SiGe Thermoelectric Film for Gas Sensor Micro-Devices
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SiGe Thermoelectric Film for Gas Sensor Micro-Devices Woosuck Shin, Kazuki Tajima, Yeongsoo Choi, Noriya Izu, Ichiro Matsubara, Norimitsu Murayama National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan, ABSTRACT A gas sensor micro-device using both thermoelectric film and catalyst film has been developed on the platform of micro-hotplate. Thermoelectric thin film of B-doped SiGe was deposited on the Si3N4/SiO2/Si substrate by helicon sputtering method and thermal annealing was carried out to crystallize the as-deposited amorphous-like film. With increasing the annealing temperature and time, the crystallization of the SiGe thin film progressed, resulting in high carrier mobility and large absolute value of Seebeck coefficient. The hydrogen sensitivity of the micro-thermoelectric gas sensors was investigated. 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. Recently, our research group has demonstrated the basic operation of novel thermoelectric hydrogen sensor (THS) [1-4], whose working principle takes advantage of both thermoelectric, TE, effect and selective catalytic reaction. Currently, the development of THS was focused on the microfabrication of micromachined THS, µ-THS, that demonstrate an array of highly desirable features, such as low power consumption, high sensitivity, fast response speeds and amenability to mass production, as reviewed for the micro gas sensor [5]. In our previous researches [6-8], we have demonstrated the successful hydrogen detection with the thermoelectric hydrogen sensor using the thin film P-doped SiGe of large Seebeck coefficient value prepared by conventional RF-sputtering method. However, the film of low resistivity became important as the line width of thermoelectric pattern is reduced for the microfabrication. In this study, B-doped SiGe thin film was prepared using helicon sputtering technique and the gas response of the µ-THS with this film was demonstrated. EXPERIMETAL The Si- wafer ((100) p-type, Shinetsu Inc.) of 0.35 mm thickness was used as a substrate. The wafers had multi-layers of 250 nm-Si3N4 / 80 nm-SiO2 on both the front and backsides. The front-side multi-layers were used as an electrical insulation underneath the heater and sensing layer electrodes, and also became the membrane after the back-side etching. The backside layers, on the other hand, were used as an etch mask for KOH etching. First, a sintered target of 1 at% boron-doped Si0.8Ge0.2(p-type) with the purity of 99.997% was used for the RF magnetron
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sputtering deposition of SiGe thin film with th
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