Au-In bonding below the eutectic temperature

A bonding method using Au-In alloy which requires a low process temperature of 200°C to produce high temperature (454°C) bonds is reported. Multiple layers of Au and In are deposited on semiconductor wafers in one vacuum cycle to reduce In oxidation. The semiconductor dice are then bonded to substrates coated with Au. Above 157°C, the indium layer melts and dissolves the Au layers to form a mixture of liquid and solid. The solid-liquid interdiffusion process continues until the mixture solidifies to form the Au-In bond. A scanning acoustic microscope (SAM) was used to determine the excellent bonding quality before and after thermal shock tests while an energy dispersive X-ray (EDX) was employed to determine the composition of the resulting bonds. The resulting bond has an unbonding temperature greater than 545°C. Due to the low process temperature, the stress on the bonded structure caused by thermal expansion mismatch is reduced. This type of bonding is useful when bonding at a low temperature is followed by a subsequent higher temperature process