New method to correct for the influence of organic contamination on intensity ratios in quantitative XPS

A new method is proposed to correct for attenuation effects by adventitious organic contamination in quantitative XPS. The corrected intensity ratio IoX/IoY involving two different substrate peaks is determined as the slope of the plot of IX/IC1s as a function of IY/IC1s for samples covered with organic overlayers of varying thickness. The thickness of deposited organic overlayers does not need to be perfectly uniform provided that it does not vary by more than ±40%. The sample surface can either be flat or present a random roughness (e.g. powders). Shadowing effects accompanying overlayer non-uniformity are taken into account by a simple model involving effective thickness and fractional coverage, which is shown to enhance greatly the accuracy of computed intensity ratios. Constraints of the method and possible implementations were examined with an NaCl powder covered with adventitious contamination, polystyrene (PS) and polydimethylsiloxane (PDMS). The most straightforward implementation of the method would be to record repeatedly the peaks of interest while decreasing the adventitious contamination overlayer thickness by thermal desorption. The error on IoNa2s/IoNa1s is +10%, with the apparent overlayer thicknesses varying from 1.1 to 0.3 nm. The method is applied to check the reliability of the spectrometer transmission functions reported previously by Weng etal.