Applied Surface Science 256 (2010) 7222–7227 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses M. Ehrhardt a , G. Raciukaitis b , P. Gecys b , K. Zimmer a,∗ a Leibniz-Institute of Surface Modification, Permoserstr. 15, 04318 Leipzig, Germany b Laboratory for Applied Research, Institute of Physics, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania article info Article history: Received 3 March 2010 Received in revised form 19 April 2010 Accepted 14 May 2010 Available online 11 June 2010 PACS: 81.55.Xi 81.65.Cf 42.62.Cf 79.20.Ds 81.05.J Keywords: Laser Fused silica Etching LIBWE Picosecond Toluene Structuring abstract The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for struc- turing transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF 2 , MgF 2 ), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t p = 10 ps) lasers at UV wavelengths ( 1 = 355 nm and  2 = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with differ- ent slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a  1 = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths ( 1 = 266 nm and  2 = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm 2 , respectively. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The laser-induced backside wet etching is an innovative etch- ing technique used for microstructuring of fused silica, sapphire, CaF 2 , BaF 2 , and other transparent dielectrics [1–3]. The method was initially investigated by Wang et al. [4,5] and is character- ized by producing low threshold fluence and low roughness of the etched surface. Usually, transparent dielectrics were are patterned by using laser ablation and by exploiting the laser radiation of a short wavelength, for example a F 2 laser, whose photon energy is higher than the band gap of these materials. Ultrashort pulse lasers were also used in dielectric patterning. Another structuring approach for such materials is dry etching, but this requires pho- tolithographic masking. LIBWE is the only laser processing method which provides the flexibility as well as the low threshold fluence and the nanometre-scaled depth control which were necessary for ∗ Corresponding author. Tel.: +49 341 235 3287; fax: +49 341 235 2584. E-mail address: martin.ehrhardt@iom-leipzig.de (K. Zimmer). the production of micro-optical elements such as Fresnel-lenses or diffractive gratings. LIBWE with nanosecond lasers has been investigated in a num- ber of studies [2,3,5–8] and the influence of various experimental parameters on the etch rate and surface morphology was exam- ined for a variety materials. However, the etch mechanism of LIBWE with nanosecond lasers is still in discussion. Commonly, the etching has been explained with the following thermal etching approach [2,9,10]: the backside of the sample is in contact with a highly absorbent liquid. The laser beam then penetrates the trans- parent sample and is absorbed in a small liquid volume close to the sample’s backside surface. Consequently, a thin layer of the sample surface drastically warms up through heat diffused from the laser-heated liquid to the surface, causing the layer to reach temperatures up to the melting or softening point of the sample material. A number of laser-induced secondary processes in the liquid, for example, bubble formation and collapse, have also to be considered. Such mechanical processes generate pressure and stress onto the softened surface of the sample and finally cause the removal of the material. However, the principle background 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.05.055