Low Temperature Au/Sn Solid-Liquid Diffusion Chip-to-Chip Bonding for MEMS Encapsulation

Solid-liquid diffusion bonding (SLID) is a promising 3D chip stacking technology but with long bonding time. In this study, a temperature gradient bonding process (TGB) was used for Au/Sn SLID bonding, which can achieve high bond strength even with a short bonding time. A series of optimized designs were carried out to improve bonding strength. By optimize the current density of electroplating Au, and using evaporation to prepare Sn , lower surface roughness of Au and Sn can be obtained. Then HCOOH was introduced during the bonding process to treat the Sn surface and remove Sn oxides. The temperature of hot end and cold end was separately set to 250 °C/ 150 °C with the bonding pressure of 10 MPa. The bonding time maintained for 5 minutes. Electrical resistance was first tested. The bonding interface quality were investigated by shear strength testing. The average shear strength of bonding chips with HCOOH treatment after the bonding process was about 24 MPa higher than that without HCOOH. Through the fracture analysis, there were four types of fracture locations, namely, the interfaces between SiO2 substrate and Al wiring, between Al wiring and Ti/Au, between Ti/Au and AuSn, and between AuSn and Au
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Sn. The main fracture mode might be brittle fracture. Sn spillover was not detected by cross-section milling and scanning electron microscopy (SEM). No IMC overflowed outside the bonding interface. The Energy Dispersive Spectroscopy (EDS) results showed the gray phase near the two ends of the substrate was mainly AuSn, and the light gray phase in the middle was mainly Au
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Sn.