nanostructured oxides on porous silicon microhotplates for nh3 sensing

Nanostructured oxides on porous silicon microhotplates for NH 3 sensing R. Triantafyllopoulou a, * , X. Illa b , O. Casals b , S. Chatzandroulis a , C. Tsamis a , A. Romano-Rodriguez b , J.R. Morante b a NCSR ‘‘Demokritos’’, Institute of Microelectronics, 15310, Aghia Paraskevi, Athens, Greece b 7-EME/CeRMAE/IN[2]UB, Department of Electronics, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain Received 5 October 2007; received in revised form 18 December 2007; accepted 27 December 2007 Available online 2 January 2008 Abstract Low power micromachined gas sensors based on suspended micro-hotplates are presented in this work. The sensors were fabricated using Porous Silicon Technology. Two different metal-modified nanostructured sensitive materials were deposited on top of the active area of the micro-hotplates, using the micro-dropping technique: SnO 2 :Pd and WO 3 :Cr. For the characterization of both gas sensors, measurements in NH 3 ambient took place, in isothermal mode of operation. Improved sensors characteristics were obtained for SnO 2 :Pd sensors, compared to WO 3 :Cr, for these operating conditions. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Gas sensors; Porous silicon; Micro-hotplates; Nanostructured metal oxides; NH 3 sensing 1. Introduction The last four decades the sensitivity of semiconductive metal oxides in gas sensing under atmospheric conditions, has been intensively studied. Solid state chemical sensors are one of the most common devices employed for the detection of hazardous gases. The detection of NH 3 is of high interest for application in various areas such as agri- culture, industrial chemistry, environmental quality, auto- motive and medical applications. Ammonia sensing can be achieved by using conductometric gas sensors. The sens- ing mechanism is based on conductivity changes of the sen- sitive material, which is deposited on the top of the active area of the sensors, and corresponds to electrical modifica- tions caused both by ammonia and the by-products of the oxidation reaction of ammonia at the surface. Moreover, it has been demonstrated that the sensitivity towards various gases can be increased by using metal additives and by decreasing the crystallite size of the catalytic material [1]. In this work, we present measurements of low power gas sensors in NH 3 ambient. The sensors are based on suspended Porous Silicon micro-hotplates, for low power consumption. In order to enhance sensor sensitivity, additive-modified nan ostructured metal oxides were used as sensitive materials (SnO 2 :Pd and WO 3 :Cr), fabricated by a sol-gel process and deposited via micro-dropping. Taking into account that the occupational exposure limit for NH 3 is 50 ppm, the measurements occurred mainly in low concentrations of NH 3 , in order to detect the gas below this limit. 2. Experimental The fabrication process of suspended Porous Silicon micro-hotplates has been reported in detail elsewhere [2]. The micromachined sensors consisted of Porous Silicon membranes and a heater of doped polysilicon, which was embedded between two insulating layers. Ti/Pt layers were 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.038 * Corresponding author. Tel.: +30 210 650 3113; fax: +30 210 651 1723. E-mail address: roubini@imel.demokritos.gr (R. Triantafyllopoulou). www.elsevier.com/locate/mee Available online at www.sciencedirect.com Microelectronic Engineering 85 (2008) 1116–1119 deposited and patterned to serve as electrodes and contact pads, while the release of the devices was performed in a High Density Plasma reactor. After the fabrication of the micro-hotplates, the deposition of two different sensitive materials took place, using the micro-dropping technique [3]. Additive-modified nanostructured metal oxides were prepared by a sol-gel solution and then were deposited by micro-dropping on the suspended devices, as shown in Fig. 1. The sol-gel process for the preparation of the SnO 2 :Pd and WO 3 :Cr sensitive materials, is reported else- where [4,5]. The two sensitive materials were deposited on the micro-hotplates as follows: at first, nanopowders were mixed with an organic solvent, in order to obtain good adhesion to the substrate. A meniscus is formed and then, when the meniscus reaches the micro-hotplate, the paste is deposited by capillarity. Finally, the paste is heated up in order to remove the organic solvent. Both materials were thermally treated at a temperature of 300 °C, using the device’s heater, for 12 hours, in order to modulate and activate the sensitive material before the gas measurements. The use of micro-hotplates gives the opportunity to fabricate gas sensor arrays that incorporate varying sensitive materials, operating with very low power consumption. Fig. 2 shows an array of gas sensors with various sensitive materials, deposited by the micro-drop- ping technique. The fabrication of micro-dropped sensors has been mainly reported in closed type membranes, while in this work we focus on suspended Porous Silicon micro- hotplates. 3. Results Characterization of the gas sensors was performed at isothermal mode of operation, by keeping constant the power supplied to the heater. For the micro-hotplates used in the present work, a temperature increase rate of $ 21 °C/ mW has been estimated, based on combined electrical results as well as IR measurements. As a consequence, high operating temperatures can be achieved with low power consumption, e.g. a temperature of 300 °C is achieved with a supply of about 13 mW. The sensors were introduced into the test chamber and were exposed in various concen- trations of NH 3 in dry air, both at low (2–15 ppm) and at high (100–500 ppm) concentrations, while the working temperature of the sensors ranges from 200 °C to 350 °C. The exposure an d recovery time of the sensors was 15 min and 30 min respectively. The sensitivity of the sensors was defined as the ratio R air =R NH 3 , where R air is the sensor resistance in dry air and R NH 3 the electrical resistance in the targeted gas. Fig. 3, shows the response of the sensors with SnO 2 :Pd metal oxide, deposited by micro-dropping, for two different temperatures. We notice that the sensitivity of the sensors Fig. 1. SEM image of a micro-hotplate on top of its active area SnO 2 :Pd is deposited by micro-dropping. Fig. 2. SEM image of sensors array of various sensitive materials (SnO 2 :Pd and WO 3 :Cr). Fig. 3. Comparison of the sensitivity of gas sensors with undoped sputtered SnO 2 sensitive material and sensors with micro-dropped SnO 2 :Pd sensitive material. R. Triantafyllopoulou et al. / Microelectronic Engineering 85 (2008) 1116–1119 1117 increases with gas concentration and temperature. In the same figure, measurements of sensors using undoped SnO 2 deposited by RF sputtering are also reported for comparison. We notice that the sensitivity of the sensors increases with gas concentration and temperature. In the same figure, measurements of sensors using undoped SnO 2 deposited by RF sputtering are also reported for comparison. We notice that the response of sputtered SnO 2 sensors towards NH 3 , is lower compared to micro- dropped sensors, even when they are operated at higher temperatures (400 °C) in our case. Such a behavior is expected due to the nanostructured nature of the micro- dropped materials. Fig. 4 shows the resistance of SnO 2 :Pd and WO 3 :Cr sen- sors for various gas pulses, as NH 3 concentration increases from 2 ppm to 15 ppm. We notice that a good saturation level is obtained for both materials, for the exposure and the recovery phases as well as a good baseline, when no gas is present. In Fig. 5 the sensitivity of the gas sensors with SnO 2 :Pd and WO 3 :Cr is reported, for different operat- ing temperatures. SnO 2 :Pd gas sensors exhibit higher sensi- tivity in detecting NH 3 compared to WO 3 :Cr sensors, in agreement with the literature [6], with low power consump- tion. We notice different temperature dependence for the response for each material. In the case of WO 3 :Cr, sensor response is increased as the temperature raises from 290 °C to 350 °C. However, in the case of SnO 2 :Pd the sig- nal obtained at 280 °C is significantly higher than that obtained at 350 °C. Such a behavior is not unusual for metal oxide sensors and is attributed to the mechanisms of gas adsorption and desorption on the surface of the cat- alytic material. In principle, a metal oxide can adsorb oxy- gen from the atmosphere both as O 2 À and O À species. The adsorption of O À is more reactive and thus makes the material more sensitive to the presence of a reducing gas, such as NH 3 . At relatively low temperatures the surface preferentially adsorbs O 2 À and the sensor response is con- sequently low. As the tempe rature increases the dominant process becomes the adsorption of O À and hence the sensi- tivity of the material increases. When the temperature increases too much, then desorption of all the oxygen ionic species adsorbed previously occurs and the sensitivity decreases again [7]. 4. Conclusions In this work, low power micromachined gas sensors based on suspended micro-hotplates were fabricated and characterized. Two different metal-modified nanostruc- tured sensitive materials were deposited on top of the active area of the micro-hotplates, using the micro-dropping tech- nique: SnO 2 :Pd and WO 3 :Cr. Porous Silicon micro-hot- plates were used for low power consumption. Characterization of gas sensors was performed for various NH 3 concentrations and operating temperatures, using iso- thermal mode of operation. Improved charact eristics were obtained for SnO 2 :Pd sensors, compared to WO 3 :Cr, for these operating conditions. Acknowledgments This work was partially supported by the Greek General Secretariat of Research and Technology (PENED, Con- tract 04ED630), by the Spanish Ministry of Educa tion and Science through the CROMINA project (TEC2004- 06854-C03-01) and by the European Union through the GOODFOOD project (IST-1-508774-IP). References [1] D.G. Rickerby, M.C. Horrillo, J.P. Santos, P. Serrini, Nanostructured Materials 9 (1997) 43–52. Fig. 4. Typical graph of the resistance of both gas sensors with SnO 2 :Pd and WO 3 :Cr sensitive materials, for various pulses of low concentrations of NH 3 : 2-15 ppm. Fig. 5. Sensitivity of gas sensors with SnO 2 :Pd and WO 3 :Cr micro- dropped sensitive materials, in various temperatures, for low concentra- tions on NH 3. 1118 R. Triantafyllopoulou et al. / Microelectronic Engineering 85 (2008) 1116–1119 [2] R. Triantafyllopoulou, S. Chatzandroulis, C. Tsamis, A. Tserepi, Microelectronics Engineering 83 (4–9) (2006) 1189–1191. [3] J. Cerda ` , A. Cirera, A. Vila ` , A. Cornet, J.R. Morante, Thin Solid Films 391 (2) (2001) 265–269. [4] I. Jimenez, M.A. Centeno, R. Scotti, F. Morazzoni, J. Arbiol, A. Cornet, J.R. Morante, J. Mater. Chem. 14 (2004) 2412–2420. [5] A. Cirera, A. Vila, A. Cornet, J.R. Morante, Mater. Sci. Eng. C 15 (2001) 203–205. [6] A.M. Ruiz, X. Illa, R. Diaz, A. Romano-Rodriguez, J.R. Morante, Sensors and Actuators B 118 (2006) 318–322. [7] B. Karunagaran, Periyayya Uthirakumar, S.J. Chung, S. Velumani, E K. Suh, Materials Characterization 58 (2007) 680–684. R. Triantafyllopoulou et al. / Microelectronic Engineering 85 (2008) 1116–1119 1119 . WO 3 :Cr. Porous Silicon micro-hot- plates were used for low power consumption. Characterization of gas sensors was performed for various NH 3 concentrations. Nanostructured oxides on porous silicon microhotplates for NH 3 sensing R. Triantafyllopoulou a, * , X. Illa b ,