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Curve-fitting in XPS using extrinsic and intrinsic background structure

2000; Elsevier BV; Volume: 106; Issue: 1 Linguagem: Inglês

10.1016/s0368-2048(99)00089-4

1873-2526

J. E. Castle, Hans Chapman-Kpodo, Andrew Proctor, Anna Maria Salvi,

X-ray Spectroscopy and Fluorescence Analysis

It has been shown by Tougaard that the extrinsic background to a photoelectron peak is very small at the position of the peak. Most of the background step generated by the peak arises from other forms of energy loss which we have suggested is intrinsic to the photo-excitation process. We have defined this intensity, using a specific modified Shirley background in the immediate vicinity of the peak, and hence fixed it for a given element and chemical state by means of a shape parameter. In this paper we describe an examination of the passive film on stainless steel by ARXPS in which the fixed intrinsic backgrounds are used in fitting oxide and metal for iron and chromium and also for oxygen and carbon. By using peaks of identical shape, position and background for each angle it becomes easy to spot the behaviour of the residual extrinsic background, which as expected depends on angle. In this work we demonstrate the use of a polynomial function, that approximates the Tougaard background, included in the fitting process. This inclusion allows to define a parameter of the polynomial which represents the background slope and use this to assign different background tails to each component in a multiple peak. The behaviour of the extrinsic background can, by this means, be compared with the changing ratio of peak areas as a function of the take-off angle. This work on the behaviour of the extrinsic component of the background within one or two peaks widths of the peak enables surface structures to be placed in depth-wise order with only a small extension to the peak-fitting routines that are normally employed in X-ray photo-electron spectroscopy. We conclude that within the background close to the peak there lies information on the layer thickness and that this might be developed into a reliable method for the estimation of overlayer thickness.