Corrosion Mechanisms of Cu Wire Bonding on Al Pads

In microelectronic devices, Cu wire bonding on Al-alloy pads suffers from reliability failures that are due to the corrosion of intermetallic compounds (IMC) at the bonding interface. The issue is exacerbated in harsh operation environments with high temperature and high humidity. We studied Cu wire bonding on AlSi bond pads that was packaged and subjected to an un-biased Highly Accelerated Stress Test (uHAST). The most Cu-rich IMC at the bonding interface associated with the failure had been completely corroded, while the least Cu-rich IMC survived. The passivity of the surface oxide of an IMC is compromised likely due to the inherent structural incompatibility between the component aluminum oxide and copper oxide, and consequently the loss of structural integrity of the surface oxide, which makes the IMC less resistant to corrosion than both Al and Cu. The Cu/Al ratio of an IMC decides its ratio of the cathode-to-anode surface areas exposed to the electrolyte, and is expected to be the major factor to determine the corrosion rate of the IMC. The most Cu-rich IMC accordingly exhibits the highest corrosion rate among the IMC; in addition, its interface with the Cu wire has the highest Cu/Al ratio and therefore the highest corrosion rate in the film stack at the bonding interface. Therefore the most Cu-rich IMC is subjected to corrosions along not only the laterally inward direction, but also the vertically downward direction, which exacerbates the preferential corrosion of it. The mechanisms correlate with the widely reported reliability improvements induced by Pd-coating of the wire and other methods. On the outlook to address the reliability issue, both coating of the wire with Ni and replacing Cu wires with Al wires possesses the potential to improve the corrosion resistance, since Ni behaves thermodynamically similar to Pd in the ternary system with Cu and Al, and there will be no IMC formation with the use of Al wires. Both methods, nonetheless, are yet to be verified. To complement, a review of redox reactions generally involved in wire bonding is given, which likewise reveals that the oxidation of an IMC preferentially takes place for the more active element of Al.