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Development of a Miniaturized NO2 Gas Sensor Based on Nanoparticles WO3 Thin Film on Interdigitated Electrodes Dzung Viet Dao G-Device Center Kansai BEANS (Bio Electromechanical Autonomous Nano Systems) Kusatsu, Shiga, JAPAN

Ling-Han Li Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Japan (Present affiliation: Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan)

Takeshi Hashishin, Jun Tamaki Dept. of Applied Chemistry, Ritsumeikan University Kusatsu, Shiga, JAPAN

Kyoji Shibuya, Susumu Sugiyama G-Device Center Kansai, BEANS Kusatsu, Shiga, JAPAN

Abstract—A miniaturized semiconducting metal oxide NO2 gas sensor with integrated micro heater is reported in this paper. The sensor has the size of 1×1×0.4mm3 and detects NO2 based on the electrical conductance change of porous nanoparticles WO3 thin film, which is deposited on the interdigitated Pt electrodes. Integrated micro heater designed to provide the operating temperature above 200oC for the sensing material WO3. In this work, a 2 μm-thick Si diaphragm is suspended on four surrounding beams and separated from the substrate, therefore, the thermal isolation is very efficient. Furthermore, the thermal stress is also reduced in the diaphragm, because it could expand or contract easily in all directions when the temperature changed. The fabrication is simple since the heater and electrode are fabricated in the same process. The sensor can detect dilute NO2 gas as low as 0.05 ppm with high sensitivity up to 13. The design and electro-thermo-structural coupled field simulation as well as the fabrication and testing of the sensor are presented.

I.

electrode [3]. The micro- and nano-gap effects were explained as follows [2]. The contribution of resistance at electrode– grains interface to total sensor resistance becomes larger when the gap size is decreased. The resistance change at electrode– grain interface is much larger than that at grain boundary when the microsensor is exposed to NO2. Thus, the sensitivity to NO2 gas was increased with decreasing the gap size of the electrode.

Pt Heater

Pt Electrode WO3

INTRODUCTION

Recently, the concern in the exhaust problem of pollutant gases is growing, among which the oxidant gas such as NO2 becomes serious. For example, it has bad effects on human’s respiratory system as well as nerve system and moreover, accounts for the photochemical smog and acid rain. Therefore, micro gas sensors with high sensitivity to dilute NO2 gas (0.04-0.06 ppm), small size, robustness and low power consumption are important for practical NO2 monitoring system. Tungsten Trioxide (WO3) was found to be an excellent sensing material for NO2 detection [1]. Recently, Tamaki et al reported ultrahigh-sensitive WO3-based NO2 gas sensor which could detect dilute NO2 below 0.05 ppm with extremely high sensitivity and excellent response-recovery characteristics based on micro-gap effect [2] and nano-gap interdigitated Au

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(a)

WO3

Pt heater (b)

Si(1.8μm)

Pt (0.15μm) SiO2(0.2μm) SiO2(0.5μm) Si(400μm)

Figure 1. Configuration of the micro NO2 gas sensor: (a) bird-view showing electrode and heater on the diaphragm (b) cross-sectional view of the sensor.

This paper presents the development and characterization of a miniaturized 1x1mm2 WO3-based micro NO2 gas sensor

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IEEE SENSORS 2010 Conference

with micro-gap interdigitated Pt electrode and integrated Pt micro heater. Fig. 1 shows the schematic drawings of the sensor. The Pt interdigitated electrode and integrated micro heater are in the same plane of a thermally isolated silicon diaphragm with the size of 410×410×2 μm3, which is suspended on four tiny silicon beams. The sensor detects NO2 based on the electrical conductance change of porous WO3 deposited on the Pt interdigitated electrodes at elevated temperature. We have applied the WO3 grain models [2] to prove that along with nano gap effect of the electrodes, the sensitivity increases with larger electrode-grain boundary length. The multi physical coupled field FEM simulation was also carried out to study the thermally induced deformation of the diaphragm and the performance of the integrated micro heater, which provides homogeneous temperature (above 200oC) for the sensing material. The sensor has been fabricated using silicon micromachining technology and the performance of the sensor has been tested. The measured sensitivity was 13 for dilute NO2 down to 0.05 ppm, i.e. much higher than those of the prior work with integrated heater [4-8]. II. A.

DESIGN AND SIMULATION OF MICRO NO2 GAS SENSOR

also be evaluated. The 2D grain network model can be considered as a resistance network and solved by the nodeanalysis method. The calculation results of relationship between sensitivity and electrode length is shown in Fig. 2, i.e. the longer the electrode length, the higher the sensitivity. Therefore, interdigitated shape of the sensing electrode is introduced to enlarge the effective length of the electrode and the gap between electrodes is supposed to be less than 1 μm. B. Electro-thermo-structural simulation The performance of the sensor’s micro heater is simulated by using ANSYS multi-field solver. First of all, the heat transfer analysis is carried out to find the necessary gap size between the sensing diaphragm and bulk silicon frame in order to achieve good thermal isolation. The results showed that with gap size larger than 15 μm, heat isolation can be achieved well. The connecting beams between silicon frame and diaphragm are designed to be as long and narrow as possible to suppress thermal conduction to the frame. Then, electro-thermo-structure coupled-field simulation for the diaphragm with micro heater is carried out. The FEM (finite element method) model is constructed based on the design shown in Fig. 1 and the results are shown in Fig. 3 and Fig. 4.

WO3 grain sensing model The WO3 grains model [2] for estimating the electrode size

1 effect on NO2 sensing is based on three assumptions that: ○ the WO3 film is mainly constructed by the grains with 2 the total resistance of film is dominantly diameter of 20nm, ○ 3 the determined by the bottom part between electrodes, and ○ resistance change happens in the grains-grains and grainselectrode boundaries with the resistance change of grainselectrode boundary is larger.

Figure 3. Temperature distribution on the diaphragm of the sensor.

o

Diaphragm temperature ( C)

500

w

Beam-diaphragm 400

Closed diaphragm

300 200 100 0 5

10

15 Power (mW)

20

Figure 2. Sensitivity vs. electrode length under 0.1 ppm NO2 gas. Sensitivity rises with larger electrode length. The length of the electrode is proportional to the number of grain N, i.e. w = N×20 nm.

Figure 4. Relationship between diaphragm temperature and power consumption.

Therefore, the so called nano gap effect occurs, which means the NO2 gas sensitivity increases quickly with the gap between electrodes becomes less than 1 μm. We can expand the 1D model to 2D network model so that the relationship between the sensitivity to NO2 gas and electrode length can

Fig. 4 shows the relationship between the power consumption of the heater and the maximum temperature of the diaphragm. For example, the micro heater can provide a temperature of 250oC for the diaphragm at 12.5 mW power consumption with the temperature deviation less than 1oC

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across the whole diaphragm. The structure deformation of the diaphragm is less than 1.55 μm with maximum Von Mises stress is less than 555 kPa. The sandwich structure of SiO2/Si/SiO2 is responsible for the small structure deformation due to the mismatch of the thermal expansion of the materials [9, 10]. To illustrate the efficiency in thermal insulation of the beam-diaphragm structure, the simulation result for the closed diaphragm structure, i.e. the diaphragm attached directly to the frame without beam, is also graphed in Fig. 4. III.

FABRICATION OF THE SENSOR

microscope and Appendot-FemtoJet. The sensor was then dried at 35oC for 24 hours. Finally, the sensors were put under 300oC for 3 hours for calcinations and H2WO4 film was self-annealed to form WO3 thin film on the platinum electrode as shown in Fig. 6c. Fig.6d shows the nanoparticles WO3 thin film with high porosity on the platinum electrode, which will facilitate the penetration of gas into the sensing material WO3 and increase the contact surface between WO3 and gas particles. These features are very important for high sensitivity of the sensor.

Figure 5 shows the fabrication process flow of the micro sensor.

Oxidation on SOI wafer

Surface ICP etching

Patterning Photo resist and sputtering

Back etching using ICP

Lift off

Etch out the SiO2 to release

Figure 6. Fabrication results: (a) 1x1mm2 micro NO2 gas sensor with integrated micro heater, (b) Close-up image of electrode and heater on released diaphragm, (c ) WO3 deposited on electrode, and (d) Nanoparticles of WO3 thin film on Pt electrode. Etching out SiO2 to make windows

Dropping thin film WO3

IV. Figure 5. Fabrication process of the micro NO2 gas sensor with integrated micro heater.

SOI (silicon on insulator) wafer with Si device layer of 1.8 μm, buried silicon dioxide layer of 0.5 μm and Si substrate of 400 μm was used. Firstly, the SOI wafer was oxidized by thermal oxidation. Then lift-off process was carried out to form platinum heater and sensing electrodes on the silicon dioxide layer. After that alignment lithography was done and surface silicon layer were etched out by BHF, then the Si device layer was etched away by ICP-RIE process to form the diaphragm and cantilevers. Next, the back side was sputtered with Aluminum to form the mask for the backside ICP-RIE etching through the substrate layer of the SOI wafer. Finally, the SiO2 BOX layer was etched away by vapor HF and the diaphragm was released (as shown in Figs. 6a and 6b). The dimensions of one electrode finger are 2×0.15×250 μm3 (W×T×L), and the gap between two electrodes is 1.5 μm. On the other hand, the H2WO4 suspending solution was prepared. It was dropped on the interdigitated-shape electrodes manually with support of ZEISS Stemi 2000-C optical

CHARACTERIZATION OF THE SENSOR

The characterization of the sensor was carried out using a sealed gas chamber equipped with valves and mass flow controllers system so that the NO2 gas, air, and N2 gas could be introduced separately and mixed as shown schematically in Fig. 7. A simple electrical circuit similar to a voltage divider was used to monitor the resistance change of the WO3 through a reference resistor r. The relation between the resistance R of WO3 and the output voltage V is given by the inset equation in Fig. 7. The gas sensor was tested under 3 different concentrations of NO2 gas. The sensitivity is calculated under different gas concentration detection.

S=

⎛ 1 ⎞ = ⎜ − 1⎟ ⎟ Ra ⎜⎝ Vg ⎠

Rg

⎛1 ⎞ ⎜⎜ − 1⎟⎟ ⎝ Va ⎠

(1)

where Va denotes the voltage of the reference resistor before the NO2 is introduced, and Vg indicates the voltage on the resistor after the NO2 is introduced. Fig. 8 shows the

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output voltage curves measured across the reference resistor corresponding to the change of resistance of the gas sensor. The sensor shows the sensitivity (S=Rg/Ra) of 87 to 1 ppm, 17 to 0.1 ppm and 13 to 0.05 ppm NO2 concentrations at the operating temperature of diaphragm of 200oC. As mentioned above, the environmental standard of NO2 gas is 0.04-0.06 ppm, therefore, this gas sensor shows quite a good potential in practically monitoring the environmental NO2 gas.

V.

SUMMARY

In this paper, the development and evaluation of a 1x1mm2 semiconducting metal oxide micro NO2 gas sensor with integrated micro heater has been reported. WO3 grain model had been applied to evaluate the electrode-grain boundary length and the micro-gap effect on the sensitivity of the sensor. Furthermore, the multi-physics electro-thermo-structural coupled-field FEM (finite element method) simulation had been carried out to study the stress/strain and deformation of the beam-diaphragm structure, as well as the performance of integrated plane micro heater on the sensing diaphragm. The sensors have been fabricated using SOI wafer based on micro machining process and the performance of it had been characterized. The sensor could detect dilute NO2 gas at low concentration down to 0.05 ppm with high sensitivity of 13. The integrated micro heater can provide a temperature above 200oC with low power consumption. 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 the electrode-grains boundary length, the two important aspects dominate the sensitivity of the NO2 gas sensor. ACKNOWLEDGMENT

R=(E/V-1)r

This work was partially supported by BEANS (Bio Electromechanical Autonomous Nano Systems) Project, Japan.

Figure 7. Experiment setup for evaluation of NO2 detection of the sensor.

REFERENCES [1]

Figure 8. Response transients of the sensor to 1 ppm, 0.1 ppm and 0.05 ppm NO2 concentrations at the operating temperature of 200oC.

The micro gas sensor shows high sensitivity and quite quick response to NO2 gas in a wide concentration range. However, the recovering time is still long compared with 1 response time. It may be because of the following reasons: ○ 2 gap of the sensing the medium characteristics get worse, ○ 3 the fact that gold electrode is electrode is relative large, and ○ superior to platinum electrodes in reducing medium recovery. The voltage of the recovery process is a little lower than that of the starting one because the NO2 adhered to the sensing material might be not completely removed at 200oC. Therefore, the voltage differences in the high concentration are larger than in the lower concentration. This effect can be improved by further heating up the sensing materials to a higher temperature, e.g. above 250oC.

M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, “Tungsten oxidebased semiconductor sensor highly sensitive to NO and NO2”, Chem. Lett. 1991 (1991) pp. 1611–1614. [2] J. Tamaki, A. Miyaji, J. Makinodan, S. Ogura, S. Konishi, “Effect of micro-gap electrode on detection of dilute NO2 using WO3 thin film micro sensors”, Sensors and Actuators B 108 (2005), pp. 202-206. [3] J. Tamaki, T. Hashishin, Y. Uno, Dzung V. Dao, S. Sugiyama, “Ultrahigh-sensitive WO3 Nanosensor with Interdigitated Au Nanoelectrode for NO2 Detection”, Sensors and Actuators B, Vol.132, (2008), pp. 234–238. [4] C. Zamania, K. Shimanoe, and N. Yamazoe, “A new capacitive-type NO2 gas sensor combining an MIS with a solid electrolyte”, Sensors and Actuators B Vol. 109, Issue 2, (2005), pp. 216-220. [5] K.Sadek and W.Moussa, “MEMS Design for Fabrication”, Proceedings of the 2005 International Conference on MEMS, NANO and Smart Systems (ICMENS’05) 0-7695-2398-6/05© 2005 IEEE. [6] K. Kanda, T. Maekawa, “Development of a WO3 thick-film-based sensor for the detection of VOC”, Sensors and Actuators B Vol. 108 (2005), pp. 97–101. [7] J. Tamaki, A. Hayashi, Y. Yamamoto, M. Matsuoka, “Detection of dilute nitrogen dioxide and thickness effect of tungsten oxide thin film sensors”, Sensors and Actuators B 95 (2003) pp. 111–115. [8] X. He, et al., “NO2 sensing characteristics of WO3 thin film micro gas sensor”, Sensor and Actuators B, Vol 93, Issues 1-3, (2003), pp. 463467. [9] G. Wiche , A. Berns, H. Steffes, E. Obermeier, “Thermal analysis of silicon carbide based micro hotplates for metal oxide gas sensors”, Sensors and Actuators A 123–124 (2005) 12–17 [10] Ling-Han Li, Dzung V. Dao, J. Tamaki, S. Sugiyama and Y. Nakano, “Development of an ultra small semiconducting metal oxide NO2 gas sensor with integrated micro heater”, Proceedings of 4th Asia-Pacific Conference on Transducers and Micro-Nano Technology (APCOT), (2008), Taiwan.

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