Characterization of hermetic wafer-level Cu-Sn SLID bonding

A flux-less copper-tin (Cu-Sn) solid-liquid inter-diffusion (SLID) bonding process, providing a cost-effective hermetic vacuum sealing at wafer-level, has been investigated. Observations have been made indicating that the storage time of Cu-Sn plated wafers before bonding is critical with regard to voiding. Growth of the intermediately formed intermetallic compound (IMC), Cu6Sn5, was investigated as a possible cause. Room temperature aging of Cu-Sn plated wafers prior to bonding was performed as well as annealing of un-bonded Cu-Sn plated wafers. The presence of large Cu6Sn5 and Cu3Sn crystallites which nearly depleted the Sn was observed by optical microscopy after annealing. If large Cu6Sn5 grains from opposite contact planes meet at the bond interface, voids are predicted to be formed during the subsequent stages of liquid inter-diffusion and solidification. Implications on the Cu-Sn bonding strategy based on the results are presented.