High Sensitivity and Low Power Consumption Gas Sensor Using MEMS Technology and Thick Sensing Film

Transcription

1 Journal of the Korean Physical Society, Vol. 45, No. 5, November 2004, pp High Sensitivity and Low Power Consumption Gas Sensor Using MEMS Technology and Thick Sensing Film Nak-Jin Choi, Jun-Hyuk Kwak and Duk-Dong Lee School of Electronic & Electrical Engineering, Kyungpook National University, Daegu Jeung-Soo Huh Department of Materials Science and Metallurgy, Kyungpook National University, Daegu Jae-Chang Kim Department of Chemical Engineering, Kyungpook National University, Daegu Kwang-Bum Park, Joon-Sik Park, Kyu-Sik Shin and Hyo-Derk Park Nanomecatronics Center, Korean Electronics Technology Institute, Kyung-Ki (Received 7 July 2004) Thick films based on tin oxide are fabricated on Si substrate and their gas response characteristics for test gas are examined. Sensing materials are added Al 2O 3 based on SnO 2. In general, sensing materials are deposited on alumina substrate because of easy process but this type of sensor has high power consumption. Two heater types using a micromechanical system (MEMS) are designed to reduce the power consumption and are simulated. The sensor consists of one heater, two temperature electrodes and one sensing electrode. Sensor dimensions are 8 mm 8 mm 0.5 mm, and sensing-film dimensions are 2.7 mm 2.7 mm. Test gas is dimethylmethyl phosphonate (DMMP) gas that is the simulant gas of nerve agent, one of the chemical warfare agents. Sensitivities are measured with different amounts of Al 2O 3, with different operating temperatures and with different concentrations. High sensitivity is acquired by using thick sensing film, and low power consumption, 130 mw at 300 C, is achieved by using the MEMS technology. PACS numbers: 06 Keywords: Tin oxide, MEMS, Thick film, DMMP I. INTRODUCTION Many investigators have tried to detect gas by utilizing various kinds of sensors including metal oxide semiconductor sensors (MOS), quartz-crystal microbalance sensors (QCM), electrochemical sensors, surface acoustic wave sensors (SAW), etc [1 5]. SnO 2 -based semiconductor sensors, one of the above mentioned sensors, have many advantages such as high sensitivity, fast response, and good reproducibility including long-term stability and are not greatly affected by ambient disturbances, at least compared with other kinds of sensors [6]. Gas sensors are divided into thick-film and thin-film types [7 9]. The former show high sensitivity at low level concentration, because they have large specific surface available for chemisorption reactions, but they have problems in stability [10]. The latter have high stability and repeatability, but lower sensitivity [11]. Also, almost all sensors ddlee@ee.knu.ac.kr have film deposited on alumina substrate, but these sensors consume high power, due to large substrate thickness. In this study, to improve the sensitivity thick sensing films based on tin oxide are deposited on Si substrate by using the screen printing method, and two different devices with different heater structure are designed and fabricated to reduce the power consumption by using back-surface etching, one of the MEMS technologies [12, 13]. Then, devices are simulated by simulator program and response properties are examined for test gas. II. EXPERIMENTS 1. Fabrication and Design of MEMS Structure Semiconductor sensors need a heater because of the necessity for high thermal energy in the gas response

2 Journal of the Korean Physical Society, Vol. 45, No. 5, November 2004 Fig. 2. Process for making sensing materials. Fig. 1. Fabrication flowchart of micro gas sensor. (a) Deposition Si 3N 4 layer; (b) Patterning heater (mask #1); (c) Deposition insulation layer; (d) Patterning electrode (mask #2); (e) Patterning insulation layer (mask #3, 4); (f) Etching back surface of Si by KOH; (g) Deposition sensing materials. In this study, micro heaters of two different types were designed. Each device consisted of one heater, two temperature sensors, and one sensing electrode. The fabrication flowchart of the micro gas sensor is shown in Figure 1. SiN x thin film of 2 µm thickness for the membrane was fabricated by the LPCVD process on 4 inch 100 p- type wafer with 500 µm thickness. Ta thin film of 3000 Å thickness and Pt thin film of 2000 Å thickness were deposited by a sputtering process. Photoresist film AZ1512 of 1.2 µm thickness was coated by spin-coater and patterned by mask and a ultraviolet lithography process. By a reactive ion etching process (RIE), Pt film was etched to form a Pt heater and temperature sensors apart from the photoresist- coated region. Passivation layers for protection from conduction between heater and sensing electrode were formed from SiO 2 films by using the PECVD process. Photoresist film AZ1512 of 1.2 µm thickness for opening pad area was coated by spin-coater and patterned by mask and a ultra- violet lithography process. By a RIE process, SiO 2 film was selectively etched, except for the photoresist-coated region. Also, SiN x thin film on the back surface was selectively etched by using the same process as above mentioned for membrane formation after silicon bulk micromachining. The fabricated membrane size was 2.7 mm 2.7 mm. 2. Sensor Preparation Sensing materials were made by adding aluminum oxide with 0, 4, 12, and 20 wt.%, based on SnO 2. Figure 2 shows the process for making sensing materials. All samples were dried at 120 C for 24 h, followed by grinding and calcination at 600 C for 1 h in a furnace. They were sintered at 800 C for 1 h after deposition on SiN x membrane by a screen-printing technique. The sensors were tested after a stabilizing time of 3 days at 400 C. Overall dimensions of the device are 8 mm 8 mm 0.5 mm, and size of sensing film is 2.7 mm 2.7 mm. Materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and specific surface area analyzer (BET) analysis. 3. Gas Measurement Micro gas sensors were examined for testing. For dimethylmethyl phosphonate (DMMP) gas that is a simulant gas of nerve agent, one of the chemical warfare agents [14,15]. They were tested by a flow-through system with different amounts of Al 2 O 3, with different operating temperatures and with different gas concentrations. Gas concentration in the test chamber was controlled by mass flow controller (MFC). Total mass flow was fixed at 1000 ml/min. Changes of sensing film resistance were acquired by using data acquisition (DAQ) board (E6024 NI Co., USA) [16]. The DAQ board used can simultaneously acquire 16 channels of analog input and 500,000 samples every 1 sec. III. RESULTS 1. Characteristics of Sensing Materials The fabricated sensing materials were characterized by SEM (Hitachi Co., JP) for surface morphology and thickness, by XRD (Rigaku Co., Japan) for particle size, and by BET for specific surface area.

3 High Sensitivity and Low Power Consumption Gas Sensor Using Nak-Jin Choi et al Fig. 3. SEM photographs. (a) SnO 2 and (b) SnO 2 with added 4 wt.% Al 2O 3. Figure 3 shows SEM photographs of surface morphology after thermal treatment. Surface morphology of the materials was comparatively uniform [17]. Crystal sizes of materials were from 25 to 45 nm by XRD analysis, and their specific surface areas were from 6 to 9 m 2 /g by BET analysis [17]. The peaks of all fabricated materials coincide with the SnO 2 peaks of the JCPDS card. 2. Thermal Analysis of Micro Heater Fig. 5. Results of simulated temperature distribution through the membrane. (a) Swirl type; (b) type. The micro heaters were designed in two types. One is the swirl type, and the other is type. Figure 4 shows Fig. 6. Photographs of prepared sensor. (a) Front surface view of sensing device (b) bottom view of sensing device with etched back surface (c) screened complete sensor. Fig. 4. Schematics of two micro heater types. (a) Swirl type; (b) Type. schematics of the two micro heaters designed. The micro heaters consist of two temperature sensors, which measure the temperature of the micro heater and silicon die, and one sensing electrode which measures the resistance of the sensing material. The finite element model analysis program ANSYS was employed for thermal and structural simulation. Figure 5 shows the results of simulated temperature distribution through the membranes of the two types. The bottom bar of each figure indicates specific temperature. Figure 5(a) and (b) show the application of the same voltage, 2.5 V, to simulate the temperature distribution of the two types of heater. A thermal convection condition was considered inapplicable. The thermal distribution of the swirl type is better than that of the type. The heater of swirl type showed good uniformity of heating area through the whole membrane, but type showed a higher temperature than the swirl type at the same applied voltage. Figure 6 shows photographs of the fabricated devices. Figure 6(a) and (b) are front view and bottom view of the sensing device, respectively. And Figure 6(c) shows the complete sensor, which is printed by a screen-printing

4 Journal of the Korean Physical Society, Vol. 45, No. 5, November 2004 Fig. 8. Graphs of Al 2O 3 amount vs sensitivity at different operating temperatures. Fig. 7. Temperature variation vs power consumption for two types. (a) Swirl type; (b) type. method, one of the thick film deposition techniques. Figure 7 shows temperature variation vs power consumption for the two micro heaters. This graph shows real heater temperature as applied power changes, by infrared thermometry (IR308, Minolta Co. Japan). This instrument can measure from 250 to 800 C, and target spot size is 1.3 mm at 100 mm distance. The temperature of the heater linearly increased in proportion to applied power. At a power consumption of 130 mw, the temperatures of the two micro heaters showed 170 C and 300 C respectively. The micro heater of type consumed less power than that of swirl type. The type micro heater is selected for the sensor, because tin oxide sensors show high sensitivity at high operating temperature. 3. Characteristics of Gas Response The response to simulant gas was examined with sensing films at different operating temperatures and gas concentrations. The definition of sensitivity is shown in Equation 1. S(%) = R g R a R a 100, (1) Where R a and R g mean resistance of air atmosphere and gas atmosphere, respectively. As an example, if R a and R g are 500 and 250 kω, respectively, the sensitivity is 50 %. A negative value means that the resistance of the sensor decreases after reaction between sensing material and gas. Figure 8 shows graphs of Al 2 O 3 amount Fig. 9. Sensitivity change with DMMP gas concentration for SnO 2 with added 4 wt.% Al 2O 3. vs sensitivity at different operating temperatures. Gas concentration was fixed at 0.5 ppm. As shown in the figure, SnO 2 with added 4 wt.% Al 2 O 3 gave high sensitivity. At over the 4 wt.%, the sensitivity decreased for the test gas. Also, a 300 C temperature presented higher sensitivity than other temperatures. In general, when the operating temperature rises, gas desorption on the surface becomes faster. Therefore, sensitivity is proportional to temperature increase up to 300 C, because adsorption is faster than desorption. However, sensitivity is inversely proportional to temperature increase at over 300 C, because adsorption is slower than desorption. Fig. 9 shows sensitivity change with DMMP gas concentration for SnO 2 with added 4 wt.% Al 2 O 3. When the gas concentration increases, the sensitivity increases logarithmically. This results from the limitation of gasreaction sites on the sensor surface. So, sensitivity increases at low concentration because the sensor surface has many reaction sites. However, sensitivity is saturated at high concentration because many sites have already reacted with gas. Figure 10 shows the real response curve for a repetition test by the LabVIEW program.

5 High Sensitivity and Low Power Consumption Gas Sensor Using Nak-Jin Choi et al ACKNOWLEDGMENTS The authors acknowledge the financial support of the Confrontation to Chemical and Biological Terror and National Research Laboratory Program of the Ministry of Science & Technology. REFERENCES Fig. 10. Real response curve for repetition test by Lab- VIEW program. Conditions were fixed at 300 C operating temperature and SnO 2 with added 4 wt.% Al 2 O 3. A gas concentration of 0.5 ppm is injected twice, and is vented out by fresh air. The first sensitivity showed 67 %, and the second showed 68 %. The error between two saturated sensitivity values was below ± 3 % of full scale. Thus, the device shows good repeatability. IV. CONCLUSIONS Thick film gas sensors based on tin oxide are fabricated on Si substrate and their characteristics for test gas are examined. Sensing materials are added; Al 2 O 3 based on SnO 2. Two heater types, namely swirl type and type using MEMS are designed to reduce power consumption and are simulated. Power consumed in the type heater was about 130 mw at 300 C. Test gas is dimethylmethyl phosphonate (DMMP) gas. Sensitivities are measured with different gas concentrations, at different operating temperatures and with different amounts of Al 2 O 3. High sensitivity is obtained, over 70 %, for SnO 2 96 wt.% + Al 2 O 3 4 wt.% at 300 Cin 0.5 ppm. Repeated measures were very good, with ±3 % of full scale. High sensitivity is acquired by using thick sensing film, and low power consumption is achieved by using MEMS technology. [1] J. W. Gardner, Sensors and Actuators B 4, 109 (1991). [2] T. Nakamoto and A. Fukuda, Sensors and Actuators B 10, 85 (1993). [3] H. Nanto, T. Kawai, H. Sokooshi and T. Usuda, Sensors and Actuators B 13-14, 718 (1993). [4] T. J. Lee, H. Y. Song and D. J. Chung, J. Korean Phys. Soc. 42, 814 (2003). [5] W. Y. Chung, J. Korean Phys. Soc. 41, L181 (2002). [6] Toru Maekawa, Kengo Suzuki, Tadashi Takada, Tetsuhiko Kobayashi and Makoto Egashira, Sensors and Actuators B 80, 51 (2001). [7] D. S. Lee and D. D. Lee, J. Korean Phys. Soc. 35, 1092 (1999). [8] Y. S. Lee, B. S. Joo, N. J. Choi, B. H. Kang and D. D. Lee, J. Korean Phys. Soc. 37, 862 (2000). [9] N. J. Choi, C. H. Shim, K. D. Song, B. S. Joo, J. K. Jung, O. S. Kwon, Y. S. Kim and D. D. Lee, J. Korean Phys. Soc. 41, 1058 (2002). [10] N. J. Choi, B. T. Ban, J. H. Kwak, W. W. Baek, J. C. Kim, J. S. Huh and D. D. Lee, J. Korea Institute of Military Science and Technology 6, 819 (2003). [11] J. W. Lim, D. W. Kang, D. S. Lee, J. S. Huh and D. D. Lee, Sensors and Actuators B 77, 139 (2001). [12] F. K. Shan, G. X. Liu, B. I. Kim, B. C. Shin, S. C. Kim and Y. S. Yu, J. Korean Phys. Soc. 42, 1157 (2003). [13] J. Y. Park, D. S. Lee, J. H. Lee and Y. H. Bae, J. Korean Phys. Soc. 43, 963 (2003). [14] T. C. Marrs, R. L. Maynard and F. R. Sidell, Chemical warfare agents : Toxicology and treatment (John Wiley & Sons, New York, 1996), p. 1. [15] J. C. Lee, J. Korea Solid Wastes Engineering Society 16, 207 (1999). [16] D. Y. Kwak, LabVIEW T M Control of computer based and measurement solution (Ohm, Seoul, 2002). [17] N. J. Choi, T. H. Ban, J. H. Kwak, W. W. Baek, J. C. Kim, J. S. Huh and D. D. Lee, J. Korean Institute of Electrical and Electronic Material Engineers 16, 1218 (2003).