High Sensitive Detection of Volatile Organic Compounds Using Tin Oxide Semiconductor Sensors with Hot-Wire-Type Structures
Authors: Ohishi Tatsuya , Kitagawa Yasuhisa , Mitsuhashi Hirokazu , Nakatani Mikiya 
Published: May 2020

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