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Comprehensive Study of Copper Nano-Paste for Cu-Cu Bonding

Cu-Cu bond is a prefer solution for shrinking interconnect pitch as it offered lower interconnect resistance with no intermetallic compound risk. Major challenge in Cu-Cu bond is the formation of copper oxide and also the long thermal process. Several approaches had been proposed in the industry to overcome the challenges are diffusion bonding, ultra-sonic bonding, Cu nano-paste, use of Formic acid and argon plasma for copper surface treatment and bit grinding process to enable good Cu-Cu bond. In this paper, we explore the use of Cu nano-paste as the bonding medium between Cu bump and Cu pad. Cu nano-paste is an attractive solution for Cu-Cu bond as it deploy similar approach as conventional solder bump flip chip process. The flux used in conventional solder bump flip chip process is replaced by Cu nano-paste and the conventional reflow oven is replaced by Formic Acid Chamber. We had demonstrated good formic acid process in dealing with Cu nano-paste in terms of low resistance value and prestige copper surface.