Development of Hybrid Bonding Process for Embedded Bump with Cu-Sn/BCB Structure

High density, fine pitch wafer level bonding technology is necessary for heterogeneous integration. However, underfill would be a challenging when bonding pitch decreases. Hybrid bonding technology with metal-to-metal and passivation-to-passivation bonding simultaneously was considered as a feasible solution. In this work, a hybrid bonding scheme with asymmetric embedded Cu-Sn/Cu bumps structure was investigated. Photosensitive BCB used as passivation materials was coated on only one side of bonding pairs. This asymmetric structure allows for precise alignment. BCB curing and Cu-Sn Solid-liquid-diffusion bonding were both achieved under the temperature of 250°C. The thickness of BCB and metal was concisely designed considering the volume shrinking of polymer during bonding process. To achieve void free hybrid bonding interface, efforts were mainly paid to some key aspects. Surface pre-treatment were firstly introduced to ensure no oxidation and residual polymer on the surface of bumps before bonding. A combination of plasma and formic gas treatment were adopted. Furthermore, considering coefficients of thermal expansion (CTE) mismatch of BCB and embedded bumps, a compatible bonding process with well-matched temperature and stress was proposed and optimized to get a void-free bonding interface under the guidance of independent variable test result. Bonding interface microstructure was observed and mechanical test, electrical test of Kelvin and daisy-chain structure were performed to evaluate bonding quality, also reliability test which included HTST and TCT had been carried out. The results confirmed that embedded bump hybrid bonding would be a convincing technology for future 3D integration.