The emerging interconnect material, Co, has been investigated as metal bonding material in this work. The Co-Co direct bonding has been successfully developed at both chip-level and wafer-level with excellent bonding results, including good bonding interface, strong mechanical strength, and high electrical performance. The behavior of inter-diffusion between Co bonding layers could be verified by the TEM/EDX analyses. Moreover, the post-bonding annealing process has been discussed, showing the negligible influence on Co-Co bonding. Finally, Co has been successfully developed as a metal passivation layer to achieve a low temperature Cu bonding process at 200°C. The achievements reported in this paper explore the applications of next-generation interconnect materials in 3D IC technologies.