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Postprint Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar / O 2 mixtures

The ion flux obtained during reactive magnetron sputtering of an Al target in Ar/ O 2 gas mixtures was studied by energy-resolved mass spectrometry, as a function of the total and O 2 partial pressures. The positive ions of film-forming species exhibited bimodal energy distributions, both for direct current and radio frequency discharges, with the higher energy ions most likely originating from sputtered neutrals. For the negative oxygen ions a high-energy peak was observed, corresponding to ions formed at the target surface and accelerated towards the substrate over the sheath potential. As the total pressure was increased the high-energy peaks diminished due to gas-phase scattering. Based on these results, the role of energetic bombardment for the phase constituent of alumina thin films are discussed.