Low temperature Si/Si wafer direct bonding using a plasma activated method

Manufacturing and integration of micro-electro-mechanical systems (MEMS) devices and integrated circuits (ICs) by wafer bonding often generate problems caused by thermal properties of materials. This paper presents a low temperature wafer direct bonding process assisted by O₂ plasma. Silicon wafers were treated with wet chemical cleaning and subsequently activated by O₂ plasma in the etch element of a sputtering system. Then, two wafers were brought into contact in the bonder followed by annealing in N₂ atmosphere for several hours. An infrared imaging system was used to detect bonding defects and a razor blade test was carried out to determine surface energy. The bonding yield reaches 90%–95% and the achieved surface energy is 1.76 J/m² when the bonded wafers are annealed at 350 °C in N₂ atmosphere for 2 h. Void formation was systematically observed and elimination methods were proposed. The size and density of voids greatly depend on the annealing temperature. Short O₂ plasma treatment for 60 s can alleviate void formation and enhance surface energy. A pulling test reveals that the bonding strength is more than 11.0 MPa. This low temperature wafer direct bonding process provides an efficient and reliable method for 3D integration, system on chip, and MEMS packaging.