We have found that controlled Ar ion bombardment enhances the degradation of a-Si:H based surface passivation of c-Si surfaces. The decrease in the level of surface passivation is found to be independent on the ion kinetic energy (7–70 eV), but linearly proportional to the ion flux (6×1014–6×1015?ions?cm?2?s?1). This result suggests that the ion flux determines the generation rate of electron–hole pairs in a-Si:H films, by which metastable defects are created at the H/a-Si:c-Si interface. Possible mechanisms for the ion induced generation of electron–hole pairs are discussed.