Three-dimensional integrated-circuit (3D IC) is one of the most important electronic packaging technologies nowadays. Cu-to-Cu through-silicon-via interconnection is a crucial process in 3D IC. Direct Cu-to-Cu bonding has been a challenging process for the industry due to its strict requirements on processing conditions and corresponding high processing costs. Micro-bumping is a widely used method for Cu-to-Cu bonding. Cheap and fast process are its advantages, and it has been a mature process in electronic industry. However, intermetallic compounds (IMCs) usually form at solder joints, which are brittle and electrical resistant. Filler materials that help forming Cu-to-Cu joints with better reliability and lower costs are demanding. In this article, a critical review on Cu-to-Cu interconnection technology is presented and an approach for fabricating ductile Cu-to-Cu 3D IC joints is proposed.