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Microelectronic Engineering 85 (2008) 1116–1119 www.elsevier.com/locate/mee
Nanostructured oxides on porous silicon microhotplates for NH3 sensing R. Triantafyllopoulou a,*, X. Illa b, O. Casals b, S. Chatzandroulis a, C. Tsamis a, A. Romano-Rodriguez b, J.R. Morante b b
a NCSR ‘‘Demokritos’’, Institute of Microelectronics, 15310, Aghia Paraskevi, Athens, Greece 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: SnO2:Pd and WO3:Cr. For the characterization of both gas sensors, measurements in NH3 ambient took place, in isothermal mode of operation. Improved sensors characteristics were obtained for SnO2:Pd sensors, compared to WO3:Cr, for these operating conditions. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Gas sensors; Porous silicon; Micro-hotplates; Nanostructured metal oxides; NH3 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 NH3 is of high interest for application in various areas such as agriculture, industrial chemistry, environmental quality, automotive and medical applications. Ammonia sensing can be achieved by using conductometric gas sensors. The sensing mechanism is based on conductivity changes of the sensitive material, which is deposited on the top of the active area of the sensors, and corresponds to electrical modifications 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
*
Corresponding author. Tel.: +30 210 650 3113; fax: +30 210 651 1723. E-mail address:
[email protected] (R. Triantafyllopoulou).
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.038
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 NH3 ambient. The sensors are based on suspended Porous Silicon micro-hotplates, for low power consumption. In order to enhance sensor sensitivity, additive-modified nanostructured metal oxides were used as sensitive materials (SnO2:Pd and WO3:Cr), fabricated by a sol-gel process and deposited via micro-dropping. Taking into account that the occupational exposure limit for NH3 is 50 ppm, the measurements occurred mainly in low concentrations of NH3, 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
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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 SnO2:Pd and WO3:Cr sensitive materials, is reported elsewhere [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-dropping 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 microhotplates. 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
Fig. 1. SEM image of a micro-hotplate on top of its active area SnO2:Pd is deposited by micro-dropping.
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Fig. 2. SEM image of sensors array of various sensitive materials (SnO2:Pd and WO3:Cr).
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 concentrations of NH3 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 and recovery time of the sensors was 15 min and 30 min respectively. The sensitivity of the sensors was defined as the ratio Rair =RNH 3 , where Rair is the sensor resistance in dry air and RNH 3 the electrical resistance in the targeted gas. Fig. 3, shows the response of the sensors with SnO2:Pd metal oxide, deposited by micro-dropping, for two different temperatures. We notice that the sensitivity of the sensors
Fig. 3. Comparison of the sensitivity of gas sensors with undoped sputtered SnO2 sensitive material and sensors with micro-dropped SnO2:Pd sensitive material.
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increases with gas concentration and temperature. In the same figure, measurements of sensors using undoped SnO2 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 SnO2 deposited by RF sputtering are also reported for comparison. We notice that the response of sputtered SnO2 sensors towards NH3, is lower compared to microdropped 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 microdropped materials. Fig. 4 shows the resistance of SnO2:Pd and WO3:Cr sensors for various gas pulses, as NH3 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 SnO2:Pd and WO3:Cr is reported, for different operating temperatures. SnO2:Pd gas sensors exhibit higher sensitivity in detecting NH3 compared to WO3:Cr sensors, in agreement with the literature [6], with low power consumption. We notice different temperature dependence for the response for each material. In the case of WO3:Cr, sensor response is increased as the temperature raises from 290 °C to 350 °C. However, in the case of SnO2:Pd the signal 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 catalytic material. In principle, a metal oxide can adsorb oxygen from the atmosphere both as O2 and O species. The adsorption of O is more reactive and thus makes the material more sensitive to the presence of a reducing gas, such as NH3. At relatively low temperatures the surface
Fig. 5. Sensitivity of gas sensors with SnO2:Pd and WO3:Cr microdropped sensitive materials, in various temperatures, for low concentrations on NH3.
preferentially adsorbs O2 and the sensor response is consequently low. As the temperature increases the dominant process becomes the adsorption of O and hence the sensitivity 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 nanostructured sensitive materials were deposited on top of the active area of the micro-hotplates, using the micro-dropping technique: SnO2:Pd and WO3:Cr. Porous Silicon micro-hotplates were used for low power consumption. Characterization of gas sensors was performed for various NH3 concentrations and operating temperatures, using isothermal mode of operation. Improved characteristics were obtained for SnO2:Pd sensors, compared to WO3:Cr, for these operating conditions. Acknowledgments This work was partially supported by the Greek General Secretariat of Research and Technology (PENED, Contract 04ED630), by the Spanish Ministry of Education and Science through the CROMINA project (TEC200406854-C03-01) and by the European Union through the GOODFOOD project (IST-1-508774-IP). References
Fig. 4. Typical graph of the resistance of both gas sensors with SnO2:Pd and WO3:Cr sensitive materials, for various pulses of low concentrations of NH3: 2-15 ppm.
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