Fluxless Bonding of Bismuth Telluride Chips to Alumina Using Ag–In System for High Temperature Thermoelectric Devices

Bismuth telluride (Bi₂Te₃) and its alloys are the most commonly used materials for thermoelectric devices. In this paper, the fluxless bonding process was developed to bond Bi₂Te₃ chips to alumina substrates for high temperature applications. The silver-indium (Ag-In) system was chosen for the process development. To work with this system, the Bi₂Te₃ chips were coated with 100 nm titanium (Ti) and 100 nm gold (Au) as barrier layer and plated with 10 μm Ag layer. The Bi₂Te₃ samples were annealed at 250^°C for 200 h. No interdiffusion between Bi₂Te₃ and Ag was detected. The Ti/Au barrier layer was not affected either. It showed exceptional step coverage on the rough Bi₂Te₃ surface even after the annealing process. To prepare for bonding, alumina substrates with 40 nm TiW and 2.5 μm Au were plated with 60 μm Ag, followed by 5 μm In and thin Ag cap layer for oxidation prevention. The Bi₂Te₃ chips were bonded to alumina substrates at 180^°C. No flux was used. The resulting void-free joint consists of five regions: Ag, (Ag), Ag₂In, (Ag) and Ag. (Ag) is Ag-rich solid solution. The joint has a melting temperature higher than 660 ^°C. Due to the thick ductile Ag layer on alumina, the Bi₂Te₃ chip did not break after bonding despite its significant coefficient of thermal expansion mismatch with alumina.