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 ${\rm Cu}/{\rm Si}_{3}{\rm N}_{4}/{\rm Cu}/{\rm Ni\hbox {-}P}/{\rm Au\hbox {-}Sn}/{\rm Ni}/{\rm Ni}_{2}{\rm Si}/{\rm SiC}$ 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 $\zeta$ phase with a melting point of 522 $^{\circ}{\rm C}$ ). 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 $\zeta$ phase was first- converted into the more Au-rich $\beta$ phase. This created physical contact between the Sn and Ni atoms, resulting in brittle ${\rm Ni}_{x}{\rm Sn}_{y}$ phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of ${\rm Ni}_{x}{\rm Sn}_{y}$ in preference to the Au-rich Au–Sn phases is energetically favorable.