Experimental investigations in the intermetallic and microvoid formation in sub-200 °C Cu–Sn bonding

Abstract

This paper reports the intermetallic growth and microvoid formation in the Cu–Sn layers, which were annealed at low temperatures (sub-200°C) for durations varying from 120 to 1440 min. A 10 µm thick tin was electrodeposited on copper samples. Both Cu₆Sn₅ and Cu₃Sn IMCs were formed and had a non-uniform scalloped shaped profile but with different scallops sizes. Void growth was studied at three different locations, i.e., the Cu–Cu₃Sn interface, within the Cu₃Sn, and at the Cu₃Sn–Cu₆Sn₅ interface. The void size in these locations increased with increasing annealing durations and temperatures due to the coalescence of nearby voids. The void fraction at the Cu–Cu₃Sn and Cu₃Sn–Cu₆Sn₅ interfaces was observed to decrease, whereas the void fraction within the Cu₃Sn IMC increased with increasing annealing durations. The largest voids were seen at the Cu–Cu₃Sn interface, while the highest void fraction was found within the Cu₃Sn IMC. The overall void size and void fractions for all experimental conditions were always smaller than 3 µm² and 1.44 µm⁻¹, respectively. The obtained results can be used in the hermetic packaging of MEMS devices performed at sub-200 °C. Processing at these low temperatures result in reduced thermo-mechanical stress and also eliminate the molten tin squeezing-out from the bonding zone, which is a known issue in Cu–Sn solid–liquid inter-diffusion bonding performed at temperature > 232 °C.