Determination of SiO2 and C layers on a monocrystalline silicon sphere by reference-free x-ray fluorescence analysis

The redefinition of the kilogram will be based on Planck's constant, which can be calculated from Avogadro's constant, and hence by 'counting' the silicon atoms in a monocrystalline 28Si sphere weighing one kilogram. To reduce the influence of the surface layers on the determined value requires, amongst other issues, an accurate quantification of these layers on the 28Si spheres. Apart from the native SiO2 layer, carbonaceous contamination layers and adsorbed water molecules are expected to be crucial in determining the surface composition. The overall mass contribution of these surface layers must be determined with an accuracy below 10% in order to be able to meet the targeted uncertainty budget for the redefinition. In this work, we performed a quantification of these surface layers, applying reference-free x-ray fluorescence spectrometry using a modified manipulator within an ultra-high vacuum chamber. Using this setup, we are able to quantitatively assess the surface of the spheres on a large area (about 20%) with quantification uncertainties below the required target of 10% for oxygen and carbon in order to meet an absolute uncertainty value of the mass below 10 µg.