We have used the National Institute of Standards and Technology (NIST) Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) [1,2] to simulate photoelectron intensities for thin films of SiO_1.6N_0.4 and HfO_1.9N_0.1 on silicon with excitation by Al Kα X-rays. We considered Si 2p_3/2 photoelectrons from SiO_1.6N_0.4 and the substrate and Hf 4f_7/2 photoelectrons from HfO_1.9N_0.1. The simulations were performed for ranges of film thicknesses and photoelectron emission angles and for two common configurations for X-ray photoelectron spectroscopy (XPS), the sample-tilting configuration and the Theta Probe configuration. We determined photoelectron effective attenuation lengths (EALs) by two methods, one by analyzing photoelectron intensities as a function of film thickness for each emission angle (Method 1) and the other by analyzing photoelectron intensities as a function of emission angle for each film thickness (Method 2). Our analyses were made with simple expressions that had been derived with the assumption that elastic-scattering effects were negligible. We found that EALs from both methods were systematically larger for the Theta Probe configuration, by amounts varying between 1 % and 5 %, than those for the sample-tilting configuration. These differences were attributed to anisotropy effects in the photoionization cross section that are expected to occur in the former configuration. Generally similar EALs were found by each method for each film material although larger EALs were found from Method 2 for film thicknesses less than 1.5 nm. SESSA is a useful tool for showing how elastic scattering of photoelectrons modifies EALs for particular materials, film thicknesses, and XPS configurations.

[1] http://www.nist.gov/srd/nist100.cfm .

[2] W. Smekal, W. S. M. Werner, and C. J. Powell, Surf. Interface Anal. 37, 1059 (2005).