Effects of Cu/Al intermetallic compound (IMC) on copper wire and aluminum pad bondability

Copper wire bonding is an alternative interconnection technology that serves 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. Copper wire bonding can be a potentially alternative interconnection technology along with flip chip interconnection. However, the growth of Cu/Al intermetallic compound (IMC) at the copper wire and aluminum 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°C to 300°C for 2 to 250 h, respectively. The formation of Cu/Al IMC was observed and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) versus 1/T). The obtained activation energy was 26Kcal/mol and the behavior of IMC growth was very sensitive to the annealing temperature. To investigate the effects of IMC formation on the copper wire bondability on Al pad, ball shear tests were performed on annealed samples. For as-bonded samples, ball shear strength ranged from 240-260gf, 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 were closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to identify the phases of IMC and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was γ-Cu₉Al₄ and it provided a strong bondability.