The effect of ion bombardment on the relationship between the critical hydrogen concentration and the reactor pressure has been investigated for hydrogenated amorphous silicon (a-Si:H) deposited with the expanding thermal plasma-CVD (ETP-CVD) method. By changing the reactor pressure the ionic cluster formation in the plasma can be varied. It is observed that the decrease of the critical hydrogen concentration with increasing reactor pressure can not be compensated by ion bombardment at 14V biasing. Biasing with 20V however increases the critical hydrogen concentration nearly up to the value obtained at low pressures. This indicates that the incorporation of ionic cluster formed at elevated reactor pressures can be reduced by substrate biasing, possibly due to break-up upon impact on the substrate surface or due to processes occurring in the secondary plasma close to the substrate.