Accuracy limitations for composition analysis by XPS using relative peak intensities: LiF as an example

Although precision in XPS can be excellent, allowing small changes to be easily observed, obtaining an accurate absolute elemental composition of a solid material from relative peak intensities is generally much more problematical, involving many factors such as background removal, differing analysis depths at different photoelectron kinetic energies, possible angular distribution effects, calibration of the instrument transmission function, and variations in the distribution of photoelectron intensity between “main” peaks (those usually used for analysis) and associated substructure following the main peak, as a function of the chemical bonding of the elements concerned. The last item, coupled with the use of photoionization cross sections and/or relative sensitivity factors (RSFs), is the major subject of this paper, though it is necessary to consider the other items also, using LiF as a test case. The results show that the above issues, which are relevant to differing degrees in most XPS analyses, present significant challenges to highly accurate XPS quantification. LiF, using the Li1s and F1s XPS peaks, appears, at first sight, to be an ideal case for high accuracy. Only 1s core levels are involved, removing any possible angular effects, and it is a wide bandgap material, resulting in the main Li1s and F1s peaks being well separated from the following scattered electron backgrounds. There are, however, two serious complications: (1) the main F1s and F2s levels have a major loss of intensity diverted into satellite substructure spread over ∼100 eV KE from the main line, whereas the Li1s level has very much less diversion of intensity; (2) there is serious overlap of the substructure from F2s (∼30 eV BE) with the main line of Li1s at ∼56 eV. We report here a detailed analysis of the LiF XPS, plus a supporting theory analysis of losses of intensity from Li1s and F1s to satellite structure, based on the cluster models of LiF. We conclude that, if the overlap from the F2s substructure is correctly subtracted from Li1s, and the intensity from satellites for F1s and Li1s properly estimated, the atomic composition of the single crystal LIF may be recovered to within 5%, using the photoionization cross sections of Scofield, inelastic mean free path lengths based on Tanuma, Powell, and Penn, and the calibrated instrument transmission function. This refutes the claim by Wagner et al., based on their empirical determination of RSFs, (which applied only to the instruments and the analysis procedure they used, in 1981) that Scofield values are too low in general and, for Li1s in particular, are low by a factor of ∼2. This is important because Wagner-based RSFs (sometimes modified and sometimes not) are still embedded in quantification software on modern commercial instruments, and so analysts need to be aware of how those RSFs were obtained/modified. Incorrect use can lead to large quantification errors.