Mutual calibration by a combination of a zero-offset method (ZOM) and a length-unit traceable method (LTM) was suggested as a method to determine the true thickness of ultra-thin oxide films. Although, medium energy ion scattering spectroscopy (MEIS) and x-ray photoelectron spectroscopy (XPS) were suggested as the ZOMs, the offset value of thickness is affected by the surface contamination layer.

In this study, we compared the effect of the surface contamination layer on the thickness measurement of a series of HfO₂ films by MEIS and XPS. The surface of the HfO₂ films were contaminated by thin carbon layers with the thicknesses from 0.25 nm to 1.30 nm. In MEIS, the variation of the offset value by the surface carbon layer was negligibly small in the range of 0.020 nm. However, it was large in XPS in the range of 0.131 nm. In conclusion, MEIS can be a reliable ZOM to calibrate the offset values in the thickness measurement of ultra-thin oxide films with negligibly small effect of surface contamination.