Molecular dynamics simulations of near-surface modification of polystyrene: Bombardment with Ar + and Ar + / radical chemistries

Molecular dynamics (MD) simulations have been carried out to examine the effects of Ar + , Ar + / H , and Ar + / F bombardment of a model polystyrene (PS) surface. For bombardment with 100 eV Ar + only, the simulations show the formation of a heavily cross-linked dehydrogenated damaged layer in the near-surface region after some initial fluence, consistent with plasma and beam system experimental results. The 1-2 nm thick amorphous carbon-rich modified layer has a much lower sputter yield compared to that of the virgin PS, which has a H:C ratio of 1. Simultaneous bombardment of the damaged dehydrogenated PS layer with 300 K H or F radicals and 100 eV Ar + can facilitate the removal of the layer as well as inhibit its initial formation. The development of the steady-state dehydrogenated layer under Ar + -only bombardment results from a competition between the breaking of carbon-hydrogen bonds (which leads to dehydrogenation and subsequent cross-linking) and the breaking of carbon-carbon bonds (which leads to sputtering of polymer fragments). For the conditions presented in this study, the loss of hydrogen eventually overtakes the removal of polymer fragments, resulting in the formation of the dehydrogenated cross-linked near-surface layer. The final properties of the dehydrogenated layer from the MD simulations are compared at steady state to ellipsometric data for plasma-exposed PS samples, and the initial and final sputter yields from MD are compared to experimental beam system data.