Mechanism to improve the reliability of copper wire bonding with palladium-coating of the wire

摘要

Pd-coating of Cu (PCC) bond wires has been widely employed to improve the reliability of Cu wire bonding technology for microelectronics. Description of the underlying mechanism, however, is lacking in literature. We provide an interpretation of how the improvement is induced. After an un-biased highly accelerated stress test (uHAST), the most Cu-rich intermetallics (IMC) were Cu3Al2 under the bare Cu bond wire, and predominantly (CuPdx)Al under the PCC wire. The Pd-coating therefore had suppressed the formation of (CuPdx)(3)Al-2. The suppression stemmed from the improvement of the thermodynamic stability of (CuPdx)Al, which also decreased the kinetics of phase transformation toward (CuPdx)(3)Al-2. The cathode/anode area ratio of (CuPdx)Al is lower than that of Cu3Al2. The corrosion rate of (CuPdx)Al therefore becomes lower, and the reliability of the wire bonding increases. Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to similarly improve the corrosion resistance. Furthermore, the use of Al wires on Al alloy pads will implement a monometallic system across the bonding interface which can also enhance the corrosion resistance. The underlying correlation between the atomic ratio of (Inert Element)/(Active Element) of the IMC and the corrosion rate of the IMC revealed here is pertinent to the research in other fields including the corrosion of alloys/dealloying and nanoporosity, and the making of functional materials with nanoporosity for applications such as catalysis and advanced energy technologies.