Characterization of intermetallic compounds in Cu-Al ball bonds: Layer growth, mechanical properties and oxidation

In high power automotive electronics copper wire bonding is regarded as most promising alternative for gold wire bonding in 1^st level interconnects and therefore subjected to severe functional requirements. In the Cu-Al ball bond interface the growth of intermetallic compounds may deteriorate the electrical, thermal and mechanical properties. The layer growth and properties of these intermetallic compounds are crucial in the prediction of the long term behavior. To mimic the growth of intermetallic compounds during and after copper ball bonding, diffusion couples of aluminium and copper were annealed at 225-500°C and chemically analyzed by SEM/EDS. Also five separate intermetallic compounds were melted together from the pure elements and aged in evacuated quartz ampoules for 240 hours at 500°C. In this work values for the indentation Young´s modulus, load independent hardness, fracture toughness and volumetric densities are given. For four intermetallics in the diffusion couple indentation Young´s modulus and hardness were determined using a universal nano-indenter (UNAT). The combination of nano-indentation (10-500mN), micro-Vickers (0.5-2N) and Vickers hardness measurements (10-49N) gave hardness values over a wide range of loads. The reduction of the apparent hardness is explained by the indentation size effect. Fracture toughness was measured with Vickers indentation, volumetric densities with Archimedes method. It can be concluded that the Cu-rich intermetallics Cu₉Al₄ and Cu₃Al₂ are less sensitive to fracture and have lower average densities than the other intermetallic compounds. The volumetric decrease during formation causes internal stress. Finally, the difference in oxidation behaviour of the intermetallic compounds and the Cu solid solution was demonstrated through experiments performed on the diffusion couples.