Triple-Stacked Au/SiO₂ Hybrid Bonding with 6-μm-Pitch Au Electrodes on Silicon-on-Insulator Substrates Using O₂ Plasma Surface Activation for 3-D Integration

In this paper, we have developed multilayer bonding technology to stack complementary metal-oxide semiconductor (CMOS) circuits. The steps include repeated embedding of gold (Au) electrodes, hybrid bonding of Au and SiO₂ on a silicon-on-insulator (SOI) substrate using surface activation with O₂ plasma, and subsequent elimination of the Si substrate. Furthermore, the characteristics of Au/SiO₂ hybrid bonding via O₂ plasma are tested with the fabrication of a triple-stacked daisy-chain test device using 3-μm-diameter Au electrodes at a pitch of μ m. When the surface is activated using O₂ plasma before bonding, the bonding strength increases approximately four times stronger than that when the surface activation is performed with sequential plasmas of Ar and O₂. This additional strength ensures that the bonded interface remains intact while the layers are stacked. A prototype device is manufactured with alignment errors of approximately 0.4μm at the center and approximately 1 μm at the corners. No voids are seen in the two bonded interfaces, and more than 984 000 Au contacts are produced, each having an average resistance of 92.8 mΩ. These results indicate that the proposed multilayer stacking technology is promising for developing high-performance 3-D integrated circuits, 3-D integrated CMOS image sensors, and microelectromechanical systems based on SOI substrates.