Effects of Surface Physical Properties on Ultrasonic Cu/Cu Bonding

Due to the advantages of low electrical resistance and size miniaturization, Cu to Cu has become the important trends in microelectric interconnect fabrication. Robust Cu to Cu bonding with low temperature, ambient pressure (under N2 or air) and short processing time is strongly requested for advanced IC and substrate packaging, MEMS hermetic sealing, and flexible electronics assembly as well. Ultrasonic technique has recently be adopted in direct copper bonding, this study focuses on the influences of surface physical features on direct bonding between two copper electro-deposits. Experimental results show that joints consisting of hard bumps bonded with soft Cu surface exhibit improved bonding strength. They also indicate that the bonding temperature at 100°C with the protection of N2 and higher surface roughness both positively contribute to the joint strength.