Roles of temperature and humidity in the oxidation of sputter-deposited Cu–Ta alloys in air

Abstract Oxidation behavior of a series of Cu–Ta alloys prepared by co-deposition of RF sputtered pure copper and tantalum was studied in air. Alloy composition, oxidation temperature and relative humidity (RH) ranges examined were 5–69 at.% Ta (alloys with 22–69 at.% Ta were amorphous), 40–85°C and 20–70%, respectively. Although Cu–58at.%Ta alloy showed no apparent change, some of the alloys exposed to an air at 70°C and RH=70% for 60 h showed change in color. The Cu–5at.%Ta alloy showed bright blue, and alloys with 10–30 at.% Ta were all turned to reddish color by oxidation. Among the alloys examined, Cu–24at.%Ta alloy showed the highest oxidation rate under the above condition. The in-depth profiles of Cu–24at.%Ta alloy specimens oxidized for 6–60 h in the air of 70°C and RH=70% indicated existence of metallic Cu-layer on the top of Ta-rich oxide layer and Cu-enriched alloy layer under the oxide film. The metallic copper layer must be caused by diffusion of metallic copper from the Cu-enriched alloy layer to the top of the surface similarly to the oxidation of Cu–Zr alloys at room temperature in a laboratory air. Even Cu–Ta alloys which show very high corrosion resistance in HCl solutions does not always have high corrosion resistance to oxidation in air because of difference in surface film formation process and the resulting compositions and film structure.