Solid State Annealing Behavior of Aluminum Thin Films on Sapphire

摘要

High purity aluminum films were deposited on sapphire substrates by thermal evaporation. Solid state annealing of these specimens resulted in the development of several types of previously unobserved microstructural features. Through thickness drum-like cavities covered with a thin membrane of y-alumina were formed in the film in the vicinity of annealing hillocks. Removal of the overlying aluminum using a KOH etch revealed sub-surface, hollow oxide structures at the aluminum-sapphire interface. Clusters of such features were generally located underneath annealing hillocks. The influence of annealing heat treatment and gas atmosphere on the structure of the films were studied. It was determined that dewetting of the Al film (the initial stage in the formation of the interfacial cavities) occurred only if oxygen was present in the annealing atmosphere. Experiments showed that for samples annealed in air, the thickness of the surface oxide layer was similar to that of the wall thickness of the interfacial oxide structures. The amount of oxygen required for the formation of the interfacial features was estimated based on the feature density, the size and geometry of the features, and the type of alumina (y/ot) present. This quantity was used to estimate a value for an apparent interfacial diffusion coefficient. The findings strongly suggest that short-circuit paths develop in the annealed films that facilitate oxygen transport to the interface.