Study On Cu/SiO₂ Low-temperature Hybrid Bonding Using Two-step Cooperative Surface Activation

With Moore's Law moving to the limit and the wave of miniaturization,intelligence and multi-functional development of electronic products is launched,expanding the packaging dimension of chips from two-dimensional?2D?to threedimensional?3D?is recognized by the microelectronics industry as an effective way to shorten the interconnection length and an ideal solution to improve the chip's functional density.The Cu/SiO₂ hybrid bonding technology is compatible with the hybrid interconnection of Cu-Cu,SiO₂-SiO₂,and Cu-SiO₂,and the chip stacking can be completed without microbumps and underfill,which is the key technology for high-density 3D Si integration.However,the technology is currently protected by US Patents,and the process details have not been disclosed.Moreover,the existing bonding process flow often requires high-temperature annealing of nearly 400 °C,which is difficult to be compatible with semiconductor manufacturing processes.It is urgent to develop a low-temperature hybrid bonding technology with independent intellectual property rights.In this paper,Ar and N₂ plasma were used for surface treatment of Cu and SiO₂ wafers.The results showed that both plasmas could quickly remove organic contaminants,flatten the surface to increase the real contact area and effectively improve the density of hydroxyl?-OH?functional groups on the SiO₂ surface.However,due to the presence of trace oxygen in the plasma atmosphere,Ar/O2 plasma easily produces a small amount of oxidation on the Cu surface,while N₂/O2 will cause a large amount of oxidation and nitridation on the Cu surface,hindering the subsequent low temperature connection.To overcome the oxidation problem,this study used formic acid solution to remove the oxide film on the Cu surface,and a two-step cooperative surface activation process combining formic acid and Ar/O2 plasma was further proposed.The effects of formic acid and different sequences of two-step cooperative activation on the surface of Cu and SiO₂ wafers were investigated.The results showed that after being activated by formic acid and then treated by Ar/O2 plasma?referred to as formic acid ? Ar/O2?,the surface morphology of Cu and SiO₂ were uniformized and covered with-OH groups,which significantly increaseed the surface energy and minimized the influence of electrophilic corrosion on SiO₂.Then,the two-step cooperative activated wafers were subjected to thermalcompression bonding,and the bonding strengths of Cu-Cu and SiO₂-SiO₂ obtained at 200 °C for 30 min were 5.35 MPa and 4.28 MPa,respectively.After that,low-temperature strengthening at 200 °C was continued for 2 h to further improve the interface performance.Based on the two-step cooperative surface activation low-temperature bonding process,the solid Cu-Cu,SiO₂-SiO₂,Cu-SiO₂ homogeneous/heterogeneous and Cu/SiO₂ hybrid bonding interfaces with continuous distribution of elements in the transition layer were obtained.Combined with molecular dynamics simulation,the bonding mechanism of the hybrid interface under low temperature conditions was further clarified,including: the pre-bonding of SiO₂-SiO₂ was first realized under pressure relying on hydrogen bonding and van der Waals force,and sufficient contact was achieved between Cu surfaces;As the temperature rises,dehydration polymerization occured between SiO₂ to form partial Si-O-Si covalent bonds,and CuCu interface also condensed to realize Cu-O-Cu connections.During the annealing process,the O element in Cu-O-Cu diffused into the Cu matrix,while promoting grain growth and recrystallization and closing the nanovoids driven by Gibbs free energy,and ensuring the sufficient formation of Si-O-Si covalent bonds,thus obtaining reliable electrical connection and mechanical support.