Abstract: A method of building up an oxidized metal coating on a substrate in which the substrate is positioned adjacent a metal target in a chamber containing a low pressure mixture of inert and reactive gases; and a voltage having AC and DC components is applied to the target to sputter onto the substrate a thin layer of elemental metal and oxidized metal. The substrate is then moved to an oxidizing station in the chamber and the layer is subjected to reactive ions, as for example a beam of reactive ions, which oxidizes the elemental metal in the layer. The steps are repeated until an oxidized metal coating of a desired thickness has been built up.