Nickel-indiffused lithium niobate ridge optical waveguides fabricated by a novel self-aligned process

The fabrication process for the self-aligned technique is illustrated. At first, the patterns of waveguides are transferred onto the LiNbO₃ substrate by the photolithographic process. Then, the Ni diffusant, the diffusion barrier of Ti, and the PE masks of Si are successively deposited on the substrate by an E-beam evaporation system. By using the lift-off process, the tri-layered structure composed of Ni/Ti/Si is left on the substrate with good adhesion and exact alignment. Putting the sample into the oven, the diffusion process is performed under 800-900°C for several hours to drive Ni atoms into the LiNbO₃ substrate. With the protection of the PE mask formed by the combination of Ti and Si, the regions not protected by the PE mask will be domain-inverted in the molten benzoic acid, and then will be etched in pure HF, completing the fabrication of a Ni:LiNbO₃ ridge waveguide. It has been proved that Ni diffusion of 300 Å (for a waveguide with extraordinarily polarization at a wavelength of 1.3 μm), Ti diffusion barrier of 700 Å, and Si PE mask of 3000 Å become the best combination