Reliability of wafer-level SLID bonds for MEMS encapsulation

Optimal performance of a MEMS device often requires hermetic packaging to enable proper functionality throughout the device lifetime. Solid-Liquid Interdiffusion (SLID) bonding is an attractive way to encapsulate MEMS devices in wafer level since it provides: i) possibility to use low temperature metal bonding process, ii) metallic seal rings don´t take much out of the valuable surface area, iii) ductile metals can adopt mechanical and thermo-mechanical stresses and iv) metals have lower gas permeability compared to polymer or glass based sealing materials. Although metal bonding utilizing different binary metal-systems is becoming more common there is only a limited amount of information available on the defects of the bond and their effect on the reliability. In this work four typical defects of AuSn and CuSn seal rings, which are used for wafer-level hermetic bonding for MEMS resonators, are identified as non-uniform plating thickness, bubbles in electrodeposited metallizations, voids in bonded seal rings, and excessive amount of tin. Shear test was used to compare the mechanical properties of the interconnections in both material systems, and to evaluate the effects of different defects on failure mechanisms. In addition, the effects of thermo-mechanical treatments on the reliability performance are also studied.