Solid-Liquid Interdiffusion (SLID) bonding — Intermetallic bonding for high temperature applications

Solid-Liquid Interdiffusion (SLID) bonding has emerged as a promising bonding technique, particularly for high-temperature applications. It uses well-establish processing steps such as electroplating, and it is well suited for wafer-level processing. SLID is based on intermetallic compounds (IMCs) as the bonding medium, enabling a thermal stability at temperatures well above the bonding temperature. In this paper, we present our work on Cu-Sn and Au-Sn SLID bonding. For Cu-Sn SLID bonding, our focus has been on wafer-level processing and optimization of manufacturing parameters. We present successful flux-free Cu-Sn SLID bonding for hermetic sealing, and an experimentally obtained kinetics model for IMC formation as function of time, showing different mathematical relations above and below the melting point of Sn. The model is used for predicting the effect of a given bonding temperature profile. For Au-Sn SLID bonding, our focus has been on phase identification and verification of reliability. The bond is found to reach thermal equilibrium only after high-temperature storage, and an updated design rule for metal layer thicknesses is established. Importantly, a high reliability of Au-Sn SLID bonding is verified: Bonding materials with different Coeffitions of Thermal Expansion and exposing the assemblies to high temperature storage followed by thermal cycling, very high bond strengths (~70 MPa) and no failures are observed.