Comprehensive Analysis of a Cu Nitride Passivated Surface That Enhances Cu-to-Cu Bonding

As 3-D packaging is expected to meet new requirements for next-generation system-in-packaging (SiP), various technologies have been discussed for vertical integration. To enable high-performance vertical interconnects, Cu-to-Cu bonding is an essential process for decreasing the metal-interconnect size between vertically stacked devices and for improving system performance. However, the high-temperature and high-pressure conditions of the Cu bonding process and copper surface oxidation are important issues to be resolved for better mass production, especially for chip-to-wafer bonding. In this study, a comprehensive analysis was performed on the effects of a two-step Ar and N2 plasma treatment used to create a copper nitride passivated surface. Chemical and structural analyses of the copper nitride surface showed that this two-step plasma treatment removed nonuniformly formed native copper oxides and formed an ultrathin copper nitride passivation layer. Electrical and mechanical analyses showed that the resulting uniform conductive copper nitride surface and plastic property have the potential to improve the quality of thermocompression bonding. The results of these comprehensive analyses demonstrate the excellent ability of copper nitride to passivate copper surfaces and indicate the possibility of achieving effective Cu bonding in air.