Fine-pitch bump-less Cu-Cu bonding for wafer-on-wafer stacking and its quality enhancement

3D integration by means of face-to-face (F2F) stacking of wafer-on-wafer (WoW) is successfully demonstrated using bump-less Cu-Cu bonding on 200 mm wafers. Cu surface topology is optimized and carefully cleaned prior to bonding. Bonded Cu structures provide sufficient mechanical strength to sustain shear force during wafer thinning. Excellent specific contact resistance of ~0.34 Ω.μm² is obtained. The contact resistance is attributed to the formation of micro-voids at the bonding interface. Continuous daisy chain contains at least 16,000 contacts at 15 μm pitch is connected successfully. This provides IC-to-IC connection density of 4.4 × 10⁵ cm^-2 suitable for future wafer level 3D integration of IC to augment Moore´s Law scaling. Finally, a non-vacuum and non-corrosive method to provide temporary passivation of Cu using self-assembled monolayer (SAM) is described and improvement in the Cu-Cu bond quality is presented.