Triple-Stacked Au/SiO2 Hybrid Bonding with 6-μm-pitch Au Electrodes on Silicon-On-Insulator Substrates using O2 Plasma Surface Activation for Three-Dimensional Integration

In this study, 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- m diameter Au electrodes at a pitch of 6 m. When the surface is activated using O2 plasma before bonding, the bonding strength increases approximately four times stronger than that when 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 0.4 m at the center and approximately 1 m 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 three-dimensional (3D) integrated circuits, 3D integrated CMOS image sensors, and microelectromechanical systems based on SOI substrates.