Fluxless bonding of Si chips to Ag-copper using electroplated indium and silver structures

In this study, we present fluxless bonding process between silicon and Ag-copper dual-layer substrate using electroplated In/Ag solder. We first study nucleation mechanism of electroplated In over Ag substrate. It is interesting to find that electroplated In reacts with underlying Ag to from AgIn2 IMC layer during electroplating. A novel laminating technique is developed on Cu substrate as cladding layer. The Ag cladding on the copper substrate is a buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3ppm/°C) and Cu (17ppm/°C). To bond Si chips to the Ag layer on copper substrates, In-based alloy (InAg) is used. A fluxless bonding process is designed and developed between Si/Cr/Au/Ag and Cu/Ag/In/Ag. The bonding process is performed in 50-militorr vacuum atmosphere without any flux. The resulting joints consist of three distinct layers, i.e., Ag, Ag2In intermetallic compound and Ag layer. Microstructure and composition of the joints are studied using Scanning Electron Microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). Annealing on the bonding samples is under way to change the three layer joint into two layers of (Ag) and Ag, which would have high re-melting temperature above 700°C. This technique presents our success in overcoming the very large mismatch in thermal expansion between silicon and copper. It can be applied to mounting numerous high power silicon devices to Cu substrate for various industry applications.