Behavior of aluminum oxide, intermetallics and voids in Cu-Al wire bonds

Nanoscale interfacial evolution in Cu-Al wire bonds during isothermal annealing from 175 °C to 250 °C was investigated by high resolution transmission electron microscopy (HRTEM). The native aluminum oxide film (∼5 nm thick) of the Al pad migrates towards the Cu ball during annealing. The formation of intermetallic compounds (IMC) is controlled by Cu diffusion, where the kinetics obey a parabolic growth law until complete consumption of the Al pad. The activation energies to initiate crystallization of CuAl₂ and Cu₉Al₄ are 60.66 kJ mol^-1 and 75.61 kJ mol^-1, respectively. During IMC development, Cu₉Al ₄ emerges as a second layer and grows together with the initial CuAl₂. When Al is completely consumed, CuAl₂ transforms to Cu₉Al₄, which is the terminal product. Unlike the excessive void growth in Au-Al bonds, only a few voids nucleate in Cu-Al bonds after long-term annealing at high temperatures (e.g., 250 °C for 25 h), and their diameters are usually in the range of tens of nanometers. This is due to the lower oxidation rate and volumetric shrinkage of Cu-Al IMC compared with Au-Al IMC.