Laser Processing of Silicon Suboxide for the Fabrication of Multilevel Fused Silica Diffractive Phase Elements

UV-absorbing silicon suboxide (SiOx, x 1) films can be patterned by excimer laser ablation and subsequently oxidized to fused silica by thermal annealing. This two-step process allows for the fabrication of microstructured components made entirely of fused silica. For example, diffractive optical elements with two phase-quantized levels (binary DOEs) can be produced via structured rear-side ablation of a thin film of silicon suboxide on a fused silica substrate followed by oxidation. Such phase elements can be used as projection masks for precise laser microfabrication, e.g. for parallel nano hole drilling. In addition to these binary patterns, multilevel phase structures are produced by repeating the ablation step after recoating the structured surface with additional layers of silicon suboxide. A diffractive phase element in form of a three-level line grating exhibits a diffraction efficiency of more than 60% in the +1st order. This efficiency can be enhanced by increasing the number of recoating and ablation steps and, thus, the number of phase levels. Within the levels, a surface roughness of R a 3 nm is obtained.