Comparison of ion energies and fluxes at the substrate during magnetron sputtering of ZnO : Al for dc and rf discharges

Energy spectra of positive and negative ions have been measured in magnetron sputtering of a ZnO : Al target. The discharges have been operated in dc or rf, the latter with a frequency of 13.56 or 27.12 MHz, at the same geometry, argon pressure, and discharge power. While the average energy of the positive ions against a grounded surface increases with increasing frequency due to increased plasma potential, the average energy of the negative ions strongly decreases due to a target voltage decrease. The flux of negative ions simultaneously decreases, whereas that of the positive ions slightly increases. Combining these quantities to the energy flux onto floating substrates shows a drastic decrease of the energy flux with increasing frequency for high-energetic negative ion bombardment of the films while the low-energetic positive ion impingement is weakly increased. From the point of the energetic bombardment rf discharges are hence favoured to obtain high-quality films. The energy distributions of negative ions have a sharp peak in the dc mode but broad distributions in the rf discharges. Additionally, the latter are characterized by an overlaid peak structure that is explained by the oscillation of target and plasma potential.