Balanced Dual Platinum Micro-Cantilever Thermal Conductivity Gas Sensor Using 3-Omega Technique
Authors: Lotfi Ardalan , Navaei Milad , Hesketh Peter J. 
Published: July 2021

Introduction
Thermal Conductivity Detector (TCD) is one of the commonly used sensors in Gas Chromatography (GC) systems. The principle behind TCD sensor is joule heating which heats the surrounding gas and the thermal-physical properties of the sensor changes based on the injected sample thermal conductivity [1, 2]. This technique can detect any gas that has different thermal conductivity with respect to the carrier gas. Therefore, TCD which is a universal gas detector have variety of applications including agriculture and environmental monitoring. TCD can also detect ammonia gas which has important role in agriculture [3].
There have been considerable efforts to miniaturize gas sensors, to achieve portable systems which require low power consumption and simple to operate outside the laboratory. Among these sensors, TCD has several advantages such as simple operation principle, short response time, high corrosion resistance and CMOS compatible fabrication process. One of the limitations of this sensing method is Limit of Detection (LOD), therefore, the TCD sensors geometrical design and electrical measurement method have been continuously engineered to improve it. The concept of cantilever-based platinum balanced TCD was introduced by our group to increase the sensors working temperature and ability to tolerate high thermally induced stresses. This advancement allows us to use the 3-Omega [3-5] technique for electrical measurement of the sensor while not sacrificing the high operating temperature.
Method
The conventional TCD sensor used a single layer of thin film for both heating and sensing. In this work, a new design for the cantilever-based platinum balanced TCD is proposed that has two active layers, one for heating and another for sensing. The TCD structure composed of ALD aluminum oxide layer sandwiched between platinum layers, and these three layers are sandwiched between two aluminum oxide layers. In designing double active layers TCD, the heating layer of the sensor needed to be thick enough to be able to pass a sufficiently high current. On the contrary, for the sensing layer to be sensitive to the change of temperature, it needs to be thin enough to have a high resistance value. A lumped thermal model was developed to study the time delay between platinum layers assuming constant properties and steady-state response.
Results and Discussion
The new design had two active layers which can use different types of excitation current or voltages. It was found that the sensing layer can follow the TCD heater's temperature with a delay less than while the two active layers TCD's response time was 6 µs. By studying the four possible combinations of excitation current types, AC or DC, defined a method for adapting the 3-Omega technique to the 2-Omega technique. In the 2-Omega technique, an AC current excitation, with ω frequency, for the heating layer a DC current excitation for the sensing layer, while the lock-in-amplifier operates at 2ω to determine the sensing layer voltage. Figure 1 depicts the heating and sensing layer time dependent voltages, and Figure 2 presents the Fourier transform of those voltages. The increase in the sensing layer resistance and ease of eliminating the 1-ω frequency present in the 3-Omega technique, enhanced the LOD of the TCD sensor. and allows for the use of the full dynamic range of the lock-in amplifier. The higher precision and accuracy enhances the Limit of Detection (LOD) of the sensor which was achieved by separating the sensing and heating in a two active layers TCD design.
REFERENCES
[1] A. Mahdavifar, M. Navaei, P. J. Hesketh, M. Findlay, J. R. Stetter, and G. W. Hunter, "Transient thermal response of micro-thermal conductivity detector (µTCD) for the identification of gas mixtures: An ultra-fast and low power method," Microsystems & Nanoengineering, vol. 1, no. 1, 2015.
[2] A. Lotfi, M. Navaei, and P. J. Hesketh, "A Platinum Cantilever-Based Thermal Conductivity Detector for Ammonia Sensing Using the 3-Omega Technique," ECS Journal of Solid State Science and Technology, vol. 8, no. 6, pp. Q126-Q133, 2019.
[3] A. Lotfi, C. A. Heist, A. Warren, M. Navaei, and P. J. Hesketh, "Platinum Balanced Cantilever-based Thermal Conductivity Detector for Gas Chromatography Application," in 2019 IEEE SENSORS, 2019, pp. 1-4: IEEE.
[4] S. Kommandur, A. Mahdavifar, P. J. Hesketh, and S. Yee, "A microbridge heater for low power gas sensing based on the 3-Omega technique," Sensors and Actuators A: Physical, vol. 233, pp. 231-238, 2015.
[5] S. Kommandur, A. Mahdavifar, S. Jin, P. J. Hesketh, and S. Yee, "Metal-coated glass microfiber for concentration detection in gas mixtures using the 3-Omega excitation method," Sensors and Actuators A: Physical, vol. 250, pp. 243-249, 2016.

Figure 1