Chemical sputtering of hydrocarbon films by low-energy Ar+ ion and H atom impact

Erosion of hydrocarbon films is investigated in a particle-beam experiment employing sources for argon ions and hydrogen atoms. Sputtering by argon ions sets in above a threshold of 58 eV and reaches a yield of 0.5 at an ion energy of 800 eV. Sputtering by argon ions with an additional flux of thermal hydrogen atoms towards the surface occurs above a threshold of 1.3 eV and reaches a yield of about three at an ion energy of 800 eV and a hydrogen atom to argon ion flux ratio of 400. A pronounced dependence of the Yield on this flux density ratio is observed. It is proposed that the shift of the threshold energy as well as the change in the absolute yields is due to the process of chemical sputtering: within a collision cascade of the incident ions, broken bonds are instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the ion penetration range, which diffuse to the surface and desorb. This has important implications for the lifetime of plasma wall components in the divertor region of next step nuclear fusion devices.