Low Temperature Wafer-To-Wafer Hybrid Bonding by Nanotwinned Copper

Three-dimensional heterogeneous integration is at the core of development for advanced package technology with transcending Moore's law, evolving several applications including mobile, HPCs and AI in practice in foreseeable future. Nowadays, copper material as redistribution layer or through silicon via had widely been applied into fan-out wafer level package, package-on-package interposer integration, and embedded multi-die interconnect bridge. Recently, nanotwinned copper has been reported with distinctive properties which have shown several benefits in domain of Cu to Cu direct bonding such as high surface diffusivity and mechanical properties. It has been successfully bonded at 200°C for 1 hour within film-to-film. For further advanced technologies including 3D IC, nt-Cu has shown a potential candidate toward bonding technology. In this paper, we have proposed wafer-to-wafer hybrid bonding by using a solderless structure of nt-Cu/SiO2 hybrid structure. The nt-Cu/SiO2 hybrid structure was fabricated by electroplating, CMP and followed by wafer to wafer bonding. In view of literature, CMP has played crucial role to control roughness which further affect bonding interface. This nt-Cu/SiO2 hybrid structure with well controlled roughness with whole wafer flattened by CMP, which two step CMP process routes were conducted and achieved arithmetic average roughness of 0.3 nm with dishing 1 nm on nt-Cu. Sequentially, the nt-Cu/SiO2 hybrid structure were bonded successfully with a significantly reduced bonding temperature of 200°C which depends critically on the <111>-oriented Cu as well as continuous nt-Cu/SiO2 bonding interface. After bonding process, the morphology and structure of nt-Cu/SiO2 hybrid bonding wafers were analyzed with FIB-SEM and TEM to investigate the cross-sectional bonding interfacial circumstance followed by reliability test. It has been successfully passed TCT test and with pull strength over 10 MPa. Furthermore, the effective fabricated nt-Cu/SiO2 with high strength and well interface can be bonded with low temperature wafer-to-wafer hybrid bonding have shown a great novel candidate for heterogonous integration for future advanced package technology.