Roughness Enhanced Au Ball Bonding of Cu Substrates

Process development studies of Au ball bumping on metallographically polished Cu substrates at ambient temperature were conducted by investigating the effect of process parameters on the ball bond shear force and the extent of bonding. These studies were performed on substrates polished with 0.06-$\mu$m or 1-$\mu$m abrasive solutions so as to assess the effect of surface roughness on bondability. Response surfaces were generated to illustrate the effects of ultrasonic power, bonding force, and time on bond shear force, and process windows were defined as those parametric combinations that yielded bond shear forces of 25gf or higher. After dissolving the Cu substrate away, the etched surfaces of the Au bumps were examined for bonded areas. Au–Cu ball bonds of about 65$\mu$m diameter with bond shear force values higher than 25gf were obtained on 0.06-$\mu$m polished substrates, but at an optimum bonding time of 1000ms. Increase in surface roughness, however, reduced the bonding time considerably, and values as low as 200ms were sufficient to yield bond shear force values higher than 25gf on 1.0$\mu$m polished substrates. Bonding on 1.0-$\mu$m polished substrates not only reduced the bonding time, but also increased the maximum bond shear force and reduced the localization of bonded areas. These results suggest that a greater number of surface asperities of sufficient height on rougher substrates provide more bonding sites and hence improve the bondability.