[en] Full text: Silicon surface oxide composition and stability are important parameters in modern silicon device production. The use of silicon as a 1 kg ultra pure single crystal silicon sphere for determination of the Avogadro constant requires very precise knowledge about composition and stability of the surface oxide layer. The current definition of the kilogram in terms of a Pt/Ir artefact can be replaced by an atomic definition in terms of the Avogadro constant if the final uncertainty in sphere volume (diameter) and mass measurement is less than 1 x10^-8. A monolayer of silicon oxide on the sphere surface can contribute uncertainties of this order. In this work we demonstrate how the silicon surface oxide layer can be measured with very high resolution RBS in channelling mode using a 2 MeV He ion beam. This method uses very high RBS depth resolution in the glancing angle detection geometry (93 deg) and channelling background reduction in order to obtain the desired sensitivity to surface layer components. Results will be presented for the analysis of a 2 nm thick silicon oxide layer on a silicon single crystal substrate showing well resolved RBS signals from carbon, oxygen and silicon from the surface layer. This work is part of an ongoing international collaborative effort involving several national metrological laboratories