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 mu m Ag layer. The Bi2Te3 samples were annealed at 250 degrees 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 mu m Au were plated with 60 mu m Ag, followed by 5 mu m In and thin Ag cap layer for oxidation prevention. The Bi-2 Te-3 chips were bonded to alumina substrates at 180 degrees 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 degrees 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.