Micro patterning of fused silica by ArF- and F2-laser ablation

Abstract

Surface-relief gratings on optical materials are required for various telecommunication applications, as light couplers for planar optical waveguides, Bragg reflectors, or alignment grooves for liquid crystals. Vacuum-UV-laser ablation enables precise machining of fused silica. A comparison of ablation results obtained at 157 nm and 193 nm wavelength leads to the following conclusion: at 193 nm the surface properties (e.g. mainly roughness) have strong influence on the ablation behaviour. At 157 nm, the volume properties (e.g. mainly bulk absorption) are dominating instead. Though there are severe constraints concerning deep hole drilling at 193 nm, because irregular cracking can hardly be excluded, the generation of high resolution surface patterns is possible at this laser wavelength. As an example, sub-half-micron surface-relief gratings were fabricated on fused silica by laser ablation with nanosecond (ns) pulses at 193 nm from an ArF excimer laser. The grating relief was generated by imaging a transmission amplitude grating with a Schwarzschild objective of 25x demagnification. To provide high resolution in combination with high fluence, which is required for ablating fused silica, an off-axis mask projection scheme utilizing superposition of the zero order beam with the +1st diffraction order was applied.