Sequential plasma activation methods for hydrophilic direct bonding at sub-200 °C

We present our newly developed sequential plasma activation methods for hydrophilic direct bonding of silica glasses and thermally grown SiO 2 films. N 2 plasma was employed to introduce a metastable oxynitride layer on wafer surfaces for the improvement of bond energy. By using either O 2 -plasma/N 2 -plasma/N-radical or N 2 -plasma/N-radical sequential activation, the quartz–quartz bond energy was increased from 2.7 J/m 2 to close to the quartz bulk fracture energy that was estimated to be around 9.0 J/m 2 after post-bonding annealing at 200 °C. The silicon bulklike bond energy between thermal SiO 2 films was also obtained. We suggest that the improvement is attributable to surface modification such as N-related defect formation and asperity softening by the N 2 plasma surface treatment.