Room-temperature direct bonding using fluorine containing plasma activation

Room-temperature Si/Si wafer direct bonding has been achieved successfully without wet chemistry treatment as well as no requiring annealing. Very strong bonding strength of Si/Si pairs, close to the bulk-fracture strength of silicon, is demonstrated at room temperature thanks to adding small amount of carbon tetrafluoride (CF₄) into oxygen plasma treatment. The surface energies of bonded Si/Si wafer pairs were influenced by plasma treatment parameters. Moreover, the wafer surfaces and the bonding interfaces are analyzed to explore the bonding mechanism. Adding small amount of CF₄ into O₂ plasma does not etch the Si surface in a short time (∼60 s), but it renders fluorinated oxide grown on the Si surface, which should be less hydrophilic than the surface treated by O₂ plasma. Therefore, fewer water molecules at bonding interface may be prone to produce many covalent bonds via polymerization reaction and result in strong bonding at room temperature.