Influence of elastic-electron scattering on measurements of silicon dioxide film thicknesses by x-ray photoelectron spectroscopy

We investigate the systematic error due to neglect of elastic scattering of photoelectrons in measurements of the thicknesses of thin films Of SiO2 on Si by x-ray photoelectron spectroscopy (XPS). Calculations were made of su bstrate Si 2p photoelectron currents excited by Mg and Al K alpha x rays fo r different SiO2 thicknesses, different angles of photoelectron emission, a nd three representative XPS configurations using an algorithm based on the transport approximation. We calculated practical effective attenuation leng ths (EALs) from changes of the computed Si 2p photoelectron cur-rents. Thes e EALs were less than the corresponding inelastic mean free paths by betwee n 6.5% and 9.4%, with the difference depending on the x-ray source, the spe cific range of SiO2 film thicknesses under consideration, the XPS configura tion, and the range of photoelectron emission angles. Useful average values of the EAL were found for emission angles between 0 degrees and about 60 d egrees (with respect to the surface normal) and for silicon dioxide thickne sses such that the substrate signal was attenuated to not more than 1% and 10% of its original value. Our calculated EALs are in satisfactory agreemen t with measured EALs., For larger emission angles, the calculated EALs chan ge rapidly with SiO2 thickness, and specific values should be found for the conditions of interest. (C) 2001 American Vacuum Society.