Energy dependence of the electron attenuation length in silicon dioxide

An accurate knowledge of the attenuation lengths of photoelectrons is necessary for quantitative analysis by x-ray photoelectron spectroscopy (XPS), especially for the determination of the thicknesses of thin films. Imamura et al (2011 Meas. Sci. Technol. 22 024007) have recently reported measured values of the attenuation lengths of electrons in SiO2 in the energy range 150 to 850 eV using synchrotron x-rays that diverge systematically from the theoretical predictions. Their analysis depends on layer thicknesses, measured by x-ray reflection (XRR) analysis and the attenuation of the intensity of Si 2p elemental x-ray photoelectrons from the Si wafer substrate, through the oxide layer, along the surface normal. It is shown that both the XRR and x-ray photoemission methods, as used, involve bias effects. The bias in the XRR data is removed using the thickness differences. The bias in XPS is shown from new data on the forward focusing of the Si 2p substrate photoelectrons at 1152 eV, using Mg x-rays. This shows that the polar intensity falls faster than predicted by a simple attenuation length analysis but requires consideration of the intensity attenuation resulting from strong uncompensated elastic scattering. After these corrections, which are much greater than Imamura et al's stated uncertainties, a significantly closer agreement is found between the theoretical predictions and Imamura et al's experimental data