KOH etching fs laser processing
Abstract: 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.