Void-free room-temperature silicon wafer direct bonding using sequential plasma activation

Room-temperature Si/Si wafer direct bonding has been performed by an optimized sequential plasma activated bonding process. A shorter O₂ reactive ion etching (RIE) plasma (∼10s) treatment followed by treatment with N₂ radicals for 60s is used for surface activation. The activated wafers are brought into contact in ambient air. After storage at room temperature for 24 h, high bonding strength (∼2.25 J/m²) is achieved without requiring any annealing process. This value is close to the bulkfracture strength of silicon. Furthermore, no annealing voids are observed at Si/Si interfaces even if the bonded wafer pairs are heated from 200 to 800°C in subsequent processes. The bonding interfaces and their optical transmittances are also investigated. This void-free, room-temperature bonding technique based on sequential plasma activation is inexpensive and suitable for the microelectromechanical system manufacturing process and wafer-scale packaging.