Low temperature fluxless bonding technique using In-Sn composite

Device packages usually need more than one soldering operation to complete. For photonic and fiber optics devices and packaging, indium solder has become ever more popular due to its ductility. Indium has a relatively low melting temperature of 156 degrees C. During subsequent bonding operations, another process with a bonding temperature lower than 156 degrees C is needed. Desirable bonding temperature should be lower than 156 degrees C but higher than the maximum temperature of the solder during device operation. In this paper, we report a new alternative to the conventional solder using In/Sn multilayer composite. This is a fluxless and oxidation-free soldering technique that has a bonding temperature of 140 degrees C. In fabrication, Cr/Sn/In/Au multilayer composite is deposited on bare silicon dice in one high vacuum cycle to prevent oxidation. Immediately upon deposition, Au and In react to form a stable AuIn2 protective outer layer against oxidation. Silicon substrates are deposited with thin Cr/Au layers. The silicon die and substrate are bonded in hydrogen environment at 140 degrees C. Scanning acoustic microscopy (SAM) analysis is used to evaluate the joint quality. This bonding technique consistently achieves uniform and void-free joints. SEM and EDX analyses also are performed on the joint cross sections. The SEM image shows uniform joint thickness of 5 mu m and joint microstructure. SEM and EDX results indicate the joint is consisted of In-Sn alloy with embedded AuIn2 grains. The re-melting temperature of several joints is measured. It ranges from 125 degrees C to 150 degrees C. This shows that the joint composition is not exactly eutectic that has a melting temperature of 118 degrees C, but rather is Sn-rich. This interesting result offers a very important advantage in that it increases the temperature that the device package can handle. The fluxless feature of this technology is valuable for bonding and assembling many emerging photonic devices that simply cannot tolerate flux.