The microstructure investigation of flip-chip laser diode bonding on silicon substrate by using indium-gold solder

In recent year, the combination of III-V semiconductor devices with Si manufacturing techniques to develop optoelectronic integrated circuits (OEICs) has been widely studied. Flip-chip bonding has been used widely because superior electrical performance, proper reliability, efficient heat conduction and self-alignment are the advantages of this technology. Because optoelectronic devices are quite sensitive to temperature and stress-induced degradation, the bonding medium should be chosen to have high thermal conductivity and stress-relief. The indium (In) based alloy solders are generally recognized to provide lower melting point, longer fatigue life, and higher thermal conductivity. In this study, we have successfully developed a fluxless bonding process to manufacture In-Au microjoint between laser diode and silicon substrate. During the soldering, the solder reacts with the bonding pad metal to form the intermetallic compound at the interface. Such an intermetallic compound is crucial to the quality of solder joint. We utilized SEM, EDX, and XRD to observe and identify the intermetallic compounds. These results indicate that AuIn₂ is the main intermetallic phase and plays an important role on the quality of joints. Moreover the reliability of solder joint is strongly depended on the initial microstructure. The optimum bonding temperature is found to be about 200°C by the microstructure of the solder joint by SEM and optoelectronic characteristics (I-V and L-I) of the laser diodes. Shear force test has also been performed according to MIL-STD-883C. The results reveal the fact that all well-boned devices meet the shear force requirement. To verify the thermal stability, the bonded samples were tested by thermal shock test. The bonded specimens endure 500 cycles of thermal shock between liquid nitrogen temperature and a hot plate (80°C). To evaluate the long-term reliability, the bonded laser diodes were subjected to an accelerated aging test at 90°C for 500 h. These devices show no abrupt degradation from I-V and L-I plots and their mechanical strength is nearly unchanged as before. This shows that indium could achieve the requirements of thermal stability. The flip-chip bonding technique by using indium solder shows good feasib- ility for the integration of laser diodes on silicon substrates.