Chemical sputtering of hydrocarbon films

Abstract

Erosion of hydrocarbon films at room temperature due to argon ions and thermal atomic hydrogen is investigated in a particle-beam experiment. Physical sputtering by the ions is observed at energies ⩾200 eV and reaches a yield of 0.5 at an ion energy of 800 eV. The measured yields are in agreement with TRIM.SP computer simulations, and a threshold energy of ≃58 eV is derived for physical sputtering. Erosion by simultaneous fluxes of argon ions and thermal hydrogen atoms is observed at all energies investigated down to 20 eV and reaches a yield of about 3 at an ion energy of 800 eV and a hydrogen-atom-to-argon-ion-flux ratio of 400. It is proposed that the significant decrease of the threshold energy as well as the increase of the absolute yields is due to the process of chemical sputtering: Within a collision cascade caused by the incident ions, bonds are broken and instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the common range of ions and hydrogen atoms. Finally, the molecules diffuse to the surface and desorb. The threshold energy of chemical sputtering is on the order of typical carbon–carbon bond energies in organic compounds of several eV. Based on this mechanism a model for the energy dependence of the chemical sputtering yield is presented, which leads to good agreement with the data.