Erosion of tungsten-doped amorphous carbon films exposed to deuterium plasmas

Tungsten-doped amorphous carbon films with 0-9.5 at.% W concentration were produced by magnetron sputtering and exposed to deuterium plasmas applying different ion energies and fluences. The partial C and W erosion rates were determined from the C and W areal density changes, respectively, measured by Rutherford backscattering spectrometry. For W-doped films the erosion rate decreases with increasing W concentration and incident fluence. During deuterium plasma exposure carbon is preferentially eroded while tungsten atoms accumulate at the surface leading to the formation of a W-rich layer, which decreases the removal efficiency and leads to a continuous decrease of the erosion rate. At 30 eV/D incident energy a relatively compact W-rich layer is formed on films with higher (≥5%) W concentration which protects the carbon underneath from further erosion. For films with lower (≤2.5%) W concentration the erosion rate decreases, but the erosion process does not stop because the Wrich layer has a high porosity. Reactive neutral species can penetrate through this porous layer and react with carbon atoms below it. At 100 eV/D incident energy the erosion rate is significantly higher compared with 30 eV/D. In addition, slight tungsten sputtering is observed due to the presence of a small fraction of D + plasma ions which impinge with 300 eV/D.