Interfacial phase transitions and bonding in the Cu/Al₂O₃ system

Abstract

We have studied the microstructure and structure of the interfacial region in the Cu/Al₂O₃ system bonded using the gas—metal eutectic process. Using a number of different contrast modes in the transmission electron microscope, we have shown that there exists a very thin amorphous interfacial phase, and this has been identified as a Cu—Al—O glass. We propose a simple mechanism to explain the formation of this glassy phase at the interface. On annealing, this glass crystallizes to the CuAlO₂ and CuAl₂O₄ phases, which in turn, can decompose to Cu, Al₂O₃ and Cu₂O, depending on the local oxygen concentration. The bond strength is reduced by about 25% on annealing, corresponding to the appearance of the crystalline interfacial phases.