Microstructural characterization and mechanical performance of wafer-level SLID bonded Au-Sn and Cu-Sn seal rings for MEMS encapsulation

Special applications, such as microelectromechanical systems (MEMS), often require hermetic sealing in order to achieve a desired operation. Solid–liquid interdiffusion (SLID) bonding is an attractive method for encapsulating MEMS devices at the wafer-level, providing, e.g., high re-melt temperatures and tolerance for topographical variations. Several different SLID bond solutions have been investigated; however, there are only a limited number of published reliability studies available. In this paper, wafer-level Au-Sn and Cu-Sn SLID seal rings were mechanically characterized with shear and tensile tests. The evolution of bond microstructures and consequent effects on mechanical reliability were evaluated with a mixed flow gas test, a high temperature storage test and a thermal shock (TS) test. Virgin samples showed high mechanical strength. The Au-Sn system, with a thin Ni layer between the TiW adhesion layer and the bond, demonstrated a shear strength of 170 MPa. Cu-Sn, with a Cu-Cu₃Sn-Cu structure, exhibited a shear strength of 275 MPa. Statistically significant decreases in strength were identified after reliability tests. The shear strength of the Au-Sn bond with an (AuSn + Au₅Sn)eut structure decreased 40% in a corrosive environment. After 3000 TS cycles, the tensile strength of the Cu-Sn bond reduced by 45%. Fracture surface analysis revealed through-bond failures that were not observed previously. In cross-sectional analysis, vertical cracks were observed, which may contribute to the decrease in tensile strength.