A low temperature bonding process using deposited gold-tin composites

Abstract

Conventional bonding techniques using gold-tin preforms or paste usually require a process temperature near 320 °C to ensure complete melting of the preforms. However, a 320 °C process temperature is too high for many devices such as AlGaAs/GaAs and GaInAsP/InP laser diode chips. We report a technique which needs only 260 °C to produce nearly eutectic AuSn bonding. This technique utilizes the unique property of the gold-tin alloy system in that the 232°C tin melting point is significantly lower than the 280 °C eutectic point and solid state interdiffusion. The bonding medium consists of AuSn multilayer composite deposited directly on the object to be bonded. This technology eliminates the requirement of preforms, inhibits tin oxidation and provides good control of bonding layer thickness. Results of bonding of 3 mm x 5 mm GaAs dice show that high quality bondings are obtained as determined by a scanning acoustic microscope. The specimens underwent 40 cycles of thermal shock test between -196 °C and 160 °C without bonding degradation and die cracking. Scanning electron microscopy and energy-dispersive X-ray studies reveal interesting mechanisms of the bonding process. This new process is particularly useful for bonding electronic and optical devices which cannot take a temperature above 260 °C.