Copper on sapphire: Stability of thin films at 0.7 Tm

Abstract

Thin films of copper deposited on the (11$\text{2}$0) plane of sapphire disintegrated into clusters when annealed at 650°C under nonoxidizing conditions. The breakdown was found to initiate at processing defects in the film; this is in contrast to earlier work with zirconia films on sapphire where cavities nucleated profusely at grain boundaries due to poor wetting between zirconia and alumina. A comparison between these two cases, as well as a theoretical model, suggest that processing defects are the source of instability when the contact angle is less than (α + π/2), where α is one half of the dihedral angle formed by a grain boundary with the free surface. The breakdown of the film eventually led to separated, single crystal beads of copper. The beads coarsened and changed their orientation by solid state diffusion, leading to a highly preferred orientation. The {l11} plane of copper was found to be parallel to the sapphire surface. But the texture was planar isotropic, that is, it was rotationally symmetric around the plane normal. This result is in agreement with other observations that copper forms {111} planar isotropic texture on (0001) sapphire, and also on polycrystalline α-alumina.