Wafer-on-Wafer Stacking by Bumpless Cu–Cu Bonding and Its Electrical Characteristics

Face-to-face stacking of wafer-on-wafer is successfully demonstrated on 200-mm wafers using bumpless Cu-Cu bonding. The Cu bonding structures are fabricated using a damascene metallization process and recessed prior to bonding. The Cu surface topology is optimized and carefully cleaned prior to bonding. Bonded Cu structures are found to provide sufficient mechanical strength to sustain shear force during wafer thinning. Cu-Cu ohmic contact is formed as confirmed from the crossbar Kelvin structure, and its contact resistance is found to be sensitive to the time lapse between cleaning and bonding. The measured contact resistance is attributed to imperfections such as voids and oxide precipitates at the bonding interface.