Very High-Temperature Joints Between Si and Ag–Copper Substrate Made at Low Temperature Using InAg System

We present a fluxless bonding process between silicon and Ag-copper dual-layer substrate using electroplated indium/silver solder. The nucleation mechanism of In plated over Ag layer is first investigated. It is interesting to discover that In atoms react with underlying Ag to form AgIn₂ compound layer during electroplating. A novel Ag laminating technique on Cu substrates is developed. The Ag cladding functions as a strain buffer to manage the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/degC) substrates. To bond Si chips to the Ag layer on copper substrates, In-based alloy (InAg) is used. A fluxless bonding process is developed between Si/Cr/Au/Ag and Cu/Ag/In/Ag. The process is performed in 50-militorr vacuum to suppress solder oxidation. No flux is used. The resulting joints consist of three distinct layers of Ag, Ag₂In and Ag. Microstructure and composition of the joints are examined using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). Bonded samples are further annealed to convert the Ag₂In phase into solid solution phase (Ag). The joint has a melting temperature above 850degC. This technique presents our success in overcoming the very large CTE mismatch between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrates for various industrial applications.