Sandwiched Platinum Thin Film TCD for 3-Omega Technique for Detection of Ammonia Gas

Abstract

Over the past few decades, sensor technology has been revolutionized by exploring the disciplines of nanotechnology and nanofabrication process. As a result, environmental sensors including gas sensors have been miniaturized and become more advanced in the terms of sensitivity and accuracy. Todays, gas sensors can cover a broader range of volatile compounds with a limit of detection (LOD) as low as single ppm. Despite these advances, ammonia sensing is still challenging because of its corrosive nature. The chemical-based gas sensors for ammonia are not reliable due to several factors such as slow recovery time, sensor base-line drifting, vulnerability to degradation and false alarm [1]. On the other hand, physical-based ammonia sensing approaches have the advantage of fast response time and long-term durability [2]. The Thermal Conductivity Detector (TCD) limit of detection has been improved to 30 ppm by making use of the 3-omega measurement technique, however the sensor requires a robust geometrical design, and careful material selection to withstand the higher temperature during operation [3,4]. To enhance LOD further, a new design including design elements which eliminates the shortcomings in the current cantilever design. The temperature difference in the deposition process for Al203 and platinum layer causes the sensor to bend away from the silicon wafer surface, and be vulnerable to fluid velocity. Hence a supporting platinum layer has been added underneath of Al2O3 layer so that thermal stresses introduced during fabrication, and generated in operation are compensated. The new design of micro wire TCD also includes optimized shape to bear the high thermal stresses caused by continuous current needed for the 3-omega measurement technique, and a higher stability of the temperature coefficient of resistance with platinum film to improve reproducibility of response. [1] Azad, A. M., Akbar, S. A., Mhaisalkar, S. G., Birkefeld, L. D., & Goto, K. S. (1992). Solid‐state gas sensors: A review. Journal of the Electrochemical Society, 139(12), 3690-3704. [2] Mahdavifar, A., Navaei, M., Hesketh, P. J., Dimandja, J. D., Stetter, J. R., & McMurray, G. (2015). Implementation of a polysilicon micro electro-thermal detector in gas chromatography system with applications in portable environmental monitoring. ECS Journal of Solid State Science and Technology, 4(10), S3062-S3066. [3]Lotfi, Ardalan, Milad Navaei, and Peter J. Hesketh. "A Platinum Cantilever-Based TCD for 3-Omega Sensing of Gas Mixtures." ECS Transactions 86.16 (2018): 79-86. [4] Lotfi, A., Mahdavifar, A., Struk, D., Stetter, J. R., Navaei, M., & Hesketh, P. (2017). Ultimate Sensitivity of Physical Sensor for Ammonia Gas Detection Exploiting Full Differential 3-Omega Technique. ECS Transactions, 80(10), 1571-1578 Figure 1