Laser etching of periodic 1D and 2D submicron relief gratings on prestructured fused silica surface

Abstract

Laser-induced backside wet etching (LIBWE) allows the etching of transparent materials with pulsed UV-lasers. The laser-etched structures are characterized by a high fidelity and a low surface roughness. For the etching of periodically sub-micron structures on a solid surface interfering laser beams were used produced by projection of diffraction masks. In conjunction with LIBWE surface relief gratings were realized on planar and curved fused silica substrates in one-step direct fabrication process. The fundamentals of topography and roughness evolution of surfaces etched by LIBWE are investigated in detail. The etching of sub-μm gratings with a period of 760 nm into flat surfaces by means of interfering laser beams shows a saturation of the grating depth within 20 pulses. The decrease in height of sub-micron gratings from 125 to less than 10 nm within 15 laser pulses causes a substantial roughness reduction. The depth limitations in etching of the gratings are the result of the influence of the surface topography to the heat flow. The more efficient heating of surface peaks in contrast to the valleys results in higher etch rates and probably causes the smooth surfaces observed in LIBWE processing. The thermal diffusion length determines the structure dimension influenced by this smoothing effect. The knowledge on the effects of submicron resolved laser irradiation by LIBWE approach is from great importance for applying laser backside etching to nanometer grating fabrication. To demonstrate the capabilities of the processing approach a rectangular binary grating with a period was subsequently patterned with sub-micron relief gratings.