High-efficiency microstructuring of VUV window materials by laser-induced plasma-assisted ablation (LIPAA) with a KrF excimer laser

We report on the micro structuring of fused silica (a-SiO₂) and calcium fluoride (CaF₂) with a conventional KrF excimer laser (248 nm) by utilization of the effects in the laser-induced plasma-assisted ablation (LIPAA). Mask projection of the UV light is realized onto the rear (instead of the front) side of the UV transparent samples. The plasma generated from a metal target located behind the rear surface of the VUV window effectively assists in the ablation. In the case of fused silica, we obtain high-quality complex micro structures with structure depths even above 500 μm in aspect ratios of 1:5 and better. The ablation rate in fused silica can reach a level as high as 1 μm per pulse with this novel method, demonstrating a remarkable efficiency. While the ablation rate observed for CaF₂ remains at 50 nm per pulse, the up to 100 μm deep micro structures demonstrate an excellent quality without signs of severe cracking or stress outside the mask projected area. This technique permits high-quality micro fabrication of bio-medical, electronic and opto-electronic devices based on oxides and fluorides by use of a conventional UV laser.