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In this paper, we successfully demonstrate ultrahigh density (>; 106 cm-2) Cu-Cu interconnects of 6-μm pitch using wafer-on-wafer thermo-compression bonding. This is a significant improvement from our previous achievement of 15-μm pitch. In addition, we integrate Cu sealing frame with excellent helium leak rate to the bonded structures to promote the overall bond reliability. On top of that, temporary passivation of Cu surface using self-assembled monolayer (SAM) enhances the resistance against oxidation and particle contamination. Finally, thermal cycling test confirmed the thermal stability of the Cu-Cu daisy chain structure up to 1,000 cycles. Hence, this work opens up new opportunity for wafer level integration of Cu-Cu bonding with state-of-the-art TSV technology, enabling future ultrahigh density 3D IC applications.