Copper wire bonding is an alternative interconnection technology to serve as a viable, and cost saving alternative to gold wire bonding. Its excellent mechanical and electrical characteristics attract the high-speed, power management devices and fine-pitch applications. The copper wire bonding will be an important interconnection technology along with a flip chip technology.
However, the growth of Cu/Al IMC (intermetallic compound) at the copper wire and aluminum pad interface can induce a mechanical failure and increase a potential contact resistance. In this study, the copper wire bonded chip samples were annealed at the temperature range from 150℃ to 300℃ for 2 to 250 hours, respectively. The formation of Cu/Al IMC was observed by using optical microscope and SEM, and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) vs. 1/T). The obtained activation energy was 26kcal/mol. And the behavior of IMC growth was very sensitive to the annealing temperature. Moreover, the reaction rates of Cu/Al IMC formation was about 100 times lower than those of Au/Al IMC formation at the temperature range from 150℃ to 300℃.
To investigate the effects of IMC formation on the copper wire bondability on Al pad, Wire Pull Tests (WPT) and Ball Shear Tests (BST) were performed on annealed samples. The results of Wire Pull Strength (WPS) were independent of the change of annealing conditions and all failures took place at wire ball necks. However, Ball Shear Strength (BSS) showed large dependencies on the annealing conditions. For as-bonded samples, ball shear strength ranged about 240 ~ 260 gf, and ball shear strength changed as a function of annealing times. For annealed samples, fracture mode changed from adhesive failure at Cu/Al interface to IMC layer or Cu wire itself. The IMC growth and the diffusion rate of aluminum and copper closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to investigate a major forming IMCs and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was $γ-Cu_9Al_4$ and this corresponds well to EDS results.
