematical model for analyzing data is based on the usual equation for XRF intensity, corrected by the stratified model for the layer excitation. The final expressions are fitted to the data by using the Simplex algorithm. Three samples of silicon wafers with surface layers of copper were made by using the controlled-evaporation technique under vacuum. The final layer thicknesses were 190 Å, 400 Å, and 800 Å. For each sample, several measurements were carried out by performing an angular scan in the region of the critical angle for total reflection. Different measurements were taken immediately after evaporation, after 30 min, after 1 h, after 4 h, and after 30 min in oven at 65°. The results show small variations among the different spectra measured for each sample. The most significant variations are observed for the first measurement (after evaporation) and the last one (after the heating in oven). The mathematical model works correctly for a two-layer scheme but shows inconsistencies for more complex schemes. This indicates that the stratified model is not appropriate for continuous media due to the nature of the theoretical assumptions in which the stratified model is based. Our results show also that the surface oxidation process takes place in the first moments of exposition to air and does not progress afterwards except if the sample is heated.