Mechanisms for low-temperature direct bonding of Si/Si and quartz/quartz via VUV/O₃ activation

Direct bonding is an attractive technique for joining two mirror polished surfaces without any intermediate materials. Vacuum ultraviolet (VUV) irradiation is an effective way for cleaning surfaces as well as increasing hydrophilicity with less surface damage. However, its applications for the direct bonding of silicon and quartz glasses have seldom been explored. Therefore, in this paper, a vacuum ultraviolet/ozone (VUV/O₃) activated direct bonding method for Si/Si and quartz/quartz was developed. A defect-free interface with high bonding strength was achieved after a low-temperature annealing process at 200 °C. According to water contact angle, Raman spectroscopy, atomic force microscopy, and transmission electron microscopy measurements, we demonstrated an understanding of the beneficial surface treatments from VUV irradiation. Then two models for Si/Si and quartz/quartz bonding were established separately based on low-temperature water stress corrosion. Our studies found that the increase of surface roughness with the extension of VUV irradiation time was attributed to the oxide asperities generated on the irradiated surfaces, which possessed strong deformability under the water stress corrosion effect. The excellent bonding benefits from the saturated silanol groups formed on the VUV irradiated surfaces and the positive effects of the interfacial water on the gap closure during the annealing process.