XPS measurement of lubricant layer thickness on magnetic recording disks

We have studied the measurement of perfluoropolyether layer thicknesses by X-ray photoelectron spectroscopy (XPS). We used a model with a perfluoropolyether layer on a silicon dioxide substrate for the XPS analysis. By using the XPS Ols signal ratio of perfluoropolyether and silicon dioxide, the perfluoropolyether thickness could be determined from the following equation d(Angstrom)=16ln[7.2I(1)/I-2+1] where the O1s escape depths of organic substances and silicon dioxide were 16 Angstrom and 20 Angstrom respectively. I-1 is the Ols signal intensity of perfluoropolyether and I-2 is that of silicon dioxide. A C1s XPS signal cannot be used to measure the thickness because the amount of hydrocarbon contamination varies and the perfluoropolyether has many C1s peaks.