Atomic diffusion bonding using oxide underlayers for optical applications

Atomic diffusion bonding of quartz glass wafers using thin Ti films, with SiO₂ underlayers on wafer surfaces, provides 100% light transmittance at the bonded interface along with strong bonding energy, after post-bonded low-temperature annealing. Cross-section images obtained using transmission electron microscopy show that the bonded interface after annealing at 350 °C consists of amorphous structure including nanocrystalline grains. Structural analysis using electron energy loss spectroscopy shows that post-bonded annealing enhances oxidation of Ti with oxygen dissociated from SiO₂ underlayers, and that Ti oxides form close to TiO₂ or Ti₄O₇. This oxidation provides 100% light transmittance with high bonding strength attributable to the annealing. Moreover, we applied this technique for bonding glass and sapphire wafers using SiO₂-Nb₂O₅ underlayers, demonstrating that 100% light transmittance and control of refractive index matching are achieved simultaneously at the bonded interface.