Surface layer impurities on silicon spheres used in determination of the Avogadro constant

The Avogadro constant is required to be determined with an uncertainty of l ess than 1 x 10(-8) in order to allow an atomic definition of the kilogram, A single-crystal silicon sphere 93.6 mm diameter is used for this determin ation, A thin surface layer (typically 2 nm to 5 nm thick on flats and 10 n m or more on spheres) of contaminants such as oxide, water and hydrocarbons on the sphere can significantly affect the measurements due to corrections for density changes and to phase change on reflection in the diameter meas urement by optical interferometry. The stability of this surface layer as a function of time is also of importance because of ongoing measurements. Th e nature of this contamination has been investigated using optical ellipsom etry and ion beam analysis, It is concluded that the composition and struct ure of the surface layer are affected by a number of parameters and that th e most appropriate method of achieving the desired accuracy is to remove th e surface layer by etching and to form a hard stable coating of controlled thickness and composition. This coating may be either silicon dioxide or si licon nitride.