Molecular dynamics simulations of oxygen-containing polymer sputtering and the Ohnishi parameter

The effects of ion bombardment on polymer surfaces can be profound, with implications for all plasma-based pattern transfer processes that involve the use of polymer etch masks in lithography and etching. We report results from molecular dynamics (MD) simulations of Ar⁺ (100 eV) sputtering of oxygen-containing polymers. The MD data are compared with available experimental results, with special focus on the so-called Ohnishi parameter, which has been shown to correlate with sputtering yields for many O-containing polymers. The MD simulations match the measured data as well as the published correlations. We present a quantitative model of sputtering for these polymers that shows why the Ohnishi parameter (a function of the polymer composition) is proportional to the steady-state sputtering yield. However, we also show that the Ohnishi parameter does not correlate with yields for other polymers, including polyfluoroethylene and polyethylene. The MD simulations show that the validity of this parametrization is dependent on whether or not the sputtering of the polymer transitions between ion-induced scissioning and cross-linking at steady state.