Atomic diffusion bonding for optical devices with high optical density

An inorganic bonding method providing 100% light transmittance at the bonded interface was proposed for fabricating devices with high optical density. First, we fabricated 5000 run-thick SiO₂ oxide underlayers on synthetic quartz glass wafers. After the film surfaces were polished to reduce surface roughness, the wafers with oxide underlayers were bonded using thin Ti films in vacuum at room temperature as a usual atomic diffusion process. After post annealing at 300 °C, 100% light transmittance at the bonded interface with the surface free energy at the bonded interface greater than 2 J/m² was achieved. Dissociated oxygen from oxide layers probably enhanced Ti films oxidation, resulting in high light transmittance with high bonding strength attributable to the annealing. Using this bonding process, we fabricated a polarizing beam splitter and demonstrated that this bonding process is useful to fabricate devices with high optical density.