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

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3 Author(s)
Jong Sung Kim ; Electr. Eng. & Comput. Sci., California Univ., Irvine, CA ; Wang, P.J. ; Lee, C.C.

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.

Published in:

Components and Packaging Technologies, IEEE Transactions on  (Volume:31 ,  Issue: 4 )