Fluxless InSn bonding process at 140 °C

Abstract

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 AuIn 2 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 AuIn 2 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.