An investigation of cascade energy density effects using classical trajectory simulations of sputtering by molecular ions.

Abstract

The NPS computer simulation model was modified to study sputtering by molecular ions. The simulations were performed on a Cu(111) surface for normally incident O⁺₂ ions at 0.5, 1.25, 2.5, and 5.0 kev/ion. The molecule's angular orientation was both set at specific values and randomly determined. The normalized energy distributions of sputtered atoms for two energy ranges, 0-20 and 0-3 ev/sputtered atom, and the sputtering yield ratios were investigated. The 0-20 ev/sputtered atom energy distributions show distinct peaks at 1.8 ev which are insensitive to incident energy, energy density, and molecular orientation. Simulations indicate that there may be fine structure in the 0-3 ev/sputtered atom range with a cascade interaction unique to molecular sputtering. For a number of molecular orientations which produce very different cascade overlaps, the sputtering yield ratios do not correlate with the degree of overlap. The energy density of a cascade is not an accurate predictor of the sputtering yield ratio.