V. Rehacek

Slovak University of Technology in Bratislava, Presburg, Bratislavský, Slovakia

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Publications (54)59.99 Total impact

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    ABSTRACT: Patterning of metal oxide nanostructures with precisely controlled geometries and spacings can play an important role in the improvement of sensors for gas detection. Titanium oxide thin films were deposited on oxidized silicon substrates by reactive magnetron sputtering at room temperature. Patterning of TiO2 nanostructures was conducted by electron beam lithography combined with plasma etching. It was found that for 120 nm-thick TiO2 nanostructure formation, HSQ e-beam resists and Cr films prove to be suitable mask materials. Experimental results showed that the size of TiO2 nanostructures depends mainly on the e-beam lithography process and they can be controlled by the design geometry and the exposure dose. TiO2 nanostructures with a minimal diameter of 70 nm and spacing of 200 nm were successfully fabricated by ICP etching in CF4/Ar plasma through negative e-beam resist HSQ.
    Journal of Micromechanics and Microengineering 07/2015; 25(7). DOI:10.1088/0960-1317/25/7/074006 · 1.73 Impact Factor
  • V Rehacek, I Hotovy, M Vojs
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    ABSTRACT: In this paper, experimental knowledge is presented on the deposition of gold nanoparticles from a colloidal solution on the surfaces of TiO2 prepared on silicon wafers. Important parameters, such as TiO2 surface hydrophilization and functionalization by silane coupling agent (3-mercapto-propyl-tri-metoxy-silane) were investigated in order to obtain TiO2 surface with optimum properties for the immobilization of Au nanoparticles with a close-packed structure.
    Journal of Micromechanics and Microengineering 01/2015; 25(7). DOI:10.1088/0960-1317/25/7/074008 · 1.73 Impact Factor
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    ABSTRACT: We present results on nanocrystalline NiO chemoresistive films which are able to detect 5 ppm of ethanol vapor in air and to operate at 250 °C. NiO films with 50 and 100 nm thicknesses were prepared by DC reactive magnetron sputtering on alumina substrates previously coated by Pt layers as microheaters and interdigitated electrodes. NiO-based sensors with 50 nm thickness exhibited three times higher relative sensitivity and were approximately 2 times faster than the sensor element with 100 nm thickness in the whole ethanol concentration range. These 50 nm thick NiO films are created by nanocrystals with lateral size of about 30 nm homogenously covering the Pt/alumina surface. On the other hand, NiO films with 100 nm thickness are porous, divided in parts and lapped by smaller grains (∼15 nm) in several overlayers.
    Vacuum 09/2014; 107:129–131. DOI:10.1016/j.vacuum.2014.04.012 · 1.43 Impact Factor
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    ABSTRACT: In this work, the optimal characteristics of etching TiO2 films in an inductively coupled plasma system with CF4/Ar plasma were investigated. The maximum etch rate of TiO2 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 TiO2 and F were analysed. It was found that the etching mechanism included ion-stimulated desorption of reaction products on the TiO2 surface and formation of a solid solution and non-volatile TiF3.
    Vacuum 09/2014; 107:20–22. DOI:10.1016/j.vacuum.2014.03.025 · 1.43 Impact Factor
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    ABSTRACT: Titanium oxide thin films were deposited at room temperature by reactive magnetron sputtering in a mixture of oxygen and argon on oxidized silicon substrates. The optimal etching characteristics of TiO2 films by an inductively coupled plasma system were investigated. The maximum etch rate of TiO2 was 104 nm/min at fixed 200W of ICP power and the highest investigated value of RF chuck power of 150W. Patterning of TiO2 tip arrays by electron beam lithography and dry etching was conducted. Experimental results showed that the exposure dose optimization was a significant parameter for controlling the tip size and its shape. Pyramidal TiO2 tip arrays were successfully fabricated by dry etching in CF4/Ar plasma through a Au nanoparticle mask. The TiO2 tip arrays can be expected to have an important application in gas microsensors.
    Applied Surface Science 09/2014; 312:107-111. DOI:10.1016/j.apsusc.2014.03.174 · 2.54 Impact Factor
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    Vlastimil Rehacek, Ivan Hotovy, Marian Vojs
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    ABSTRACT: Voltammetric sensors based on bismuth film electrodes are an attractive alternative to other sensors for application in electroanalysis of heavy metals. Bismuth film electrodes can be formed by a similar method on the same substrates as mercury. These systems were used most frequently for simultaneous determination of heavy metals such as Pb, Cd and Zn by anodic stripping voltammetry. Our voltammetric sensor was fabricated on an alumina substrate. A photoresist film prepared by pyrolysis of positive photoresist S-1813 SP15 on the alumina substrate was used as an electrode support for bismuth film deposition. The influence of the Nafion membrane on the measurement sensitivity of the sensor and mechanical stability of the bismuth film were investigated. The sensor was successfully applied for determination of Pb, Cd and Zn in an aqueous solution in the concentration range of 0.2 to 10 µg L−1 by square wave anodic stripping voltammetry on an in-situ formed bismuth film electrode with Nafion-coating. Parameters of the sensor such as sensitivity, linearity, detection limit, repeatability and life-time were evaluated. In the best case, the detection limits were estimated as 0.07, 0.11 and 0.63 µg L−1 for Pb, Cd and Zn, respectively. Finally, the applicability of the sensor was tested in analysis of Pb, Cd and Zn in real samples of tap and river water using the method of standard additions.
    Electroanalysis 05/2014; 26(5). DOI:10.1002/elan.201400020 · 2.50 Impact Factor
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    ABSTRACT: The effect of NiO additives and post-deposition treatment by rapid thermal annealing on the properties of TiO2 thin films has been studied. The structural, compositional and H2 sensing parameters have been investigated by X-ray diffraction (XRD), glow discharge optical emission spectrometry (GDOES), scanning electron microscopy (SEM) and by electrical measurements. In the range of annealing temperatures from 500 °C to 700 °C crystallization started and the structure changed from amorphous to polycrystalline. The NiO modified sensing films exhibit enhanced and improved sensing behavior to hydrogen at relatively low operating temperatures. The inversion of the conductivity type of response due to different H2 concentrations has been observed. The critical H2 concentration causing break of the p-type response to n-type response was identified. The temperature dependence of this critical concentration as a function of operating temperature was studied. Moreover, change of response type due to different annealing temperatures was achieved.
    Applied Surface Science 03/2013; 269:110–115. DOI:10.1016/j.apsusc.2012.09.061 · 2.54 Impact Factor
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    ABSTRACT: Novel pyrolyzed photoresist film (PPF) electrodes for application in electroanalysis have been prepared by pyrolysis of various photoresists on alumina substrates and characterized for their electrical, microstructural and electrochemical properties. As a support, the PPF electrodes were successfully applied for the determination of heavy metals (Pb, Cd and Zn) in an aqueous solution by square wave anodic stripping voltammetry (SWASV) on in situ formed bismuth film electrodes (BiFEs) with or without Nafion-coating. Selected parameters of Nafion film preparation (thickness, curing temperature) on the analytical performance of Nafion-coated BiFEs were investigated. Since the concentration of Bi(III) used for formation of the bismuth film influences the height of the stripping peaks, the bismuth to target metals concentration ratio had to be optimized in the range from 1 to 50. 2 × 10−6 mol/L concentration of Bi(III) was chosen for the target metals concentration range of 1 × 10−8–1 × 10−7 mol/L. By using the bare and the Nafion-coated BiFEs in simultaneous determination of target metals it has been found that the analytical sensitivities for Pb(II) and Cd(II) were 2 times higher in comparison with non-coated BiFEs due to the synergistic effect of redox cycling and non-faradic cation-exchanging preconcentration in the Nafion film. In the best case, the detection limits were estimated as 3 × 10−9, 6 × 10−9 and 2.4 × 10−8 mol/L for Pb(II), Cd(II) and Zn(II), respectively.
    Electrochimica Acta 02/2012; 63:192–196. DOI:10.1016/j.electacta.2011.12.075 · 4.09 Impact Factor
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    ABSTRACT: NiO thin films were prepared by dc reactive magnetron sputtering with thicknesses of about 50 and 100 nm on alumina and KCl substrates. The deposited films were annealed in a furnace at 500°C in nitrogen atmosphere for 2 hours. From XRD and TEM investigations were found that annealed NiO thin films have a polycrystalline structure with the size of grains below 25 nm. The electrical responses of NiO sensor structures towards different ethanol, acetone and toluene concentrations have been measured. It was found that the thickness of NiO films is an important parameter in determining the sensitivity of prepared gas sensors.
    Advanced Semiconductor Devices & Microsystems (ASDAM), 2012 Ninth International Conference on; 01/2012
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    ABSTRACT: Pyrolyzed photoresist film (PPF) electrodes for application in electroanalysis were prepared on alumina substrates. These electrodes were characterized for their electrical, microstructural (by Raman spectroscopy) and electrochemical properties. As a support, the PPF electrodes were tested for simultaneous determination of Pb(II), Cd(II) and Zn(II) in an aqueous solution on in-situ formed bismuth film by square wave voltammetry (SWV). The dependence of the stripping responses on the concentration of target metals was linear in the range from 1 × 10-8 to 9 × 10-8 mol/L. The effect of activation of the PPF surface by argon plasma on analytical performance of bismuth film electrode (BiFE) on PPF support was also investigated.
    Journal of Electrical Engineering 01/2011; 62(1). DOI:10.2478/v10187-011-0008-0 · 0.42 Impact Factor
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    ABSTRACT: The multilayer compound thin films, consisted of metal oxides (TiO<sub>2</sub> and NiO) prepared by dc magnetron sputtering technique, have been studied. The structural, compositional, electrical and gas sensing properties have been investigated by XRD, GDOES and Van der Pauw method considering changes in layout, annealing temperature and addition of Au noble metal catalyst. The Au modified compound oxides exhibit fast response and enhanced sensitivity to hydrogen at low operating temperatures.
    Advanced Semiconductor Devices & Microsystems (ASDAM), 2010 8th International Conference on; 11/2010
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    ABSTRACT: The indium oxide thin films were deposited at room temperature by reactive magnetron sputtering in the mixture of oxygen and argon on silicon and oxidized silicon substrates. The influence of the oxygen flow in the reactive mixture and post-deposition annealing on the structural properties were investigated. The as deposited In(2)O(3) films showed a dominating randomly oriented nanocrystalline structure of cubic In(2)O(3). The grain size decreased with increasing oxygen concentration in the plasma. Annealing in reducing atmospheres (vacuum, nitrogen and argon), besides improving the crystallinity, led to a partial cubic to rhombohedral phase transition in the indium oxide films.
    Journal of Electrical Engineering 11/2010; 61(6):382-385. DOI:10.2478/v10187-010-0059-7 · 0.42 Impact Factor
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    ABSTRACT: Electrical characteristics and elemental depth profiles of ohmic contacts to p-GaN using Au/Ni-Mg-Ox metallization have been investigated. The objective was to examine the possibilities of increasing the charge carrier concentration in the surface region of GaN by adding Mg, thus of a p-type dopant into the Au/NiOx metallization structure. For this purpose, a Ni-Mg-Ox layer with a low concentration of Mg was deposited on p-GaN by dc reactive magnetron sputtering. The top Au layer was deposited in a similar way. The fabricated contact structures were annealed in N2. When the Ni-Mg layer in the Au/Ni-Mg-Ox/p-GaN structure was deposited in an atmosphere with a low concentration of oxygen (0.2 at%), the structure exhibited a low resistance ohmic nature. The contact resistance was lower than in the case of a Au/Ni-Ox/p-GaN structure without the Mg dopant in the metallic layer. An increase in the concentration of oxygen in the working atmosphere resulted in higher values of the contact resistance of the Au/Ni-Mg-Ox/p-GaN structure. In our opinion the ohmic nature of the contact structure is related to the existence of a metal/p-NiO/p-GaN scheme. The measured values of the contact resistance in the Au/Ni-Mg-Ox/p-GaN structure in comparison with the Au/Ni-Ox/p-GaN structure are caused by an increased charge carrier concentration in the surface region of p-GaN, which is a consequence of Mg diffusion from the Ni-Mg-Ox layer.
    Journal of Electrical Engineering 11/2010; 61(6). DOI:10.2478/v10187-010-0058-8 · 0.42 Impact Factor
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    ABSTRACT: Structural evolution of indium oxide thin films deposited at room temperature by reactive magnetron sputtering and annealing in a reducing atmosphere were investigated. The as deposited indium oxide (In2O3) films showed a dominating randomly oriented nanocrystalline structure of cubic In2O3. The grain size decreased with increasing oxygen concentration in the plasma. Annealing in reducing atmospheres (vacuum, nitrogen and argon), besides improving the crystallinity, led to a partial cubic to rhombohedral phase transition in the indium oxide films. Annealing improved the optical properties of the indium oxide film and shifted the absorption edge to higher energies.
    Thin Solid Films 06/2010; 518(16):4508-4511. DOI:10.1016/j.tsf.2009.12.018 · 1.87 Impact Factor
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    ABSTRACT: TiO2 thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO2 thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO2 thin films were tested as sensors of hydrogen at concentrations 10,000–1000 ppm and operating temperatures within the 180–200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.
    Thin Solid Films 12/2009; DOI:10.1016/j.tsf.2009.08.052 · 1.87 Impact Factor
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    Vlastimil Rehacek, Ivan Hotovy, Marian Vojs, Fedor Mika
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    ABSTRACT: Bismuth film electrodes (BiFEs) have a potential to replace toxic mercury used most frequently for determination of heavy metals (Cd, Pb, Zn) by anodic stripping voltammetry. We prepared a graphite disc electrode (0.5mm in diameter) from a pencil-lead rod and developed a nitrogen doped diamond-like carbon (NDLC) microelectrode array consisting of 50,625 microdiscs with 3μm in diameter and interelectrode distances of 20μm on a highly conductive silicon substrate as a support for BiFEs. The disc graphite BiFE was used for simultaneous determination of Pb(II), Cd(II) and Zn(II) by square wave voltammetry (SWV) in an aqueous solution. We found the optimum bismuth-to-metal concentration ratio in the solution to be 20. The dependence of the stripping responses on the concentration of target metals was linear in the range from 1×10−8 to 1.2×10−7mol/L. Detection limits 2.4×10−9mol/L for Pb(II), 2.9×10−9mol/L for Cd(II) and 1.2×10−8mol/L for Zn(II) were estimated. A bismuth-plated NDLC microelectrode array was used for Pb(II) determination by differential pulse voltammetry (DPV) in an aqueous solution. We found that the stripping current for bismuth-plated NDLC array was linear in the concentration range of Pb(II) from 2×10−8 to 1.2×10−7mol/L. The detection limit 2.2×10−8mol/L was estimated from a calibration plot.
    Microsystem Technologies 03/2008; 14(4):491-498. DOI:10.1007/s00542-007-0432-z · 0.95 Impact Factor
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    ABSTRACT: This work describes the design, simulation, fabrication and characterization of a TiN/Pt microheater prepared on a GaAs micromechanical structure as a prospective device for micro-electro-mechanical system (MEMS) sensor arrays. Electro-thermal simulation was employed to verify the properties of the designed microstructure, which confirmed achievement of the operating temperatures in the range from 470 to 600K with a heating power less than 25mW. The average temperature gradient in the active area does not exceed 0.6K/μm. Fabrication of GaAs suspended membranes was demonstrated, realized in two steps by combination of surface and bulk micromachining. Development and characterization of a microheater on a GaAs membrane is described. The mechanical stability of the heated multilayer membrane structure was tested and satisfactory mechanical stability of the hotplate was confirmed. The power consumption at an operating temperature of approximately 550K is about 30mW which is in good agreement with the value of about 22mW obtained from electro-thermal simulation. The achieved thermal resistance value is 8.43K/mW.
    Microsystem Technologies 03/2008; 14(4):629-635. DOI:10.1007/s00542-007-0470-6 · 0.95 Impact Factor
  • Sensors and Microsystems - 11th Italian Conference; 01/2008
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    ABSTRACT: In this work, analysis of three different heating systems for two types of Pt micro-hotplate is reported: GaAs bulk structure (bulk GaAs), polyimide/GaAs bulk structure (PI-GaAs) and AlGaAs/GaAs suspended membrane structure (AlGaAs/GaAs). Complex electro-thermal characterization of prepared micro-hotplates was realized. Maximal reachable temperature of suspended membrane heating structure was 260degC with corresponding power 36 mW compared with the GaAs bulk structure with maximal temperature 220degC and corresponding power 1.5 W. At temperatures and powers above maximal limits, degradation and destruction of heating meanders occurred. Power consumption P200degC of sample on GaAs bulk substrate was 850 mW, and on PI/GaAs bulk substrate 380 mW, whereas power consumption of sample prepared on AlGaAs/GaAs suspended membrane was significantly lower about 26 mW.
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    ABSTRACT: A multitude of industries use H2 either as part of their process or as a fuel. All these applications motivate nowadays the development of hydrogen sensor devices which enable its safe and controlled use. Since H2 is explosive above the lower explosion limit at 40,000 ppm, devices which permit the detection of its presence and measure its concentration become indispensable. In this work, we present a microsensor based on NiO thin films produced with dc reactive magnetron sputtering on GaAs, with an incorporated Pt heater, all on a DO-8 package ready for use. The microsensor was tested to H2 concentrations 5,000 and 10,000 ppm at different working temperatures. The change of the electrical resistance of NiO thin films was the signal for hydrogen sensing. The response of the sensor was not proportional to concentration of the gas neither to the working temperature.
    12/2007: pages 379-382;

Publication Stats

463 Citations
59.99 Total Impact Points

Institutions

  • 1998–2014
    • Slovak University of Technology in Bratislava
      • • Institute of Electronics and Photonics
      • • Department of Microelectronics
      • • Faculty of Electrical Engineering and Information Technology
      Presburg, Bratislavský, Slovakia
    • National Cancer Institute, Bratislava
      Presburg, Bratislavský, Slovakia