Mechanical issues of Cu-to-Cu wire bonding

Cu-to-Cu wire bonding provides benefits both from economical and from electrical point of view. However, since Cu is harder than Al or Au, it is expected to induce higher mechanical stresses in the substrate and more bonding problems during and/or after wire bonding. There are three steps during bonding: bond forming, ultrasonic vibration, and cooling down. In order to understand their physical nature, different techniques are applied. The bond forming process is simulated using finite element analysis (FEA) and the stress generated at the last stage (when the wire ball reaches its final shape) is presented. The grain distortion in the Cu wire bond after bonding is studied using scanning electron microscopy and orientation imaging microscopy. After ultrasonic vibration, the whole structure cools down. Due to the difference in coefficient of thermal expansion between Cu and Si, extra stress is built up. The final residual stress is measured by Raman spectroscopy. The result is compared with FEA and an excellent agreement has been achieved. The impact on the substrate by the capillary tool is clearly shown. The result also shows that the maximum tensile stress in the substrate is located near the edge of the pad and depends highly on the positioning of the wire bond.