Characterization of hermetic wafer-level Cu-Sn SLID bonding

Abstract

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), Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> , 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 Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> and Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn crystallites which nearly depleted the Sn was observed by optical microscopy after annealing. If large Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> 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.