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Face-to-face stacking of wafer-on-wafer is demonstrated successfully using bump-less Cu-Cu bonding. Using self-assembled monolayer passivation and in-situ desorption, Cu-Cu bond is enhanced in shear strength and bonding uniformity. Excellent specific contact resistance of 0.30 Ω.μm2 is obtained. Continuous daisy chain of at least 6,000 contacts at 15μm pitch is connected successfully. This provides inter-IC connection density of 4.4 × 105 cm-2 suitable for wafer level 3D integration to augment Moore Law's scaling.