Abstract
Au–Sn solid–liquid interdiffusion (SLID) bonding is a novel and promising interconnect and die attach technology for high temperature (HT) applications. In combination with silicon carbide (SiC), Au–Sn SLID has the potential to be a key technology for the next generation of HT electronic devices. However, limited knowledge about Au–Sn SLID bonding for HT applications is a major restriction to fully realizing the HT potential of SiC devices. Two different processing techniques—electroplating of Au/Sn layers and sandwiching of eutectic Au– Sn preform between electroplated Au layers—have been studied in a simplified metallization system. The latter process was further investigated in two different Cu/Si3N4/Cu/Ni–P/Au– Sn/Ni/Ni2Si/SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on as-bonded, thermally cycled, and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes, and environmental stress tests. A uniform Au-rich bond interface was produced (the ζ phase with a melting point of 522 °C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au–Sn SLID can absorb thermo-mechanical stresses induced by large coefficient of thermal expansion mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au–Sn SLID is shown to be superb, exceeding 78 MPa. However, after HT thermal ageing, the ζ phase was first converted into the more Au-rich β phase. This created physical contact between the Sn and Ni atoms, resulting in brittle NixSny phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of NixSny in preference to the Au-rich Au–Sn phases is energetically favorable.
