Surface-enhanced copper bonding wire for LSI and its bond reliability under humid environment

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Cu bonding wires, in general, are severely limited in their use compared to Au wires; such as wire oxidation, lower bondability, forming gas of N₂+5%H₂, and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application. A surface-enhanced Cu wire (EX1) has been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Stitch strength was much better under fresh conditions and maintained without any deterioration after being stored in air for a prolonged period of time. EX1 had a lifetime of over 90 days in air, although it was 7 days for the bare Cu wire. Spherical balls were formed with pure N₂ (hydrogen-free), whereas the bare Cu produced offcenter balls. Cost-effective and secure gas, pure N₂ was only available for EX1. The reliability for Cu wire bonding under humid environment was investigated in pressure cooker test (PCT). The lifetime for EX1 and the bare Cu was over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires in PCT. EX1 improves the bond reliability by controlling diffusion at the bond interface. The excellent performance of Pd-coated Cu wire, EX1 is comparable with Au wires and suitable for LSI packaging.