Low Temperature and Short Time Au/Sn Solid-liquid Diffusion Bonding for 3D Integration

High temperature and long-time solid-liquid diffusion bonding (SLID) will barrier the scaling down of multi-chip interconnection and increase the total time of 3D integration. A wafer-level temperature gradient bonding technology (TGB) was used in the Au/Sn SLID bonding to lower the temperature and shorten the bonding time. Au/Sn bonding with different pad sizes were designed. The results showed that the Au/Sn bump with different pad size effectively reduce the Sn overflow than that with same pad size. In addition, suitable electroplating current density could lower the Au bump surface roughness and evaporation process could also make Sn flat. With all these optimized design, the bonding was successfully achieved in a vacuum bonding chamber. The hot end temperature was set to 250°C and the cold end temperature was set to 150 °C with the bonding pressure of 5000 N. The bonding condition was maintained for 20 minutes. Then X-ray detection, shearing test and scanning electron microscope (SEM) were used to study the quality of the bonding interface and the overflow of Sn. Misalignment was not detected based on the wafer-level and chip-level X-ray. The chips in the middle of the bonded wafer had a higher shear force compared to the surrounding part of the wafer. SEM images indicated the fracture all occurred on the grain boundary and no dimple were founded. Daisy chain structure were observed when the cross sectional image of the bonded chip was detected.