Developments of MEMS Gas Sensors Based on Tin Oxide semiconductors: Application to Battery-Powered Household Gas Alarms
Authors: Taniguchi Takafumi , Kobayashi Chiaki , Orita Taishi , Miyata Takeshi , Dogami Naganori , Mitsuhashi Hirokazu 
Published: May 2020

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