Ion sputter depth profiling in AES and XPS
The universal method of destructive composition-depth profiling applied in AES and XPS is through the sputtering of thin sections of the sample surface by energetic ion bombardment. Here, the analysis is carried out as a succession of ion sputtering and AES/XPS analysis cycles. The advantages of ion sputter depth profiling are:
1. The information depth is variable (typically between 0.5 and 3 nm)
2. The analysis is independent of the sputtering yield (for most cases)
3. The influence of the matrix on the elemental sensitivity factor is small
4. The analyzed area is small compared to the sputtered area, thus minimizing crater edge effects.
The quantification of composition-depth profiles by ion sputtering involves the conversion of the original measured peak area data (in XPS) or peak-to-peak height data (in AES) as a function of sputter time into an atomic concentration profile as a function of depth. This would require an accurate knowledge of the sputtering rate through the material so that a calibration of the depth scale can be performed. Likewise, the intensity of the XPS or AES signal must be calibrated in terms of a local elemental concentration. The goal here is to achieve a measured concentration profile as close as possible to the “true profile” if the sputtering proceeds in an ideal “layer by layer” removal of surface atoms from the sample. In practice, however, due to the various sputtering induced topographical and compositional changes of the instantaneous sample surface as well as changes of the electron mean free path, most analysis are reported as relative atomic composition profiles as a function of the sputter time. It is possible to get an accurate conversion of the sputter time into the depth scale, but this involves extensive measurements of the sputter rate of standard reference materials with accurately known thickness and closely identical composition to the unknown sample. Some of the factors that mitigate the accuracy of a sputter depth profile analysis include: