A study on an abnormal oxidation issue of copper bonding wire

Copper (Cu) wire is a good alternative to expensive gold (Au) wire for wire bonding interconnects in many semiconductor and microelectronic applications. However, Cu wire does pose some challenges, like it is harder than Au wire and it easily oxidize in air especially with moisture. Cu oxide layer is not a passivation layer and it will continue to grow. The Cu oxide will hinder the adhesion between wire and pad and lead, causing non-stick-on-pad (NSOP) and non-stick-on-lead (NSOL). Therefore, the shelf life and floor life of Cu wire is much shorter than that of Au wire, and special package and storage techniques are needed for Cu wire. Normally, Cu wire floor life is about 7 days, which means the wire can last for 7 days in clean room after opening the vacuum package. In this work, a batch of Cu wires were found to be severely oxidized in a production clean room, where they were open and hung on the wire bonder after 2-3 days. Surface analysis was conducted on the problematic wire. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and X-ray Fluorescence Spectrometer (XPS) are employed to identify the corrosion products on the Cu wire. The results show that the corrosion products are CuCl and CuO. It is suspected that the chloride (Cl) may come from the Cl-contained floor detergent. Simulation test was conducted to verify the contamination source in the production clean room, and the simulation results match well with the actual results. It is suggested that the assembly houses should use Cl-free detergent in the production room to prevent potential Cl contamination.