Microstructural design in ultrafine interconnects under current stressing

Electromigration has a potential to be used as a process for microstructural design in ultrafine interconnects such as TSVs and microbumps. This study investigates the relationship between the electromigration and the microstructural evolution in a Cu/Sn-microbump/Cu structures through a modeling and simulation approach. The electromigration-induced polarity effect on the interfacial Cu₆Sn₅ and Cu₃Sn compounds as well as the acceleration of the Cu pad consumption are found, as previously reported in experimental studies. In addition, the simulation results predict that the Cu₆Sn₅ grains at the anode become finer when an current is applied. The polarity effect is weaker when the pad size increases from 10 μm to 20 μm. In the larger microbump, the growth of the interfacial Cu₆Sn₅ is slower, while the growth of the interfacial Cu₃Sn and the consumption of the Cu pad is faster. The growth of the interfacial IMCs and the consumption of the Cu pad at both electrodes are faster in the hourglass-shaped microbump. The interfacial Cu₆Sn₅ layer at the anode in the barrel-shaped microbumps appears to be convex, while concave in the hourglass-shaped microbump. The polarity effect becomes more pronounced when bulk Cu₆Sn₅ grains in the Sn microbump are dissolved and redeposited onto the interfacial Cu₆Sn₅ at the anode. The bulk Cu₆Sn₅ grains are predicted to migrate towards the anode driven by the electric current.