Wafer-level low temperature bonding with Au-In system

Wafer bonding at low temperature is an essential process for next generation MEMS & Sensor packaging. Optoelectronic devices, such as image sensor module and laser diode integrated circuit, need low bonding temperature, high re-melting temperature, high thermal conductivity, and stress-relaxed structure in many cases. Eutectic Au-In system was developed as a replacement of previous Au-Sn system for specific systems require bonding temperature lower than 200°C. Bonding temperature of developed Au-In system was set at 180°C, which was 100°C lower than that of Au-Sn system. Though polymer materials has been used for low temperature bonding, out-gassing and volume shrinkage during the bonding process often degraded bonding quality and accurate alignment between the wafers. Clean packaging with accurate alignment was achieved with eutectic Au-In bonding which also possessed high re-melting temperature over 450°C. Majority of the deposited metallizations to construct the system was converted to intermetallic compounds (AuIn & AuIn2) after bonding reaction. Peak temperature and duration time were varied to investigate optimum condition of wafer-level bonding and diced separate dies are used for X-ray inspection, microstructural observation of the cross-section, and shear test. The results showed that bonding parameters critically affected mechanical reliability of the bonded joint. Failure through the solder layer (unreacted pure In) resulted in higher shear strength, while clear separation between the wafer and under bump metallization (UBM) revealed low bond strength.