Optimisation of ultrafast laser assisted etching in fused silica

Ultrafast laser assisted etching (ULAE) in fused silica is an attractive technology for fabricating three-dimensional micro-components. ULAE is a two-step process whereby ultrafast laser inscription (ULI) is first used to modify the substrate material and chemical etching is then used to remove the laser modified material. In this paper, we present a detailed investigation into how the ULI parameters affect the etching rate of laser modified channels and planar surfaces written in fused silica. Recently, potassium hydroxide (KOH) has shown potential to outperform the more commonly used hydrofluoric acid (HF) as a highly selective etchant for ULAE. Here we perform a detailed comparison of HF and KOH etching after laser inscription with a wide range of ultrafast laser irradiation parameters. Etching with KOH is found to be significantly more selective, removing the laser modified material up to 955 times faster than pristine material, compared with up to 66 when using HF. Maximum etching rates for the two etchants were comparable at 320 μm/hour and 363 μm/hour for HF and KOH respectively. We further demonstrate that highly selective, isotropic etching of non-planar surfaces can be achieved by controlling the polarization state of the laser dynamically during laser inscription.