High Temperature Interconnect and Die Attach Technology: Au–Sn SLID Bonding

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/Si
₃
N
₄
/Cu/Ni-P/Au-Sn/Ni/Ni
₂
Si/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 Ni
x
Sn
y
phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of Ni
x
Sn
y
in preference to the Au-rich Au-Sn phases is energetically favorable.