Molecular size effect in the chemical sputtering of a-C:H thin films by low energy H+, H2+, and H3+ ions

We have experimentally determined total carbon yields per incident H atom in the energy range 36-300 eV/H for H{sup +}, H{sub 2}{sup +}, and H{sub 3}{sup +} projectiles incident normally on {approx}60 nm thick a-C:H films, using 2-D ellipsometry determination of erosion crater volumes ex vacuo, the separately characterized thin film carbon density, and the incident beam current integration accumulated on target during the crater evolution. During each beam exposure, methane production was monitored using in situ quadrupole mass spectrometry (QMS). The present total carbon yields/H for incident H{sub 3}{sup +} ions obtained via ellipsometry are in agreement with total mass loss measurements for H{sub 3}{sup +} by Balden and Roth over the investigated energy range. The observed methane production per incident H for the molecular ions exhibits molecular size effects over the entire energy range investigated, confirming the trend observed in the ellipsometry-based total C yields/H.