Rapid communication Dry etching characteristics of TiO 2 thin films using inductively coupled plasma for gas sensing I. Hotovy a, * , S. Hascik b , M. Gregor c , V. Rehacek a , M. Predanocy a , A. Plecenik c a Institute of Electronics and Photonics, Slovak University of Technology, Ilkovicova 3, 81219 Bratislava, Slovakia b Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovakia c Department of Experimental Physics, Comenius University, Bratislava, Slovakia article info Article history: Received 27 February 2014 Received in revised form 24 March 2014 Accepted 25 March 2014 Keywords: TiO 2 Dry etching Inductively coupled plasma X-ray photoelectron spectroscopy abstract In this work, the optimal characteristics of etching TiO 2 films in an inductively coupled plasma system with CF 4 /Ar plasma were investigated. The maximum etch rate of TiO 2 was 93 nm/min at fixed 200 W of inductively coupled plasma power and the highest investigated value of RF chuck power of 150 W. Using the X-ray photoelectron spectroscopy the chemical reactions between TiO 2 and F were analysed. It was found that the etching mechanism included ion-stimulated desorption of reaction products on the TiO 2 surface and formation of a solid solution and non-volatile TiF 3 . Ó 2014 Elsevier Ltd. All rights reserved. In recent years, great expectations in the gas-sensor field have been in novel and fascinating quasi-one-dimensional structures (nanotips, nanowires, nanobelts, nanotubes) of metal oxides (as ZnO, TiO 2 , SnO 2 , Fe 2 O 3 , In 2 O 3 , MoO 3 , WO 3 , NiO). These are single- crystalline nanostructures with a high aspect ratio, normally grown by a bottomeup approach [1]. These intriguing character- istics make the metal oxide structures potentially good candidates for chemoresistive gas sensors, reducing instabilities suffering from their polycrystalline nature associated with grain coalescence and drift in electrical properties. In spite of such benefits, also in this bottomeup approach many problems remain unsolved, such as poor repeatability due to a scarce control of the growth [2]. There are also some difficulties in growing and transferring the nano- structured metal oxide films on the transducer platform and in making good electrical contacts to these nanostructures. The tope down method is another way to achieve ordered patterns for different kinds of metal oxide materials. The use of dry etching processes allows to tailor the vertical and horizontal etch rate of a film selectively masked by a photoresist, dielectric or metal to achieve a high fidelity pattern transfer [3]. Therefore, here is the interest at developing effective plasma etching chemistry with an adequate etching rate of metal oxide thin films. Until now, there is little reported on the etch characteristics of TiO 2 , especially in high- density plasma [4,5]. The inductively coupled plasma (ICP) source provides the characteristics of the high etch rate with a reasonable etch selectivity to mask materials. Papers to date show the dependence of the TiO 2 etch rate on the operating parameters using CH 4 /H 2 , Cl 2 , SF 6 , BCl 3 /Ar and CF 4 /Ar plasmas [4e7]. Nevertheless, the relationships between the etching parameters and the chemical processes in CF 4 /Ar plasma depending on the RF chuck power have not been reported yet. In our study we investigated the etching characteristics of sputtered TiO 2 thin films using CF 4 /Ar mixture in an ICP system. First, the etch rates were measured as a function of the applied RF chuck power. Second, the chemical reactions on the etched surface were analysed by XPS. Finally, TiO 2 tip arrays were successfully fabricated by the developed etching process through an Au nano- particles mask. Polycrystalline TiO 2 films were deposited by DC reactive magnetron sputtering onto oxidized Si substrates and annealed at 500  C. The film thickness was about 200 nm as measured by Talystep. Dry etching was performed in a Vacutec 310/320 reactor utilizing the ICP source MaPE-ICPC 200 (FHR) operated at 2.4 MHz and separate capacitively coupled RF (13.56 MHz) biasing of the sample electrode. TiO 2 thin films were etched in CF 4 /Ar plasma with variable process parameters. The working gas CF 4 was admitted into the reactor chamber through an electronic flow controller at a typical flow rate of 4 sccm. In each case, Ar was added to enhance the stability of discharge and to facilitate ignition of the plasma at the low operating pressures employed. The etching * Corresponding author. Tel.: þ421 260291594. E-mail address: ivan.hotovy@stuba.sk (I. Hotovy). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2014.03.025 0042-207X/Ó 2014 Elsevier Ltd. All rights reserved. Vacuum 107 (2014) 20e22