Shape Change of Cu Thin Films on Nonreacting Metal Substrates and the Effects of Oxidation during Vacuum Annealing

The stable shape of metallic thin films on substrates depends on the energy balance of the surface and interface and is strongly influenced by surface oxidation. However, the effects of oxidation are not clearly understood and are often limited to phenomenological discussions. In this study, we aimed to quantitatively understand and predict how oxidation affects the morphology of metallic thin films on various substrates. We investigated the shape change of Cu thin films on metallic substrates (Ru, Co, Mo, W, Nb, and Ta) after annealing in a vacuum at 400 °C. The observed Cu morphology on the Ru and Co substrates was the formation of Cu islands on a thin continuous Cu layer without exposure of the substrate surface. On the W, Nb, and Ta substrates, it was Cu islands with the exposed substrate surface. To understand the experimental observation, we carried out first-principles calculations of surface and interface energies. The effect of oxidation on the surface energy was also considered. A good quantitative correlation was found between the morphology change of the Cu thin film and the oxidation tendency of the metal substrates.