Depth profiling typical spectrum

Figure 2 Typical ISS apectral data obtained from routine depth profile of cleaned washed steel. Left spectrum represents surface. Right spectrum represents about 50-A depth. Expansiona ahown top left. Relative atomic compositions plotted top right.

Chemical effects are quite commonly observed in Auger spectra, but are difficult to interpret compared with those in XPS, because additional core levels are involved in the Auger process. Some examples of the changes to be seen in the KLL spectrum of carbon in different chemical environments are given in Fig. 2.24 [2.130]. Such spectra are typical components of data matrices (see Sect. 2.1.4.2) derived from AES depth profiles (see below). 

The XPS (N-ls) spectrum of the sample degassed at 973 K shows a large band of silazane species (58%), with minor shoulders of Si-NH2 (28%) and nitride (13%) species. The spectrum of the sample, deammoniated at 1123 K, shows a completely different picture. The dominant band in this spectrum is the nitride band (62%), with minor silazane (20%) and amine (18%) shoulders. Bands, attributable to Si-NH2—NH4C1 are not observable in the XPS spectra, suggesting that the actual concentration of these species is very small and not present on the actual surface (XPS is a typical surface probe, with a depth-profile of 0.3 - 3 nm, cfr. Appendix B). 

An immediate distinction needs to be made between dynamic and static SIMS experiments. In the former, a primary ion current density of typically > 10 pA cm is used and this leads to rapid sputtering of material. The surface is eroded at a rate of order nm s and by following the intensity of chosen peaks in the mass spectrum as a function of time, a concentration depth profile can be constructed. In this mode SIMS can be very sensitive, with trace element detection limits in the ppm-ppb range. However, quantification is not straightforward. Secondary ion intensities are strongly matrix-dependent and extensive calibration procedures involving closely related standards of known composition and under identical experimental conditions must be used to extract quantitative concentrations. 

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