Alumina coatings were deposited on silicon (111 orientation) substrates by reactive ac magnetron sputtering. Film deposition was done using Al targets and three O-2/Ar gas flow rate ratios at 5 kW power. X-ray diffraction studies showed that films were crystalline and contained several phases of alumina. Secondary ion mass spectroscopy analyses were used to measure O/Al atomic ratio and Ar and H concentrations in the films. Hydrogen content in the coatings depended on the O-2 partial pressure used during sputtering and also on the arrival rate of Al and O species on the substrates and seemed to influence the crystallinity of the coatings. (c) 2007 American Vacuum Society.