Cu-Cu wire bonding challenges on MOSFET wafer technology

Essentially, this paper presents the key challenges of monometallic Cu-Cu wire bonding process characterization for MOSFET thin wafer technology with bare Cu plated bond pad, on a leadframe package. And solutions to these challenges are presented and discussed. Generally, the technical challenges of bonding a 30um bare Cu wire on a bare Cu bond pad are lifted ball, NSOP and reliability failure related Cu bond pad oxidation. Inert atmosphere poses to be an important factor as Cu oxidizes readily. In this study, the wire bonder indexing and bonding stage configuration such Nitrogen (N₂) gas input control, Cu kit, window clamp and top plate were examined for an optimum inert atmosphere control condition to prevent bare Cu free air ball (FAB) and Cu bond pad oxidation before wire interconnection is formed. The optimum N₂ setting, 10 L/min in the N₂ was obtained in this study for providing the protection to the Cu bond pad. A thin layer of 3.8nm for Cu₂O was obtained from XPS depth measurement that caused NSOP and lifted ball. The finding implied that the differences of wire bond mechanism and process parameter characteristic from conventional wire bond system. Besides that, the hardness of both Cu FAB and Cu bond pad are also critical to form reliable bonding. The different material hardness of Cu FAB formation, bonded ball deformation and Cu bond pad were examined as a reference for process characterization. The reliability performance results of the Cu-Cu wire bonded specimens will be presented and discussed. In summary the study has demonstrated that Cu-Cu wire bonding is achievable but more work has to be done to improve the package reliability performance before large scale production, and potential cost saving.