Bonding of Copper Pillars Using Electroless Cu Plating

Nowadays, many research groups are developing low-temperature and pressureless copper-to-copper interconnection bonding methods. Among them, controlled flow electroless nickel plating, which utilizes the feature of autocatalysis of electroless plating to join copper pillars, is the most promising one [1]. However, for high frequency application, nickel is a ferromagnetic material and would trigger skin effect seriously, which could greatly reduce the effective area for signal to pass through. To deal with this issue, other electroless plating materials needs to be investigated. Among them, copper is a suitable material for interconnection due to its excellent electrical conductivity. Previously, the chip-to-substrate all-copper connections were formed by joining the two pillars with electroless copper plating followed by an annealing process at 180 degrees C [2]. Although the anneal process could enhance the bond strength up to 148MPa, it contradicts the advantage of electroless plating process, which is a low temperature process. In this research, to avoid the post annealing process, we conducted the electroless copper plating under controlled flow to fabricate the interconnection. All the process temperature in our research is below 50 degrees C. The bonding result is very well without any extraneous deposition.