PAPER #ID: 1568965100 Ozone Sensors based on Layered SAW Devices with: InOr / SiNk / 360 YX LiTaO3 Structure Alexandru C. Fechete, Wojtek B. Wlodarski, Kourosh Kalantar-zadeh, Anthony S. Holland, and Anurat Wisistsora-at Abstract-Monitoring of air-quality it is a major task nowadays as the emission of pollutant gases increases particularly in urban areas. Ozone (03) is a significant toxic pollutant and presents health risks. We have developed ozone sensors using a Surface Acoustic Wave (SAW) based devices. The sensors structure consists of a lithium tantalate piezoelectric (LiTaO3) substrate with silicon nitride (SiNx) intermediate layer deposited by R.F. magnetron sputtering and E-beam evaporation techniques. A 100 nm thin film of indium oxide (InOx) deposited by R.F. magnetron sputtering provides the selectivity towards O3. This paper presents a comparative study of the sensors performance in terms of response time, recovery time and response magnitude as a function of operational temperature. Exceptionally large frequency shift as high as 56 kHz was observed for O3 concentrations as low as 50 parts per billion (ppb) in air. Microstructural characterization of the InOx thin films by Atomic Force Microscopy (AFM) is also presented. Index Terms- Indium oxide, Ozone, SAW sensors, Silicon nitride. I. INTRODUCTION OZONE is a strong, clean disinfectant and deodorizing agent. On the other hand ozone (03) is a highly reactive pollutant, toxic gas that presents health risks [1]. The analytical instruments used to detect 03 are based on techniques such as: optical spectroscopy, gas chromatography, and mass spectrometry. These are bulky, expensive and not suitable for multispot (outdoor) measurements [2]. Therefore, compact, reliable and cheap gas sensors, which can be used for field measurements under different environmental conditions, can be an efficient alternative. Employing a Surface Acoustic Wave (SAW) based device as a gas sensor presents distinctive advantages, such as: low cost, high sensitivity and reliability [3]. They can be interfaced with electronic circuits making them portable to be used for field measurements under different environmental conditions. Manuscript received May 30, 2005. This work was supported by CRC for Microtechnology, Victoria, AUSTRALIA. A.C. Fechete, W.B. Wlodarski, and K. Kalantar-zadeh are with the Sensor Technology Laboratory at the Electrical & Computer Engineering Department, RMIT University, Melbourne 3001, Victoria, AUSTRALIA (Corresponding author's tel: +61 3 99253690; fax: +61 3 99252007, e- mail: alex.fecheteaieee.org). A.S. Holland is with the Electrical & Computer Engineering Department, RMIT University, Melbourne 3001, Victoria, AUSTRALIA (e-mail: anthony.hollandArmit.edu.au). A. Wisistsora-at is with the National Electronic & Technology Centre, Pathumathani, THAILAND (e-mail: anurat.wisitsoraat nectec.or.th). Among them layered the SAW devices provide a higher mass and conductometric sensitivity comparing with their non-layered SAW counterparts. Takada reported in 1993 for the first time a conductometric 03 sensor using an indium oxide (In203) thin film [4]. Indium oxide has a unique combination of electrical and optical properties, which makes it an important material for optoelectronic and chemical sensing applications. In a non-stiochiometric form is a highly conductive semiconductor material and presents excellent sensing properties towards reducing and oxidizing gases such as hydrogen (H2) and 03 [5], [6]. Different deposition techniques e.g.: spin coating, spray pyrolysis, sol- gel method, and DC sputtering were used to enhance the sensitivity of In203 thin films to 03 [6] - [8]. The sputtering methods are the most used ones as these techniques gives the best results, producing films that are mechanically stable and with good adhesion [9]. The sol-gel method produced highly dispersed In203 films with a prominent deviation from stoichiometry. Furthermore Gurlo et al. [10] correlated the sensing properties of the In203 films to the grain size distribution and morphology of the sol-gel prepared films. Korotcenkov et al. [11] reported that In203 porous films with minimal grain sizes and maximal deviation from stoichiometry have both maximal responses to 03 and a fast response time. Doping In203 films with molybdenum and nickel ions further increases the dispersity and deviation from the stoichiometric composition [12]. Kiriakidis et al. [13] developed a low temperature sensor for 03 and NO2 gases using a dc sputtered InO0 thin film, capable of detecting 50 ppb concentrations of 03 at 50° C. More recently ultra-sensitive low temperature 03 and NO2 sensors were reported using dc magnetron sputtered InO0 thin films [14]. In this paper layered SAW based ozone sensors with an InO0 / SiNX / 360YX LiTaO3 structure are presented. The interaction of the 03 molecules with the InO0 sensing layer perturbs the electrical boundary conditions at the surface of the SAW device. Changes in the oscillation frequency of the sensor can be correlated to the corresponding 03 concentrations. The sensitivity of InO0 films can be attributed to the oxygen vacancies in the films. The 02 molecules diffuse into the pores of the films to be oxidized on the surface and as a result a conductivity change of the InO0 film will take place. By controlling the oxygen deficiency, the conductivity of InOU films can be altered [15], [16]. Silicon nitride thin film (SiNx) presents advantages for SAW device applications such as: have a high acoustic velocity, low propagation loss, high I Authorized licensed use limited to: IEEE Xplore. Downloaded on November 17, 2008 at 21:51 from IEEE Xplore. Restrictions apply.