Improved peak-fit procedure for XPS measurements of inhomogeneous samples—Development of the advanced Tougaard background method

Abstract

A new method for the fitting of X-ray photoelectron spectra using an advanced Tougaard-background model for laterally inhomogeneous samples is presented. New is the use of a separate loss function for each spectral component. Additionally, a new Five-parameter inelastic electron scattering cross section (5-PIESCS) including a variable parameter to treat the electronic band gap energy is introduced for a better modelling of the loss structures of insulators. Synthetically generated test spectra using two peaks with strongly different loss structures and measured spectra from different samples are fitted with the traditionally used Shirley background (BS), the Tougaard background for homogeneous samples (BTH), and the newly developed advanced Tougaard background for laterally inhomogeneous samples (BTI). It was found that the fit results for the peak areas and peak positions of the three methods could be strongly different. In many cases the use of the Shirley background and the Tougaard background for homogeneous samples resulted in completely wrong component areas in spite of sometimes rather satisfying residual functions and Abbe criteria. In contrast, the advanced Tougaard background for inhomogeneous samples gave excellent results for all wide range spectra including pronounced loss structures. The new source code of the UNIFIT software (Version 2016 or higher) to calculate the advanced Tougaard-background parameters for inhomogeneous samples was verified.