XPS Analysis of a ²⁸Si-Enriched Sphere for Realization of the Kilogram

The National Metrology Institute of Japan (NMIJ) will realize the new kilogram by the X-ray crystal density (XRCD) method using a ²⁸ Si-enriched sphere. This article describes an accurate thickness measurement of a surface layer on the Si sphere by X-ray photoelectron spectroscopy (XPS). We focused on the effect of crystal orientation of the core sphere on thickness determination by XPS. The changes in the intensities of the Si core sphere and a surface oxide along the main crystallographic directions were first investigated. The variations in intensities were enhanced, resulting in a large variation in thickness. By avoiding the lines of longitude of crystallographic planes with low plane indices, the uncertainty of the thickness of the oxide layer (OL) was reduced. The structure of the carbonaceous layer (CL), including ethanol and hydrocarbon sublayers on the sphere, was also investigated, and the average thickness and uncertainty budget were evaluated.