FET hydrogen-gas sensor with direct heating of catalytic metal

Available online at www.sciencedirect.com

Sensors and Actuators B 130 (2008) 94–99

FET hydrogen-gas sensor with direct heating of catalytic metal Koichi Yokosawa a,∗ , Kazuo Saitoh a , Sadaki Nakano b , Yasushi Goto b , Keiji Tsukada c a

Advanced Research Laboratory, Hitachi, Ltd., 1-280 Higashi-koigakubo, Kokubunji, Tokyo 185-8601, Japan Central Research Laboratory, Hitachi, Ltd., 1-280 Higashi-koigakubo, Kokubunji, Tokyo 185-8601, Japan c Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushimanaka, Okayama 700-8530, Japan b

Available online 26 July 2007

Abstract A method for heating field-effect-transistor (FET) gas sensors with minimum power is proposed. The time of the FET sensor response to target gases depends on temperatures of catalytic metals laminated on gate insulators. Therefore, maximum heating efficiency can be obtained by applying current to each catalytic metal directly and using those catalytic metals as heaters. FET hydrogen gas sensors have been fabricated on 7.5 mm × 3 mm × 0.73 mm silicon chips, and narrow palladium catalytic metal has been deposited and terminated by two electrodes. The FET sensor can be heated to 100 ◦ C by applying current corresponding to 0.2 W, and the response speed to 1000 ppm by volume hydrogen gas increases by about a factor of six. No morphological change caused by the heating current has been observed in the catalytic metal. Moreover, numerical and experimental simulations demonstrate that the required power and time to heat the FET sensor to 100 ◦ C can be further reduced to 20–30 mW and 1 s, respectively, by miniaturizing the senor chip to 2 mm × 2 mm × 0.15 mm. We are planning to apply the FET sensors with this heating method to our hydrogen-leak detection system that is being developed to make hydrogen energy structures safe and secure. Such detection systems consist of many sensor nodes powered by batteries, so reducing power consumption is important to extend battery lifetimes. © 2007 Published by Elsevier B.V. Keywords: FET; Heater; Power consumption; Catalytic metal; Hydrogen gas

1. Introduction As fuel-cell vehicles come into wider use, sophisticated leakage-gas monitoring systems are required in hydrogen filling stations, automobile tunnels, and underground garages for example. We have proposed a hydrogen-leakage detection system consisting of numerous compact hydrogen-gas sensors [1,2]. The system monitors the real-time leakage of hydrogen, so it detects gas leaks while it identifies the leakage location and monitors gas diffusion in real time. The system then enables automatic equipment or an operator to take timely actions, such as turning on a fan to disperse the gas or issuing an appropriate evacuation directive. We already fabricated a small-scale prototype of the detection system using sensor networks with ten sensor nodes (sensor-transmitter combinations; Fig. 1) to test the feasibility of this hydrogen-monitoring concept [1,2]. Each sensor node

∗

Corresponding author. Tel.: +81 42 323 1111x2721; fax: +81 42 327 7722. E-mail address: [email protected] (K. Yokosawa).

0925-4005/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.snb.2007.07.084

can be located anywhere that is most convenient to detect gas leaks because they are operated by battery power and wireless communications. We have utilized field-effect-transistor (FET) hydrogen gas sensors [3–10] in each sensor node because FET sensors are compact, cheap, reliable, and exhibit low power consumption. FET sensors can detect hydrogen gas even if the sensor is at room temperature; however, they are usually operated at a constant temperature above 100 ◦ C to accelerate certain chemical reactions and to prevent effects of ambient temperature and humidity, which are the same conditions under which most electrochemical gas sensors operate. The power consumed to heat a sensor is much higher than consumed to operate a sensor; hence, the heating power shortens lifetimes of batteries. Therefore, we have proposed two approaches to reduce the heating power. The first is to heat the sensor effectively; the second is to limit heating periods. In this paper, we propose a new method for minimizing the power for heating the FET gas sensor. On the other hand, we have already proposed a power-saving function to limit heating periods [11]. Under control by this function, each sensor monitors the hydrogen gas continuously at room temperature.

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Fig. 1. Photograph of fabricated sensor node. Small-scale prototype of hydrogen detection system is composed of ten sensor nodes.

Only when the sensor detects hydrogen gas, is it heated rapidly. In this paper, at first, we describe the power-saving function. Then, we propose and evaluate the new method to minimize power for heating a sensor. Next, we examine a possibility to heat the sensor rapidly. At last, we discuss the practicality of the power-saving function from the viewpoint of delay time from hydrogen appearance until transmitting data about hydrogen concentration. 2. Methods 2.1. Power-saving function Photographs and schematic diagrams of the fabricated sensor node are shown in Figs. 1 and 2, respectively. Each sensor node has two or three sensors, read-out circuits, a trigger circuit, controller, transmitter, and battery. The controller includes an eight-bit microprocessor that consumes low power (6 mW in active mode and 1 ␮W in standby mode). The transmitter sends and receives radio waves at 430 MHz. No special license

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is required to use this frequency in Japan, and the transmission distance is longer than 200 or 300 hundred meters, which is sufficient to cover the area of a hydrogen filling station, about 30 m × 30 m, for example. The power-saving function is implemented in the controller of each sensor node. The process of the function is depicted in Fig. 2. Heating the sensor and transmitting data use the most power; therefore, sensors in each node continuously monitor the hydrogen gas. While the sensors are maintained at room temperature without heating, the controller is maintained in standby mode, and the power to the transmitter is left off (Fig. 2(a)). If a sensor detects hydrogen, and the subsequent output signal of the sensor makes a trigger signal; the trigger signal switches the controller to active mode (Fig. 2(b)). The controller switches on the heater nearby the sensors and the transmitter, acquires accurate hydrogen concentration data measured by the sensor at 100 ◦ C, and sends the data to a server. That is, this function allows each sensor node to use power only when hydrogen is detected. The total delay time from the hydrogen appearance around the sensor, which is still at room temperature until data reception by the server, is a summation of the following five parameters: (1) delay time from hydrogen appearance to outputting trigger signal to microprocessor, (2) rise time of the microprocessor from stand-by mode to active mode, (3) heating time of the sensor, (4) response time of the sensor at 100 ◦ C, and (5) transmission time from the node to the server. The rise time of the microprocessor (2) is extremely short (25 ␮s), and the response time of the sensor at 100 ◦ C (4) of less than 1 s and the transmission time from the node to the server (5) of less than 0.3 s have been evaluated experimentally [2,11]. The delay time from hydrogen appearance to outputting trigger signal to microprocessor (1), which corresponds to response time of the sensor at room temperature to hydrogen gas, and the heating time of the sensor from room temperature to 100 ◦ C (3) remain to be evaluated. 2.2. Method for heating sensor

Fig. 2. Schematic diagrams of fabricated sensor node. Process of power-saving function is also depicted. Heater and communication units use power only when hydrogen is detected.

In conventional FET gas sensors, each sensor has a layer of catalytic metal laminated on a gate insulator. In hydrogen gas sensors, palladium, platinum, or alloys containing them are usually used as the catalytic metals. Previous studies [3–6] have reported that hydrogen gas is dissociated by the catalytic metal and adsorbed onto the gate insulator forming electric dipoles that shift the gate voltage. The basis of the gas sensing is the electrochemical reaction of the catalytic metal, so the higher temperature shortens the response time. The element that must be heated is the catalytic metal itself. Therefore, maximum heating efficiency can be obtained by connecting two electrodes to the catalytic metal and applying current between them, resulting in the use of the catalytic metal as a heater (Fig. 3(a)) The structure of the sensor used for these experiments is shown in Fig. 3(b). A FET without the catalytic metal is integrated in advance on a silicon chip with a diode thermometer that is 1.5 mm from the FET. The dimensions of the chip are 7.5 mm long, 3 mm wide, and 0.73 mm thick. The typical gate of the FET is 30 ␮m long and 500 ␮m wide. A 70-nm-thick, 150-␮m-

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Fig. 4. Relationships between temperatures of FET and diode against various applied powers obtained by experiments and simulations. Required powers to heat sensor to 100 ◦ C are 0.3 W (point (a): heated by conventional external heater), 0.2 W (point (b): heated by catalytic metal heater), and 30 mW (point (c): miniaturized sensor is heated by catalytic metal heater).

Fig. 3. Schematic diagram of (a) proposed heating method and (b) photographs of fabricated FET sensor chip to evaluate the method experimentally. Narrow catalytic metal Pd is deposited on gate insulator between two electrodes using a stencil mask.

wide catalytic metal (palladium) is then deposited on the gate insulator between two aluminum electrodes (1.5 mm between them) by utilizing a stencil mask. The basic performance of the FET sensor has been confirmed previously. That is, the FET sensor with palladium catalytic metal has a high sensitivity (it can detect 50 ppm of hydrogen gas) as well as a short response time (90% of response time to 100 ppm or higher concentrated hydrogen gas is less than about 1 s) while heating the sensor by an external heater [2,11]. We expected that power consumption to heat FET sensors can be minimized by the proposed heating method, and the two un-evaluated parameters of the delay time caused by the power-saving function are obtained by experiments using the FET sensor with the new heating method. From these results we are sure the power-saving function is practical, and it will extend the battery life of each sensor node. 3. Experiments and results 3.1. Heating efficiency In the first experiment, temperatures of the catalytic metal under various applied powers were evaluated. The resistance of the catalytic metal, which is about 40 ◦ C at room temperature, depends on temperature. Hence, the resistance was calibrated against temperature in advance, while the sensor chip was heated on a traditional hot plate. The temperature of the catalytic metal under the applied power was obtained from the metal’s resistance. The temperatures of the catalytic metal and diode thermometer for various applied powers are summarized in Fig. 4. The catalytic metal acts as a heater in this method, so the temper-

ature difference between the FET and diode is about 10 ◦ C. That means the FET sensor is heated selectively, so the heating efficiency is high. The required power to heat the FET sensor to 100 ◦ C can be reduced to 0.2 W (point (b) in Fig. 4) with this method, while that with a conventional method, in which an external heater is attached on the backside of the silicon substrate, is 0.3 W (point (a) in Fig. 4). That is, the required heating power can be reduced by one third. Thermal leakage from the catalytic metal to the silicon substrate is considered to be proportional to the interface area of the catalytic metal. Hence, in the future optimal design, the narrower catalytic metal should be laminated by utilizing photolithography, to provide higher thermal efficiency. Besides, in the future design, the diode thermometer has to be positioned just below the catalytic metal to measure the temperature of the catalytic metal accurately. 3.2. Effect of miniaturizing chip size We expected that miniaturizing chip size will reduce heat capacitance thus further reducing heating power. We have numerically simulated the effect of miniaturizing chip size (lines shown in Fig. 4). First, we determined the relationship between the applied power and temperature by deriving the parameter of heat loss from the chip to the environment under the condition of a chip size that was 7.5 mm × 3 mm × 0.73 mm in the simulation. Then, the heat loss parameter which should be used in the simulation was determined by fitting the experimental data to the simulated data. Next, the relationships between applied power and temperature were calculated for several chip sizes by using the obtained heat loss parameter. The numerical simulation shows that only 30 mW is necessary to heat the chip whose size is 2 mm × 2 mm × 0.15 mm to 100 ◦ C. We confirmed the simulated value by heating a dummy chip (Fig. 5 left). The chip has a 150-␮m wide, 1-mm long, and 80nm-thick Pd membrane, which simulates a catalytic metal, with aluminum electrodes laminated on a 2 mm × 2 mm × 0.15 mm silicon chip. The chip was heated to 100 ◦ C within about 1 s with only 22 mW (Fig. 5 right). The experimentally obtained relationship between heating powers and temperatures of the

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Fig. 5. Structure of dummy chip fabricated to confirm size effect on reducing required power and time to heat (left) and experimental results (right). Only 22 mW and 1 s are required to heat the chip to 100 ◦ C.

dummy chip are consistent with the numerical-simulation value (Fig. 4). 3.3. Morphologic change by applying current We observed morphologic change of the catalytic metal, which may be induced by the heating current. We applied various currents to the simulated catalytic metals on the dummy chips, as shown in Fig. 5. The applied currents of 25 mA, 50 mA, and 70 mA correspond to 25 mW, 100 mW, and 200 mW, respectively. After applying current, we examined the catalytic metal surface, which is 5 ␮m × 5 ␮m in area by atomic force microscopy (AFM) compared with a catalytic metal without a current history. No relationships between the morphologies (either maximum or root-mean-square value of the height) and applied current values were observed (Fig. 6). 3.4. Response to hydrogen gas with heating Next, the response of the FET hydrogen sensor (7.5 mm × 3 mm × 0.73 mm) with a catalytic metal heater

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Fig. 7. Time response of FET sensor to 1000 ppm by volume hydrogen gas diluted by air. Response speed increases by up to a factor of six when FET is heated by catalytic metal heater. (Inset: detail around onsets).

to dilute hydrogen gas was confirmed. The fabricated FET sensor was set in a gas flow cell [10]. The flowing gas can be switched from air (mixture of 79% nitrogen and 21% oxygen) to hydrogen diluted by the air. A higher heating power of about 0.35 W was applied to heat the catalytic metal to 100 ◦ C because the metal was cooled by the gas blowing. Drain current responses of the FET sensor with and without heating when 0.1% (by volume) hydrogen gas was blown at 1 L/min are shown in Fig. 7. The response time is significantly shortened by the heating; the maximum time deviation with heating is about six times that obtained without heating. The different gate voltages were applied on FET sensors with and without heating, so the variation of the drain currents is different. The response times are compared by using the normalized drain current. Although the gate voltage has a gradient across the gate width on heating the catalytic metal, no adverse effects were observed when detecting hydrogen gas. 3.5. Response to hydrogen gas without heating

Fig. 6. Roughness of Pd membranes with various current histories observed by atomic force microscope. No morphologic change caused by a current application was observed.

Time responses of the FET sensor to the various concentrations of hydrogen gas were evaluated without heating as well to evaluate the delay time from hydrogen appearance to outputting trigger signal to microprocessor in the powersaving function (Fig. 8(a)). The gate voltages were monitored by using read-out circuits fabricated on a 2 mm × 2 mm silicon chip [11]. The response times and saturation value of the gate voltage shifts increases with increasing hydrogen concentration. Although the response times are longer than those with heating, the gate voltage varies sharply just after the hydrogen application. In fact, time derivative values of the gate voltages peak at about 1.2 s, 0.7 s, and 0.5 s after applying hydrogen concentrations of 100 ppm, 1000 ppm, and 10,000 ppm hydrogen, respectively, as shown in Fig. 8(b), where Fig. 8(c) plots the peak time of the time derivatives of the gate voltage. These results demonstrate that even though the FET hydrogen sensors are at room temperature, the time derivative values of the output of the sensors should trigger to switch the microprocessor to active mode effectively in the process of the power-saving function.

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Fig. 8. (a) Time responses of FET sensor at room temperature to hydrogen gas with various concentrations. (Inset: detail around onsets). (b) Time derivative values of gate voltage shifts, and (c) delay times of peaks of time derivative value. The peaks appear within 1.5 s after hydrogen gas application.

4. Discussion Experiments demonstrate that the proposed heating method, in which the current is applied to the catalytic metal directly, can reduce heating power by one-third compared with a conventional external heater. No morphologic change occurs when current up to 70 mA (0.2 W) is applied to the catalytic metal, and the FET sensor detects hydrogen gas diluted by air without any adverse effect despite a gate voltage gradient across the gate width. The response time is significantly shortened in the same way as that when heated by the external heater. Likewise, numerical and experimental simulations demonstrated that the heating power can be further reduced to about 20–30 mW and heating time to about 1 s by miniaturizing the chip size to 2 mm × 2 mm × 0.15 mm. Again, we examined the total delay time from the hydrogen appearance around the sensor until data reception by the server in the power-saving function. Each parameter of the delay time is summarized in Table 1. The first of the two un-evaluated parameters, (1) delay time from hydrogen appearance to outputting trigger signal to microprocessor, Ts , is expected to be less than 1.5 s for hydrogen gas at 100 ppm or higher concentrations. This is because, even though the sensor is at room temperature, the time derivative values of the sensor output have peaks within about 1.2 s after gas application (Fig. 8(b) and (c)). The second param-

eter, (3) the heating time of the sensor to 100 ◦ C, Th , should be about 1 s after miniaturizing the sensor chip, as shown by examination with a dummy chip (Fig. 5 right). As already mentioned, (2) the rise time of the microprocessor from stand-by mode to active mode, Tw , is negligibly short (25 ␮s). Moreover, (4) the response time of the sensor at 100 ◦ C, Tr , was less than about 1 s and (5) the transmission time from the node to the server, Tt , was less than 0.3 s, which had been evaluated in our previous works. Therefore, the total delay time from hydrogen appearance to data reception by the server Ttotal = Ts + Tw + Th + Tr + Tt is estimated to be less than 4 s. This delay time should be sufficiently short to detect hydrogen gas leakage. Consequently, we conclude that the power-saving function will be practical in developing our hydrogen-leakage detection system. By installing the powersaving function in the controller of each sensor node, each sensor

Table 1 Delay times of process of power-saving function (1) Delay time to output trigger signal, Ts (2) Rise time of microprocessor, Tw (3) Heating of the sensor to 100 ◦ C, Th (4) Response time of sensor at 100 ◦ C, Tr (5) Transmission time from node to server, Tt

<1.5 s =25 ␮s ≤1 s ≤1 s <0.3 s

Total: Ts + Tw + Th + Tr + Tt

<4 s

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node can be driven by battery and then located wherever is the most appropriate position without regard to the wiring. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan. We would like to thank Yota Kikuchi who is working in Hitachi Advanced Research Laboratory for his help with the experiments. References [1] K. Yokosawa, S. Migitaka, Y. Goto, S. Nakano, A. Watanabe, D. Kamoto, K. Tsukada, Improving hydrogen-gas sensors by including an ion-exchange membrane and hydrogen detection system prototype, in: Summaries of Third International Conference on Systems, Signals, and Devices, Sousse, Tunisia, March 21–24, 2005, p. 224. [2] K. Yokosawa, S. Nakano, Y. Goto, K. Tsukada, Prototype system comprising battery-powered sensor nodes and a wireless network for detecting hydrogen leaks, in: Proceedings of the 22nd Sensor Symposium on Sensors, Micromachines, and Applied Systems, Tokyo, Japan, October 20–21, 2005, pp. 435–438. [3] I. Lundstr¨om, S. Shivaraman, C. Svensson, L. Lundkvist, A Hydrogensensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55– 57. [4] K.I. Lundstr¨om, M.S. Shivaraman, C.M. Svensson, A Hydrogen-sensitive Pd-gate MOS transistor, J. Appl. Phys. 46 (1975) 3876–3881. [5] I. Lundstr¨om, Hydrogen sensitive MOS-structures part 1: principle and applications, Sens. Actuators 1 (1981) 403–426. [6] I. Lundstr¨om, D. S¨oderberg, Hydrogen sensitive MOS-structures part 2: characterization, Sens. Actuators 2 (1981/1982) 105–138. [7] Y. Miyahara, K. Tsukada, H. Miyagi, Field effect transistor using a solid electrolyte as a new oxygen sensor, J. Appl. Phys. 63 (1988) 2431–2434. [8] N. Miura, T. Harada, N. Yoshida, Y. Shimizu, N. Yamazoe, Sensing characteristics of ISFET-based hydrogen sensor using proton-conductive thick film, Sens. Actuators B 24/25 (1995) 499–503. [9] I. Eisele, T. Doll, M. Burgmair, Low power gas detection with FET sensors, Sens. Actuators B 78 (2001) 19–25. [10] K. Tsukada, T. Kiwa, S. Migitaka, Y. Goto, K. Yokosawa, A study of fast response characteristics for hydrogen sensing with platinum FET sensor, Sens. Actuators B 114 (2006) 158–163. [11] S. Nakano, K. Yokosawa, Y. Goto, K. Tsukada, Hydrogen gas detection system prototype with wireless sensor networks, in: Proceedings of the 4th IEEE Conference on Sensors, Irvine, CA, USA, October 31–November 3, 2005, pp. 159–162.

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Biographies Koichi Yokosawa was born in Sendai, Japan, in 1962. He received the BS and MS degrees in physics from the faculty of science, Hokkaido University in 1984 and 1986, respectively, and the PhD degree in systems and information engineering from the faculty of engineering, Hokkaido University in 1998. He joined the Central Research Laboratory, Hitachi, Ltd. in 1986. Since then he has been engaged in the research and development of SQUIDs and biomagnetometer systems, and ultrasonic diagnostic probes. He has been with the Advanced Research Laboratory, Hitachi Ltd., since 2004, where he is the leader of a national project to develop a hydrogen gas detection system. His research interests and activities cover sensors and sensing systems using superconductor, dielectronics, and semiconductor. Kazuo Saitoh received the BS, MS, and PhD degrees in physics from University of Tsukuba, Japan, in 1984, 1986, 1989, respectively. In 1989, he joined the Central Research Laboratory, Hitachi Ltd., Tokyo, Japan, where he had been engaged in the research of novel superconductive device and physics such as the electron wave device. He had been on secondment to the Superconductivity Research Laboratory (SRL-ISTEC) from 1995 to 1998, where he had been engaged in the research of single flux quantum (SFQ) devices and circuits using high temperature superconducting materials. Since 1998, he joined Advanced Research Laboratory, Hitachi Ltd., where he has been engaged in the research of Superconductive Electronics; his current interests are SFQ circuits, superconductive analog device and SQUID application. Sadaki Nakano was born in Matsuyama, Japan in 1969. He received the BE degrees in computer science from Tokyo University Agriculture and Technology in 1993. He joined the Central Research Laboratory, Hitachi Ltd., Tokyo, Japan, in 1993, where he was involved in the researches of information devices, microprocessors, and sensor network systems. Yasushi Goto was born in Ibaraki, Japan. He received the BS and MS degrees in electrical engineering from Nagaoka University of Technology in 1988 and 1990, respectively. He joined the Central Research Laboratory, Hitachi Ltd., Tokyo, Japan, in 1990, where he was involved in the research of ULSI fabrication process. He has been with the Advanced Research Laboratory, Hitachi Ltd., since 2003. His current interests are micro-electro-mechanical devices and process. Keiji Tsukada was born in Kumamoto, Japan in 1954. He received Dr. Eng. and the PhD degrees from Tsukuba University in 1990, and 2001, respectively. He joined the Central Research Laboratory, Hitachi Ltd. in 1982, where he was involved in the study of integrated solid-state chemical sensor for blood analyses. He was with the Superconducting Sensor Laboratory from 1991 to 1996. He was involved in the research and development of SQUID’s and multichannel SQUID system. He was with the Central Research Laboratory, Hitachi Ltd. from 1996 to 2003. He was a Project Leader of the SQUID application research group. He is presently a Professor of Department of Electrical and Electronic Engineering, Okayama University. He is involved in the research of gas sensor and superconducting sensor devices, and their applications.