The thickness measurement of ultra-thin SiO2 films thinner than 1 nm was studied by X-ray photoelectron spectroscopy (XPS). Amorphous SiO2 thin films were grown on amorphous Si films to avoid the thickness difference due to the crystalline structure of a substrate. SiO2 thin films were grown by ion beam sputter deposition under oxygen gas flow and the thickness was measured by in situ XPS. The attenuation length was determined experimentally by a SiO2 film with a known thickness. The straight line fit between the measured thickness using XPS and the nominal thickness showed a good linear relation with a gradient of 0.969 and a small offset of 0.126 nm. The gradient measured at the range of 3.4-0.28 nm was very close to that measured at sub-nanometer range of 1.13-0.28 nm. This result means that the reliable measurement of SiO2 film thickness below 1 mn is possible by XPS. (c) 2005 Elsevier B.V. All rights reserved.