KrF-excimer laser etching at the interface to liquids has been studied for the fabrication of well-defined micro-structures in UV-transparent materials. The etch rates and the surface morphologies of the investigated materials including oxides (fused silica, sapphire) and fluorides (CaF2,MgF2) depend on the laser parameters, the used solution, and the material itself. The etch threshold fluence and the etch rates show a dependency on the liquid solvent (acetone, cyclo-hexane, tetrachloroethylene) and the concentration of the dissolved pyrene absorber molecule (0-0.4 mol/l). For quartz and sapphire two fluence regions could be distinguished. In the low fluence region, up to 1.5 J/cm(2), low etch rates up to 25 and 20 nm/pulse were found, whereas in the high fluence region, up to 3.5 J/cm(2), the rates increase to 200 and 300 nm/pulse for fused silica and sapphire, respectively. The best machining results with low surface roughness and steep walls were achieved in fused silica in the low fluence range. The examination of etched crystalline samples by RBS spectroscopy shows an amorphization of the surface after the laser interaction. Mask projection and scanning contour mask techniques were applied for the fabrication of three-dimensional micro-structures. (C) 2002 Elsevier Science B.V. All rights reserved.