  Citation: Butkut ˙ e, A.; Merkininkait ˙ e, G.; Jurkšas, T.; Stanˇ cikas, J.; Baravykas, T.; Vargalis, R.; Tiˇ ck ¯ unas, T.; Bachmann, J.; Šakirzanovas, S.; Sirutkaitis, V.; et al. Femtosecond Laser Assisted 3D Etching Using Inorganic-Organic Etchant. Materials 2022, 15, 2817. https://doi.org/ 10.3390/ma15082817 Received: 11 March 2022 Accepted: 7 April 2022 Published: 12 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Femtosecond Laser Assisted 3D Etching Using Inorganic-Organic Etchant Agn˙ e Butkut ˙ e 1,2, * , Greta Merkininkait ˙ e 1,3 , Tomas Jurkšas 1 , Jok ¯ ubas Stanˇ cikas 2 , Tomas Baravykas 1 , Rokas Vargalis 1 , Titas Tiˇ ck ¯ unas 1 , Julien Bachmann 4 , Simas Šakirzanovas 3 , Valdas Sirutkaitis 2 and Linas Jonušauskas 2 1 Femtika Ltd., Saul ˙ etekio Ave. 15, LT-10224 Vilnius, Lithuania; greta.merkininkaite@femtika.com (G.M.); tomas.jurksas@femtika.com (T.J.); tomas.baravykas@femtika.com (T.B.); rokas.vargalis@femtika.com (R.V.); titas.tickunas@femtika.com (T.T.) 2 Laser Research Center, Vilnius University, Saul˙ etekio Ave. 10, LT-10223 Vilnius, Lithuania; jokubas.stancikas@ff.stud.vu.lt (J.S.); valdas.sirutkaitis@ff.vu.lt (V.S.); linas.jonusauskas@ff.vu.lt (L.J.) 3 Faculty of Chemistry and Geoscience, Vilnius University, Naugarduko Str. 24, LT-03225 Vilnius, Lithuania; simas.sakirzanovas@chf.vu.lt 4 Chemistry of Thin Film Materials, Department of Chemistry and Pharmacy, IZNF, Friedrich-Alexander University of Erlangen-Nürnberg, Cauerstr. 3, 91058 Erlangen, Germany; julien.bachmann@fau.de * Correspondence: agne.butkute@femtika.com Abstract: Selective laser etching (SLE) is a technique that allows the fabrication of arbitrarily shaped glass micro-objects. In this work, we show how the capabilities of this technology can be improved in terms of selectivity and etch rate by applying an etchant solution based on a Potassium Hydroxide, water, and isopropanol mixture. By varying the concentrations of these constituents, the wetting properties, as well as the chemical reaction of fused silica etching, can be changed, allowing us to achieve etching rates in modified fused silica up to 820 μm/h and selectivity up to ∼3000. This is used to produce a high aspect ratio (up to 1:1000), straight and spiral microfluidic channels which are embedded inside a volume of glass. Complex 3D glass micro-structures are also demonstrated. Keywords: selective laser etching; 3D laser microfabrication; glass microprocessing 1. Introduction Glass micro-processing using femtosecond (fs) lasers is a vast field with capabilities of producing different structures with arbitrary geometries [1]. This can be achieved using various light-matter interaction regimes. II type modification-based selective laser etching (SLE) stands out among all of them due to the possibility of producing arbitrary shaped 3D glass structures which can be on the surface of the glass samples or embedded inside the volume of the sample. This technology was also shown to be suitable for processing crystals [2–4], making it even more appealing. The underlying idea behind SLE is that the etchant etches laser-induced modified volume of dielectric substantially faster than unmodified material. To characterize it, two primary parameters can be used: etching rate, which is the speed at which modified mate- rial dissolves in the etchant, as well as selectivity, which denotes the ratio between etching rates of modified and unmodified glass. Due to different requirements dictated by various applications where SLE is used, different etching rates and selectivities might be desired. As a result, an extensive variety of works have been dedicated to understanding the under- lying physical and chemical mechanisms, which might lead to different etching rates and selectivities [5]. Most of them concentrate on changing laser exposure parameters [3,5,6]. In the absolute majority of works, aqueous Hydrofluoric acid (HF) or Potassium Hydrox- ide (KOH) solutions are being discussed [5]. Nevertheless, varying only these parameters, it is hard to achieve selectivity which would be higher than ∼1400 [6]. This limits SLE usage as a true 3D manufacturing technique which could rival additive 3D printing [7]. Materials 2022, 15, 2817. https://doi.org/10.3390/ma15082817 https://www.mdpi.com/journal/materials