In general, three types of backgrounds are used: 1) a simple straight line or linear background, 2) the Shirley background in which the background intensity at any given binding energy is proportional to the intensity of the total peak area above the background in the lower binding energy peak range[1] (i.e. the background goes up in proportion to the total number of photoelectrons below its binding energy position) and 3) the Tougaard background (or Tougaard universal cross-section approach) which is a methodology for integrating the intensity of the background at a given binding energy from the spectral intensities to higher kinetic energies[2]. The simple linear background suffers from large peak area changes depending on the position of the chosen end points and is the least accurate. The Tougaard background is the most accurate but suffers from complications in practical use, particularly if there are numerous peak overlaps. The Shirley background falls somewhere in between for accuracy, however its ease of use has resulted in its widespread adoption.
References:
[1] M.P. Seah Quantification of AES and XPS in Practical Surace Analysis by Auger and X-ray Photoelectron Spectroscopy ed. D Briggs & M.P.Seah , John Weley & Sons, Chichester UK, 1983 p. 204.
[2] Neal Fairly, XPS lineshapes and Curve Fitting in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy ed. D Briggs & JT Grant, IM Publications, Chichester UK, 2003, p. 398.
