Fabrication of SiO2 Phase Gratings by UV Laser Patterning of Silicon Suboxide Layers and Subsequent Oxidation

UV transparent phase masks are used in various laser applications like fabrication of Bragg gratings in optical fibers or micro patterning by high power laser ablation. Normally they are fabricated by a costly lithographic process including e-beam writing and reactive ion etching. We propose a new fabrication method based on UV laser ablation. The process consists of three steps. First, a silicon suboxide coating (SiOx with x < 2) with a predefined thickness is deposited on a fused silica substrate. Second, due to its strong UV-absorption, this coating can be removed in defined areas by excimer laser ablation at 193 nm or 248 nm leading to the desired phase pattern in form of a binary depth profile. Third, by applying a thermal annealing process, the remaining SiOxcoating is oxidized to UV-transparent SiO2, resulting in a UV-grade surface relief element. The precisely defined interface between substrate and layer allows for ablation with exact depth control and perfect optical surface quality. Such SiO2 phase masks feature a large processed area, high efficiency for VUV to NIR radiation and can be customized e.g. for perfect zero order suppression. Applications of such phase gratings for materials processing with a UV-femtosecond laser are demonstrated. Using the phase gratings in a mask projection configuration, submicron patterns are created in a variety of materials.