Fluxless InSn bonding process at 140 °C

Device packages usually need more than one soldering operation to complete. For photonic and fiber optic device packaging, indium solder is often used due to its ductility. Since indium has a relatively low melting temperature of 156.6 °C, the subsequent bonding operation requires a process temperature that is lower than 156 °C. In this paper, we report a new bonding process at 140 °C based on an indium–tin multilayer composite. This is a fluxless bonding technique. In fabrication, a chromiurn–tin–indium–gold multilayer composite is deposited on silicon die in one high vacuum cycle to prevent oxidation. Immediately upon deposition, gold and indium react to form a stable AuIn2 protective outer layer against oxidation. Silicon substrates are deposited with thin chromium–gold layers. The silicon die and substrate are bonded in a hydrogen environment at 140 °C. Scanning acoustic microscopy (SAM) analysis is used to evaluate the joint quality. This bonding technique consistently achieves uniform and void-free joints. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analyses also are performed on the joint cross sections. The SEM image shows a uniform joint thickness of 5 μm and the joint microstructure. SEM and EDX results indicate the joint consists of In–Sn alloy with embedded AuIn2 grains. The re-melting temperature of the joint is found to be 125–150 °C, which shows that the joint composition is not exactly eutectic that has a melting temperature of 118 °C, but rather is Sn-rich. The success of this design illustrates the solid-state interdiffusion between Sn and In to form a thin layer of Sn–In eutectic alloy. The fluxless feature of this technology is valuable for bonding and assembling many emerging photonic devices that simply cannot tolerate fluxes.