Thin die stacking by low temperature In/ Au IMC based bonding method

Low temperature bonding technology is developed using In-alloy on Au at a low temperature below 200°C forming robust intermetallics (IMC) joints with high re-melting temperature (>300°C), so that after bonding the IMC joints can withstand the subsequent processes without any degradation. Using similarly solder system and methodology, chips to wafer (C2W) bonding method has been developed, as such chips are temporary bonded onto wafer before the final bonding. The chips are bonded onto the wafer by two sequential bonding condition; temporary followed by a final bonding, which is 200/90°C (chip/wafer) for 20 sec and 180/180°C for 5 mins. The IMC joints are 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. These IMC joint showed a tensile strength of 120~330 N (23.5~38.8 MPa). Based on this study, the 3 stacked dice with 8 × 8 mm² dies with ~1700 I/Os of 80 um solder bumps were fabricated in a chip to wafer stacking method. It showed uniform bonding all over the die in each layer with relatively good tensile strength achieved. Furthermore, it also underwent 3 times reflow test at 260°C. The IMC joint was examined after going through the reflows test and the bonded samples exhibited neither de-lamination nor any changes in the microstructure.