Development of novel intermetallic joints using thin film indium based solder by low temperature bonding technology for 3D IC stacking

Low temperature bonding technology was developed using In-alloy on Au at a low temperature below 200degC forming robust intermetallics (IMC) joints with high remelting temperature (>300degC), so that after bonding, the IMC joints can withstand the subsequent processes without any degradation. Process parameters on the solder joint were optimized extensively in bonding and annealing process (temperature, time, and pressure). The joint fabricated at an optimal condition, which is 180degC for 45sec followed by annealing at 120degC for 12hrs, was evaluated in terms of microstructure and compositional observations by means of scanning electron microscope (SEM) and transmittance electron microscope (TEM). As a result, it was confirmed that the joint was completely occupied with the Au-In based IMC phases. And the re-melting temperature was measured as above 400degC by using Differential Scanning Calorimetery (DSC) and Thermo-Mechanical Analysis (TMA). This IMC joint showed a high bonding shear strength (>20MPa) and a low electrical resistance (<100mOmega). Based on this study, the 3 stacked dice with 8times8 mm² dies with ~1700 I/Os of 80um solder bumps were fabricated in a chip to chip stacking method. It showed uniform bonding all over the die in each layer and the high bonding strength of ~40 MPa and passed the 3 times reflow test at 260degC. The IMC joint reliability was examined. After going through the multiple reflows at 260degC, the bonded samples exhibited no delaminating and no changes in the bonding strength and the electrical resistance.