Fabrication of Smooth Ridge Optical Waveguides in ${\rm LiNbO}_{3}$ by Ion Implantation-Assisted Wet Etching

Very smooth relief microstructures were fabricated in X-cut Lithium Niobate by using an improved ion implantation-assisted wet etching technique. The substrates were implanted with 5 MeV Cu ions at a fluence of 1× 10¹⁵ cm^-2 through a masking layer. Three kinds of layers were patterned and tested: Au, positive photoresist and SU-8 negative photoresist. The damaged regions were then etched with a HF solution at a rate of 100 nm/s. The process can be repeated to obtain higher aspect ratios. The relief structures fabricated with this technology are presented and discussed. In order to explore a possible application of our technique optical waveguides were created in the best quality structures by means of a multi-step carbon ion implantation process with ten different energies followed by an annealing at 280°C for 30 minutes. In this way a step-like enhanced extraordinary refractive index profile was obtained inside the ridges extending from surface to a depth of 2.75 μm. Optical characterization @ 660 nm of the waveguides yielded a propagation loss of 0.23 ± 0.09 dB/cm. The very good results of the best structures make them very promising for integrated optics and acousto/opto-ίuidics.