Three-dimensional SiO2 surface structures fabricated using femtosecond laser lithography

We report the fabrication of three-dimensional (3-D) SiO2 surfaces using femtosecond-laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. Using pattern transfer of photoresist structures written by femtosecond laser-induced nonlinear absorption, SiO2-based Fresnel lens arrays with 3-D surfaces were obtained for this study. Using the open-aperture z-scan method, the femtosecond laser two-photon absorption coefficient of the KMPR resist was estimated as 17-23 cm/TW, assuming that single-photon absorption was negligible. By adding O-2 to the etching gas (CHF3) during pattern transfer, the surface roughness of the transferred structures was reduced to RMS 16.90 nm, which corresponds to one quarter of that without adding O-2. When 632.8-nm-wavelength light was coupled to the lenses with 3-D surfaces, the focal length was measured as 2790 mu m, which agreed well with the theoretical value.