Micromechanical fabrication of low-power thermoelectric hydrogen sensor

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Abstract

To detect gaseous hydrogen is of critical importance to acceptance and utilization of hydrogen as an energy carrier. Micro-machined sensors are a new generation of sensor technology combining existing integrated circuit fabrication technology with novel deposition and etching processing. This sensor structure provides a platform where the operating temperature can be rapidly changed to achieve desired response characteristics. We prepared the micro-thermoelectric hydrogen sensor (micro-THS) with the combination of the thermoelectric effect of SiGe thin film and the Pt-catalyzed exothermic reaction of hydrogen oxidation. We have focused on reducing the power consumption by modifying the micro-sensor design with micro-heater on a suspended thermal isolation structure. Hydrogen response properties of the micro-THS were also investigated. The power consumption of the micro-THS was greatly reduced by fabrication of the micro-heater on membranes. The effective area of the sensor was heated up on the micro-heater to 100 °C with the power consumption of 0.34 W. The response time, T90 of the micro-THS was faster than those of other type sensors such as semiconductor-type. The operating temperature at 100 °C detected hydrogen for the concentration from 0.01 to 3% in air, with a good linearity of voltage signal versus gas concentration. Pt catalyst only showed the high catalytic activity to hydrogen; hence, high hydrogen selectivity was achieved.

Introduction

Hydrogen is the most attractive and ultimate candidate for a future fuel and an energy carrier, because it burns cleanly, does not require a fuel processor in fuel cells, and is producible from renewable energy resources, such as electricity from solar cells. A common need in this technology area is the ability to detect and monitor gaseous hydrogen, but hydrogen gas sensors that can quickly and reliably detect H2 over a wide range of oxygen and moisture concentrations are not currently available.

The commercialized combustible gas sensors including hydrogen sensor are mostly semiconductor-type metal oxides, for example, stannic oxide and other metal oxides, which detect gases variations in their resistance. These sensors need to be heated around 400 °C in order to keep proper operation, and principally have poor selectivity to hydrogen gas because it respondes to other combustible gases, such as methane and carbon monoxide [1]. Recently, there are also some highly hydrogen selective sensors with different operation principles, such as palladium-gated FET [2], palladium coated optical sensors [3] and hot-wire type [4]. All the devices mentioned above normally respond to very low concentration of hydrogen, however, their signals saturate over about 1%.

Recently, our research group has demonstrated the basic operation of novel thermoelectric hydrogen sensor (THS) [5], [6], [7], [8], [9]. This sensor takes advantage of both thermoelectric (TE) effect and selective catalytic reaction. On the basis of the very well-known knowledge that platinum has a high selectivity for H2 oxidation, it was used as catalyst layer and SiGe was used as thermoelectric layer in the process of the thin-film type sensor device. However, bulk-type THS had some disadvantages such as high power consumption about 1 W for 100 °C, large heat capacity and slow response speed about 50 s.

Therefore, we focused on microfabrication technique with novel deposition and etching processing which is used for integrated circuit fabrication to apply for preparation of micromachined thermoelectric hydrogen sensor (micro-THS). This sensor structure provides a platform where the operating temperature can be rapidly changed to achieve desired response characteristics, known as a micro-heater. The micro-THS with the micro-heater on a suspended thermal isolation structure which is called by membrane has led to hydrogen sensors that demonstrate highly desirable features, such as low power consumption, high sensitivity, fast response speeds and amenability to mass production.

Section snippets

Experimental

To define some criteria for the design of substrates and membranes are very important for low power consumption, high mechanical strength of double membranes, well-controlled temperature distribution of the catalyst, and also thermoelectric layers. At the same time, the design rules for this thermoelectric micro-device includes the thermo-voltages, the voltages induced by thermoelectric effect, which are used as sensor signals, requiring well-controlled temperature gradients. The appearance and

Results and discussion

Heat transfer mechanisms that are expected to determine the heating power consumption of the hotplate devices are solid-state heat conduction through the silicon suspensions, again through the packages as well as heat conduction in the surrounding air thermal radiation. In order to heat up the micro-THS, the device was suspended by Au wire, separating the device form the test chamber. As the amount of heat and the resulted voltage were directly related to the existence of hydrogen in air and

Conclusions

We have prepared, a noble micromachined gas sensor with the combination of a micro-heater on membranes made by anisotropic wet etching of silicon substrate and thermoelectric hydrogen sensor. The thermoelectric hydrogen sensor selectively detects the hydrogen gas working with the thermoelectric effect of SiGe thin film and the Pt-catalyzed exothermic reaction of hydrogen oxidation. The implantation of this sensor device on the micro-heater with a suspended thermal isolation structure has led to

Kazuki Tajima studied material science and engineering and finished his master's course in 2000 at Tohoku University in Japan and received doctorate in 2003 in metallurgy from Tohoku University. He has been a member of the research staff at AIST in Japan since 2003. His research activity includes the thin film for gas sensor technology with micro-systems.

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Kazuki Tajima studied material science and engineering and finished his master's course in 2000 at Tohoku University in Japan and received doctorate in 2003 in metallurgy from Tohoku University. He has been a member of the research staff at AIST in Japan since 2003. His research activity includes the thin film for gas sensor technology with micro-systems.

Fabin Qiu received his BS degree and ME degree from school of electronic science and engineering, Jilin University, China in 1992 and 1995, respectively, and received his PhD degree from chemistry department, Jilin University in 1999. He has been a professor in Jilin University since 2001. His current interests include the micro-fabrication of gas sensor, the synthesis of functional electronic material, the preparation of poly-Si thin film and its application in the electronic devices.

Woosuck Shin studied material science and engineering and finished his master's course in 1994 at KAIST in Korea. After receiving doctorate 1998 in applied chemistry from the Nagoya University in Japan, he has been employed at AIST, Nagoya, Japan. His research activity includes the gas sensor technology with micro-systems.

Noriya Izu received his BEng, MEng and PhD in engineering degrees in materials science and processing from Osaka University in 1995, 1997, and 2001, respectively. He is currently a research scientist in the Sensor Integration Group at Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology in Nagoya, Japan. His research interests include environmental sensors and defect chemistry.

Ichiro Matsubara received his BS and MS degrees in polymer chemistry from Osaka University in 1985, 1987, respectively, and his PhD in science in inorganic and physical chemistry from Osaka University in 1994. He is currently a research scientist in the Sensor Integration Group at Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Japan. His research interests include functional oxide materials, organic–inorganic hybrid materials, and environmental sensors.

Norimitsu Murayama is a leader in the Sensor Integration Group at Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, AIST. He received his BS, and MS degrees in electronics from Kyoto University in 1982, and 1984, respectively, and his Dr Eng degree in materials science and engineering from Tokyo Institute of Technology in 1992. His research interests focus on novel processing for conductive ceramics including oxide superconductors, oxide thermoelectrics, and gas sensors.

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