Void Formation in Low-Temperature Electroplated Cu-Sn Stack for Hermetic Packaging

Electrodeposited Cu-Sn based solid-liquid interdiffusion (SLID) bonding is a popular technique used for the hermetic encapsulation in various MEMS applications. However, the micro-void formation remains a serious problem that critically degrades the bond strength and the long-term reliability of the MEMS devices. In this study, the void formation and intermetallic growth in electrodeposited Cu-Sn stack layer at temperatures below and above the melting point of tin, i.e., 232 °C are reported. The samples were fabricated on 2-inch silicon substrates using standard microfabrication techniques. Voids were observed to be larger in samples annealed at temperatures above the melting point of tin. This was due to the enhanced Kirkendall's effect as well as higher dissociation and segregation of incorporated impurities in the electrodeposited metals at higher processing temperatures. Successful bonding of electrodeposited tin over the copper pad (bottom-side) to the bare copper pad (top-side) was demonstrated at low temperature, i.e., 150 °C with the minimum voids at bond interfaces and within the intermetallic compounds.