Bonding of Bi2Te3 chips to alumina using Ag-In system for high temperature applications

Bismuth telluride (Bi₂Te₃) and its alloys are the most commonly used materials for thermoelectric devices. In this research, Bi₂Te₃ chips of 9 mm × 9 mm were coated with 100 nm titanium (Ti) and 100 nm gold (Au) as barrier layer and plated with 10 μm Ag layer. 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 in 0.1 torr vacuum with 100 psi static pressure. Despite significant difference in coefficient of thermal expansion (CTE) among materials used, the resulting joint did not break. It consists of five regions: Ag, (Ag), Ag₂In, (Ag), and Ag, and has a melting temperature higher than 660°C. The bonded sample was annealed at 250°C for 200 hours. The barrier layer and the joint remain of high quality without any breakage. After annealing, the Ag₂In compound region turns to a Ag-rich alloy layer with melting temperature higher than 690°C. Our bonding results demonstrate the superior characteristics of Ag-In system in high temperature applications. The success of this research opens the door of building thermoelectric modules for power generating or cooling applications, which require long-term operations at high temperature on the hot side.