Effect of Si contamination of a reactor on the determination of surface loss coefficients of hydrogen atoms

Abstract

Surface loss coefficients of hydrogen atoms (H) are of great importance, in particular, for numerical simulations of industrial plasma processes. In this work, the surface loss coefficient of H atoms was obtained for stainless steel and tungsten as a function of the plasma treatment time. The surface loss coefficients were determined by measuring the density distribution of H atoms, close to the investigated surfaces, using two-photon absorption laser-induced fluorescence, while H atoms were generated in an electron-cyclotron resonance plasma at 5 Pa. In parallel, the surface element composition was monitored using x-ray photo-electron spectroscopy (XPS). An exponential decay of the surface loss coefficient as a function of the plasma treatment time was observed on both surfaces due to surface contamination by silicon (Si) detected thanks to the XPS measurements. Possible sources of Si contamination were explored, among which the employed commercial plasma source turned out to be such, whereupon a new Si-free chamber was built. In the Si-free reactor, values of 0.34±0.02 and 0.33±0.02 for the surface loss coefficients of hydrogen atoms were obtained on stainless steel and tungsten, respectively. Cross-comparison with XPS measurements confirmed the absence of any Si contamination. This work demonstrates the influence of Si contamination of the sample surfaces on the surface loss coefficient, highlighting the necessity for surface and vacuum characterization when reporting on surface loss coefficient measurements. Better characterization and reporting of a surface state and composition would allow for more conclusive data and easier comparison between different setups.