Oxidation behavior of sputter-deposited Cu-Ta alloys in air

Surface region of a series of Cu-Ta alloys prepared by sputter-deposition were characterized by XRD, XPS and AES. Just after preparation, Ta was enriched in the topmost region as an oxide film while Cu was enriched under the surface film. The surface film thickness on alloys increased by a 5 h air-exposure. The surface film thickness after 5 h exposure to air plotted against alloy composition had a large maximum at 22 at.% Ta. The surface films consisted of (Cu, Ta)Ox(OH)y·nH2O type compound where Cu and Ta were enriched in low and high Ta alloys, respectively, after 5 h exposure although just Ta was enriched in the surface film on as-prepared alloys. Enrichment of Cu in the metallic state under the surface film was observed on all the alloys. When the oxidation proceeds on low Ta alloys, enriched metallic Cu under the surface film diffuses through the surface film to the top surface to form copper oxide film. On high Ta alloys, however, the Ta-rich surface oxide film on high Ta alloys is protective enough to prevent diffusion of Cu through the surface film. The composition dependence of the oxidation behavior of Cu-Ta alloys in air is largely related to the protectiveness of the initial oxide film.