Correlation between outward diffusion of additives and oxidation of Cu(3.9 at. % Ti) and Cu(2.3 at. % Ta) thin films

Thin films of pure Cu and Cu alloys (with 3.9 at. % Ti or 2.3 at. % Ta) are deposited on SiO₂/Si substrates by magnetron sputtering. Upon annealing at 700 °C in vacuum for 30 min, Ti in the Cu(3.9 at. % Ti) films will mostly diffuse to the free surface, but the majority of Ta in the Cu(2.3 at. % Ta) film remains within the Cu layer. The outward diffusion of Ti or Ta strongly influences the oxidation of Cu in air. The degree of Cu oxidation was determined by glancing incident angle x-ray diffraction and the normalized sheet resistance. Oxidation test shows that the Cu(3.9 at. % Ti) films exhibit a superior oxidation resistance when oxidized at 200 °C in air, especially for the Cu(3.9 at. % Ti) film which has been preannealed at 700 °C in vacuum. But the Cu(2.3 at. % Ta) film with the same preannealing treatment only slightly improves the oxidation resistance, in comparison with the pure Cu film. The correlation between outward diffusion of additives and oxidation of Cu in the Cu(3.9 at. % Ti) and Cu(2.3 at. % Ta) thin films is discussed.