Detailed investigation of ultrasonic Al–Cu wire-bonds: II. Microstructural evolution during annealing

Scanning and transmission electron microscopy were used to study the interface composition and morphology of copper wire-bonds heat-treated at 175 C for 2, 24, 96, and 200 h in argon. Detailed morphological and compositional characterization of the Al–Cu heat-treated interfaces was conducted on site-specific specimens prepared by focused ion beam milling. Discontinuous intermetallic grains with varying size and morphology were found to grow in regions where they originally nucleated during the bonding process. The main intermetallic phase was Al2Cu, which was found to grow via solidstate diffusion. In specimens heat-treated for 96 and 200 h, the Al4Cu9 phase was also detected. Void formation at the Al–Cu bonds heat-treated up to 200 h was not found to be a source of bond failure.