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We examined the response behavior of SnO2 based semiconductor gas sensors to ethanol. In this study, the sensing layer was fabricated from mixed solutions of tin acidic standard solution with zinc or titanium acidic standard solution. The sensitivity of SnO2 sensors prepared from acidic standard solutions to ethanol was higher than that of metal-doped SnO2 sensors prepared from SnO2 powders. The sensitivity of Zn-doped SnO2 sensors was depressed in the humidified environment, while the sensitivity of Ti-doped SnO2 sensors was not affected by humidity even if the relative humidity was high. SnO2 films prepared from SnO2 powders composed of particles with 100 nm mean particle diameter. By using tin standard solution as a material of film, semiconductor films formed of fine particles having 10 nm mean particle diameter was obtained. This film formation method using acidic standard solutions can control the thickness of film easy, and the thin SnO2 film with around 100 nm thickness could be obtained.

Introduction Detection of VOCs (volatile organic compounds) has been required because it is detrimental to human health. In particular, in working environment with organic solvents, the need for VOC detectors has grown to reduce the damage to workers. For this purpose, we have developed SnO2 semiconductor sensors with hot-wire-type structures. The hot-wire-type semiconductor sensor can detect ppm concentration gas with a simple structure suitable for mass-production, resulting in practical use in various applications [1]. In this work, we have achieved high sensitivity and selectivity for VOCs by optimizing SnO2 loaded with several metal oxides. Moreover, the newly developed VOC sensor has excellent durability against the poisonous siloxane, leading to commercial products thanks to these advantageous characteristics. To improve convenience for portable use of VOC detectors, we have developed VOC sensors fabricated by micro electro mechanical systems (MEMS) technology to save power consumption [2]. The power consumption of the MEMS VOC sensor was greatly reduced in comparison with conventional hot-wire-type sensors, without sacrificing high sensitivity to VOCs. The low power consumption of the MEMS sensor makes it possible to extend the battery life, which would lead to further spread use of the VOC detectors. Experimental Figure 1 shows the structure of a hot wire semiconductor sensor, which consists of a platinum wire coil and a sintered SnO2 bead. A solution containing several metals was dropped and immersed in the sintered SnO2. Thereafter, metal oxides were supported on SnO2 by current heating to fabricate a hot-wire semiconductor VOC sensor. The coil served as both a heater and electrodes for the semiconductor bead; the total resistance of the sensor (Rs) was approximated by a parallel electric circuit consisting of the coil resistance and the semiconductor one (Fig.2). For the evaluation of the characteristics, the sensor output voltage (V s) was obtained by incorporating the sensor into a bridge circuit and applying a voltage. The operating temperature of the VOC sensor was controlled to be approximately 450 ℃. The sample gas sensitivity was calculated as difference between V s in mixture of air and sample gas (V s gas) and V s in clean air (V s air). Figure 3 shows a cross-sectional schematic of the MEMS VOC sensor. In the MEMS sensor, a Pt micro-heater/electrodes, patterned by using photolithography, was constructed on about 100 μm square insulating membrane cross-linked to the Si substrate. A sensitive layer of SnO2 with thickness of a few tens µm was deposited on the Pt micro-heater by thick film technology. After sintering the SnO2 thick film on the platinum pattern, several metal oxides were added. Results and Conclusions Figure 4 shows the typical gas sensitivity characteristics of the fabricated hot-wire-type semiconductor sensor to various VOCs. The sensor showed high sensitivity for vapors such as acetone, ethyl acetate, and toluene in a concentration range that affects human health. On the other hand, the sensor showed low sensitivity to interfering gases such as methane and hydrogen. These results demonstrate that the sensors have sufficient selectivity for practical use. Optimization of loaded metal oxides successfully controlled oxidation activity of SnO2 optimally, resulting in improved VOC sensitivity and further reduced interfering gas sensitivity. Furthermore, the sensor showed significantly better durability against toxic siloxane gas than the conventional one, resulting in long-life use in real field. The improved durability against siloxane poisoning is attributable to the controlled oxidation activity of SnO2 by loaded metal oxides. The developed sensor is useful for monitoring VOC as a practical detector that can measure in real time. Next, we investigated the MEMS VOC sensors. Quick thermal response owing to the miniaturized structure enabled us to achieve working temperatures (~500 °C) of the sensor only in 30 milliseconds. Thus, we could operate the MEMS sensor in a pulsed voltage mode, in which averaged power consumption was less than 1 mW. We confirmed that the MEMS sensors, even with the pulsed voltage operation, reproduced high sensitivity to VOCs obtained in the conventional sensor. In addition, the MEMS sensor exhibited long term stability for more than two years. The MEMS VOC sensor would contribute to the improvement of working environment with the spread use of VOC detectors. In conclusion, we have successfully developed hot-wire-type semiconductor VOC sensors ready for commercial use. We have also developed VOC sensors fabricated by MEMS technology to save power consumption. References [1] K. Fukui, Detection and measurements of odor by sintered tin oxide gas sensor, Sensors and Actuators B: Chemical 5, 27-32 (1991); doi: 10.1016/0925-4005(91)80215-6 [2] D. Briand and J. Courbat, Micromachined semiconductor gas sensors, in Semiconductor Gas Sensors (Second Edition), 413-464 (2020); doi: 10.1016/B978-0-08-102559-8.00013-6 Figure 1

Introduction The first commercial battery-powered household gas alarm in the world was launched in 2015 in Japan by cooperation of companies including New Cosmos Electric [1]. For this purpose, we developed tin oxide semiconductor gas sensors by using micro-electro-mechanical systems (MEMS) technology [2]. On the basis of the success in the MEMS methane sensor, we have applied this technology to detecting various gases such as hydrogen. In this work, we present structure and sensing properties of the sensors, focusing on mainly methane sensing. Typical MEMS gas sensors require three or four wires: one pair for heating and the other pair for sensing. Our sensors, however, require only two wires, which are used for both heating and sensing. The unique simple structure, which we refer to as a hot-wire-type one [3], is preferable for mass-production. An optimized methane sensor with this structure exhibited good sensitivity, high selectivity of methane over interfering gases, and good long-term stability for more than five years. Low power consumption of the sensor (50 μW) enabled us to commercialize a battery-powered household gas alarm, which is suitable for monitoring gas leak hazards in various locations. Experimental Figure 1 shows a cross-sectional schematic of the MEMS methane sensor. The sensor was fabricated as follows. First, a Pt micro-hotplate was fabricated with the MEMS technology. A Pt thin film was sputter-deposited on an insulating layer grown on a Si substrate. The Pt film was patterned into a micro-heater by using photolithography. The micro-heater not only heated a sensitive layer to high temperatures suitable for sensing but also worked as electrodes that detected change in resistance of the sensitive layer. For thermal insulation, The Si substrate below the Pt micro-heater was chemically etched to form a hollow structure. We adopted a suspended-membrane-type structure in which suspension beams were L-shaped. This structure dramatically improved thermal durability in comparison with other structures such as those with straight suspension beams. Next, a sensitive layer of tin oxide with thickness of a few tens μm was deposited on the Pt micro-hotplate by thick film technology. The sensitive layer was further covered with a catalyst layer of Pd-loaded alumina with thickness of a few tens μm. The catalyst layer not only improved selectivity of methane owing to catalytic combustion of interfering gases but also reduced hydroxyl poisoning under humid ambient. Finally, the sensor element mounted on a metal header with wiring was enclosed into a filter cap with adsorbents for reducing influence of interfering gases. Sensor output voltage (V s) values were measured with the Wheatstone bridge circuit, as shown in Fig. 2. Difference between V s in mixture of air and sample gas (V s gas) and V s in clean air (V s air) was defined as sensitivity to sample gas (sensitivity = V s gas - V s air). Results and Conclusions We first present power consumption of the MEMS methane sensor. Thanks to the miniaturized structure and good thermal insulation, heating for only 30 milliseconds was sufficient for achieving high temperature of ~500 °C suitable for methane sensing. Consequently, the methane sensor could be operated in a pulsed voltage mode, leading to ultralow power consumption of approximately 50 μW on average. This value is sufficiently low to operate the methane sensor with a battery for more than five years. All the data shown in this work were taken with the pulsed voltage mode. Figure 3 shows typical gas sensitivity of the methane sensor for various gases. The sensor exhibited high sensitivity for methane. On the other hand, the sensor showed low sensitivity to gases except methane. Notably, no sensitivity was observed for ethanol vapor, which is a serious interfering gas in residential use. Good selectivity of methane over the interfering gases indicates that the interfering gases were effectively removed by catalytic combustion in the catalyst layer and filtering in the cap with the adsorbents. Next, we investigated long term stability of the methane sensor. The sensor exhibited excellent durability to siloxane gases owing to filtering in the cap, resulting in stable operation for more than five years. The long term stability of the sensor together with the low power consumption enabled us to commercialize a battery-powered household gas alarm. We believe that the alarm is suitable for monitoring gas leak hazards in various locations, where AC power supply might be unavailable [4]. Finally, we briefly mention extension of the MEMS sensor technology to other gases. An important application is to detect hydrogen, which is a clean energy source. To sense hydrogen selectively, a sensitive layer of tin oxide thick film was coated with a thin silica layer [3]. Figure 4 shows sensing characteristics of MEMS hydrogen sensor. Obviously, the sensor exhibited high sensitivity to hydrogen and excellent selectivity over interfering gases because of physical molecular sieving effect of the silica layer [3]. The MEMS hydrogen sensor would contribute to safe use of hydrogen in the future. To conclude, we have successfully developed MEMS gas sensors ready for commercial use with battery operation. In collaboration with other companies, we launched the first commercial battery-powered household gas alarm in the world in 2015. References [1] Available online (in Japanese): http://www.osakagas.co.jp/company/press/pr_2015/1222923_15658.html [2] D. Briand and J. Courbat, Micromachined semiconductor gas sensors, in Semiconductor Gas Sensors (Second Edition), 413-464 (2020); doi: 10.1016/B978-0-08-102559-8.00013-6 [3] A. Katsuki and K. Fukui, H2 selective gas sensor based on SnO2, Sensors and Actuators B: Chemical 52, 30-37 (1998); doi: 10.1016/S0925-4005(98)00252-4 [4] Available online: https://www.newcosmos-global.com/news/2701/ Figure 1