Thermodynamic study on the corrosion mechanism of copper wire bonding

• We calculate Pourbaix diagrams of Cu–Al bonding at humid environment with chloride.
• We rationalize the corrosion kinetics of Al-metal IMCs by cation transport model.
• High thermal stability of IMCs can enhance bonding corrosion resistance.
• Doping highly charged elements in Al pad can suppress aluminum diffusion across barrier.
• Reducing chloride absorption by inhibitors can enhance corrosion resistance.

In order to achieve longer life reliability of copper bonding, it is crucial to understand the corrosion mechanism of copper wire bonding under humid environments. Although it has been revealed that the formation of Al–Cu intermetallic compound (IMC) dominates the bond reliability, the mechanism remains unclear despite considerable efforts by experiments. While Pourbaix (Eh–pH) diagrams have long been used to study the corrosion mechanism of pure metals, the alloy Eh–pH diagrams are still lacking. In this work, by assessing various thermodynamic data of Cu–Al–Cl–H2O system, we construct the Pourbaix diagrams associated with Cu bonding involving IMCs and study the corrosion behaviors under various environmental parameters such as temperatures and chloride concentrations. We further rationalize the corrosion kinetics of various Al-metal IMCs on the basis of the cation transport model via the calculation of the chemical potentials of aluminum in IMCs. Our study suggests a guideline to enhance bonding corrosion resistance, i.e., improving the thermal stability of IMCs and suppressing the Al vacancy diffusion in Al2O3 layer by doping highly charged element(s) in Al pad. The results yield greater insight into the corrosion mechanism which may lead to improving reliability in Cu bonding.