Sub-micron period grating structures in Ta₂O₅ thin oxide films patterned using UV laser post-exposure chemically assisted selective etching

A high-resolution and low-damage method for patterning relief structures in thin Ta₂O₅ films by chemically assisted UV laser selective etching is presented. The method is based on the initial exposure of the Ta₂O₅ films to pulsed UV radiation (quadrupled Nd:YAG laser at 266 nm) at fluences below the ablation threshold, for the creation of volume damage in the exposed areas. Subsequent immersion of the exposed sample in a KOH solution results in selective etching of the UV-exposed areas, developing relief structures of high quality. Interferometric exposure was used for the patterning of such gratings with periods of the order of 500 nm in films with a thickness of 100 and 500 nm. The behaviour of the patterning process is studied using diffraction efficiency measurements and AFM scans. Diffraction efficiency increases by a factor of ≈63, compared to the undeveloped structure, were obtained for gratings exposed with 1000 pulses of 30 mJ/cm² energy density, which were developed in a KOH solution. The etching method presented is being applied to the fabrication of gratings in optical waveguides.