Accurate determination of the surface normal for the reliable measurement of ultra-thin SiO₂ thickness by x-ray photoelectron spectroscopy

Measurement of the thickness of SiO₂ films on crystalline Si substrates was the subject of the key comparison (K-32) of the Surface Analysis Working Group of the Consultative Committee for Amount of Substance. X-ray photoelectron spectroscopy (XPS) gave the most reproducible results that are consistent with other reflectivity based methods. In addition, XPS is free from surface contamination effects. However, the emission angle of photoelectrons is one of the main sources of the uncertainty in the measured thickness. In this report, we propose a simple and reliable procedure to determine the surface normal for accurate control of the emission angle using an amorphous SiO ₂ overlayer on an amorphous Si substrate. The surface normal can be accurately determined from the condition that the estimated thickness of a SiO₂ overlayer on an amorphous Si substrate measured must be the same for different emission angles. With the proposed calibration procedure, the surface normal can be determined precisely and the uncertainty of the electron emission angle can be evaluated.