Low Power High Concentration Gas Sensor Based on 3ω-Method Using a Suspended Nanowire Heater
Authors: Cho Wootaek , Kim Taejung , Kim Beomsang , Lee Seungwook , Shin Heungjoo 
Published: July 2021

Introduction
These days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio.
Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3].

Method

The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a.

Results and Conclusions

The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration.

References

[1] Kommandur, Sampath, et al. "A microbridge heater for low power gas sensing based on the 3-Omega technique." Sensors and Actuators A: Physical 233 (2015): 231-238.
[2] Kommandur, Sampath, et al. "Metal-coated glass microfiber for concentration detection in gas mixtures using the 3-Omega excitation method." Sensors and Actuators A: Physical 250 (2016): 243-249.
[3] Lim, Yeongjin, et al. "Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform." Nanoscale research letters 8.1 (2013): 492.

Figure 1