Low-Temperature Wafer-Level Bonding with Cu-Sn-In Solid Liquid Interdiffusion for Microsystem Packaging

This work demonstrates the potential use of Cu-Sn-In metallurgy for wafer-level low-temperature solid-liquid interdiffusion (LT-SLID) bonding process for microelectromechanical system (MEMS) packaging. Test structures containing seal-ring shaped SLID bonds were employed to bond silicon and glass wafers at temperatures as low as 170 degrees C. Scanning acoustic microscopy (SAM) was utilized to inspect the quality of as-bonded wafers. The package hermeticity was characterized by cap-deflection measurements and evaluated through finite element modelling. The results indicate the bonds are hermetic, but residual stresses limit the quantitative analysis of the hermeticity. The microstructural studies confirm the bonds contain a single-phase intermetallic Cu6(Sn,In)5 that remains thermally stable up to 500 degrees C. This work shows Cu-Sn-In based low-temperature bonding method as a viable packaging option for optical MEMS or other temperature-sensitive components.