Micromachined NH3 Gas Sensor with ppb-level Sensitivity Based on WO3 Nanoparticles Thinfilm

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1 Micromachined NH3 Gas Sensor with ppb-level Sensitivity Based on WO3 Nanoparticles Thinfilm Author Dao, Dzung Viet, Shibuya, Kyoji, Tung, Bui Thanh, Sugiyama, Susumu Published 2011 Conference Title Proc. Eurosensors XXV, published on Procedia Engineering DOI Copyright Statement The Author(s) This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0) License ( which permits unrestricted, noncommercial use, distribution and reproduction in any medium, providing that the work is properly cited. You may not alter, transform, or build upon this work. Downloaded from Link to published version Griffith Research Online

2 Available online at Procedia Engineering 25 (2011) Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece Micromachined NH 3 Gas Sensor with ppb-level Sensitivity Based on WO 3 Nanoparticles Thinfilm Dzung Viet DAO a*, Kyoji Shibuya b, Tung Thanh Bui a, Susumu Sugiyama a a Ritsumeikan University, Nojihigashi, Kusatsu, Shiga, JAPAN b Horiba, Ltd., 2 Miyanohigashi, Kisshoin, Minami-Ku, Kyoto, Japan Abstract We present a micromachined gas sensor which can detect dilute NH 3 gas at ppb (parts per billion) concentration based on chemoresistive effect of semiconducting tungsten trioxide (WO 3 ). The sensor structure consists of a poly-si diaphragm-heater suspended on four tiny beams, which are connected to a frame with overall chip size of mm 3. Thanks to the diaphragm-beam structure, the power loss due to thermal conduction effect decreases and uniformed temperature on whole diaphragm is obtained. The sensor has been fabricated and tested. Thermal efficiency of the heater was measured to be 10 o C/mW. The optimal working temperature was found to be around 300 o C, and the sensor could detect NH 3 gas at ppb concentration in a background of wet air. The nano-gap effect of the interdigitated electrodes in improvement of the sensitivity was also confirmed. Our estimation based on the measurement results and system noise showed a limit-of-detection of NH 3 gas could be down to 2.5 ppb concentration Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Keywords: Gas sensor, NH 3, WO 3, micromachine 1. Introduction Most of ammonia (NH 3 ) presents in the atmosphere is emitted from chemical plants, cultivated farmland (fertilizer), and motor vehicles [1]. No health effects have been found in humans exposed to typical environmental concentrations of NH 3. However, exposure to high levels of ammonia can cause irritation and serious burns on the skin and in the mouth, throat, lungs, and eyes [1]. The United States * Corresponding author. Tel.: ; fax: address: dzung@se.ritsumei.ac.jp Published by Elsevier Ltd. doi: /j.proeng Open access under CC BY-NC-ND license.

3 1150 Dzung Viet DAO et al. / Procedia Engineering 25 (2011) Occupational Safety and Health Administration has set an acceptable 8h exposure limit at 25 ppm by volume. Whereas health and environment issues are defined in ppm range, NH 3 was found to affect semiconductor fabrication at ppb level. The ppb-level concentrated NH 3 can change the photochemical properties of photoresist - defects. It is also photo-reactive and can deposit on optical surfaces of lithography systems causing haze. NH 3 is emitted into wafer process from various semiconductor processes including wafer cleaning, as well as from human body. In this work, we developed a micromachined gas sensor which can detect NH 3 at ppb-level concentration for real-time monitoring airborne NH 3 in cleanroom to optimally control the ventilation fan system and, ultimately, to save the energy. Although many principles for detecting NH 3 gas were reported so far, such as metal-oxide based sensors, catalytic NH 3 sensors, conducting polymer NH 3 sensors, we focus on the metal-oxide based NH 3 gas sensors, because it provides high sensitivity, small size, reliability, integration capability, mass production and low cost. Although metal-oxide based sensors have been studied extensively, the sensors which could detect NH 3 at ppb concentration are still very rare [2-4]. 2. Design and Simulation of the Sensor The structure of the gas sensor chip is schematically shown in Fig. 1. The total chip size is 1mm 1mm 0.3mm (LWT). The sensor consists of a 400μm 400μm 1.5μm (LWT) poly-si diaphragm heater suspended on four small beams to create a thermal isolation for the heater. The sensor detects NH 3 gas based on the electrical conductance change of WO 3 nanoparticles thinfilm, which is deposited on top of micro gap interdigitated Pt electrodes on the poly Si heater. Since the poly-si is stable up to very high temperature [5], and the diaphragm can almost freely deform in all directions, the thermal-induced stress in the diaphragm is much smaller compared to that in the closed-diaphragm [6], so the structure is thermally robust. Based on FEM simulation, thicknesses of SiN and TEOS-SiO 2 are decided so that the residual stresses in these layers cancel each other to reduce the out-of-plane deformation of the diaphragm. The flat diaphragm heater facilitates the patterning of micro/nano gap interdigitated electrodes on top of the diaphragm, and provides uniformed temperature profile in whole diaphragm. The widths and gaps between two adjacent fingers of the interdigitated electrodes are 5μm and 3μm, 2μm and 1μm, as well as 2μm and 0.5μm, respectively. Pt electrode Poly Si diaphragm heater Poly Si beam WO 3 Poly ) 3. Fabrication Si 3N 4(80nm) SiO 2(300nm) Pt (150nm) TEOS-SiO 2 ) ) (a) (b) Fig. 1. Configuration of the gas sensor: (a) bird-eye view, and (b) Cross sectional view The sensor has been fabricated based on micromachining technology, includes thinfilm depositions, photolithography, wet and dried etching processes as shown in Fig.2 (1-11). The fabricated chip is shown in Fig. 2 (12). The WO 3 hexagonal crystal nanoparticles thinfilm was formed by dropping WO 3 suspending solution obtained by a hydrothermal synthesis of H 2 WO 4 on top of the interdigitated electrodes followed up with thermal treatments [7].

4 Dzung Viet DAO et al. / Procedia Engineering 25 (2011) (1) SiO 2 and SiN growth on Si (6) Contact hole opening (10) Backside wet etching (2) poly Si deposition, ion implant (7) Bonding pad formation (11) SiO 2 etching from backside (3) Pt deposition and patterning (4) TEOS isolation layer deposition (8) TEOS, poly Si & SiN etching (5) Pt comb-electrode patterning (9) Backside SiN, SiO 2 etching (12) Fabricated sensing chip SiSi3N4SiO2 Poly-SiTEOS-SiO2 PtAu 4. Characterization Fig. 2. Fabrication process of the gas sensor (1) - (11), and the fabricated sensor chip (12). Firstly, the poly-si heater is characterized. The thermal efficiency of the heater was measured to be about 10 o C/mW and the heater could operate well up to 500 o C. The characterization system of the gas sensor is shown in Fig. 4. By suitably mixing NH 3 with O 2, N 2 and wet air, NH 3 with desirable concentration in background of wet air can be produced for characterization process. This mixing gas flows through the sensor at a flow rate of 50ml/min. The sensor package is placed in a temperaturehumidity controlled chamber with the relative humidity of 30% at 25 o C. The gas-induced resistance change of WO 3 of the sensor is measured on interdigitated electrodes, and the sensitivity is calculated by S = (R /R NH3-1). Optimal operating temperature of the sensor was found to be around 300 o C as shown in Fig.5. Figs. 6 and 7 show the dependence of sensitivity on electrodes gap. It can be seen that smaller the gap, higher the sensitivity. Based on a sensor response to 500 ppb concentration in a background of wet air, a limit-of-detection of the sensor (gap 3 μm) to NH 3 gas was calculated to be 2.5 ppb. 5. Conclusions Development and evaluation of a 1x1mm 2 WO 3 based NH 3 gas sensor with poly-si heater has been reported. Multi-physics electro-thermo-structural coupled-field FEM simulation has been carried out to investigate the stress/strain and deformation of the multi-layer heater. The sensors have been fabricated based on micro machining process and its performance has been characterized. The poly Si micro heater can provide a temperature above 500 o C with thermal efficiency of 10 o C/mW. The sensor could detect

5 1152 Dzung Viet DAO et al. / Procedia Engineering 25 (2011) dilute NH 3 gas at low concentration down to 500 ppb with high sensitivity of 2.3. The sensitivity was enhanced by several factors, such as uniformed working temperature, crystal structure of WO 3 nanoparticles, nano-gap effect of the interdigitated electrodes, and so on. Specifically, the optimal working temperature was found to be about 300 o C. The sensitivity was 6 times increased when the gap size of electrode decreased from 5μm to 0.5μm. Future work will focus on further narrowing the gap between the electrodes to take the advantage of nano-gap effect as well as to increase electrode-grains boundary length, the two important factors dominate the sensitivity of the NH 3 sensor. 5 ppm, N 2 balance NH 3 N 2 O 2 AIR 500CCM N2 O2 1LM Temperature and humidity controlled chamber Temperature & humidity sensor Bubbler (T o controlled chamber) NH 3 sensor (DUT) Sensor control/ Measurement circuit Data IN/OUT Unit 50 ml/min PC Sensitivity (R air /R NH3 ) Sensor Temperature () Fig. 4. Schematic view of characterization system. 5.0E E E E E E E E E E+04 AIR NH3 500ppb AIR S=2.3 S=1.8 S= E Time (min.) gap size m Fig. 6. Sensor responses versus gap sizes of electrode (NH 3 ppb; heater temperature is 240 o C). m m Fig. 5. Sensitivity versus heater temperature (NH ppb concentration). Sensitivity (R air /R NH3-1) Fig. 7. Sensitivity versus gap size (NH ppb; heater temperature is 240 o C). Acknowledgements This work was partially supported by NEDO (New Energy and Industrial Technology Development Organization) in Japan through the BEANS (Bio Electromechanical Autonomous Nano Systems) Project. The authors would like to thank Mr. Y. Nakata and M. Matsuhara of Horiba Ltd for their valuable contributions to the fabrication process of this sensor. References [1] B. Timmer, W. Olthuis, Albert van den Berg, Sensors and Actuators B 107 (2005) [2] X. Wang, N. Miura, N. Yamazoe, Sensors and Actuators B 66 (2000) [3] V Srivastava, K. Jain, Sensors and Actuators B 133 (2008) [4]Y. Wang, Q. Mu, G. Wang, Z. Zhou, Sensors and Actuators B 145 (2010) [5] M. Ehmann, P. Ruther, M. von Arx, O. Paul, J. Micromech. Microeng. 11 (2001) [6] Dzung V. Dao, L.-H. Li, T. Hashishin, J. Tamaki, K. Shibuya, S. Sugiyama, IEEE SENSORS2010, USA, (2010) [7] J. Tamaki, Y. Nakata, Y. Yamagishi, WIPO Patent, publication No. WO/2009/ (2009).