Silicon micro-electromechanical resonator for enhanced photoacoustic gas detection

Abstract

Quartz-enhanced photoacoustic spectroscopy (QEPAS) [1] is one of the most efficient ways to obtain sensitive, selective, robust gas sensors. The main drawback of QEPAS comes from usage of quartz tuning fork (QTF) as a mechanical transducer. QTF is not specifically design for photoacoustic gas sensing and its further integration is limited. As a solution we propose a silicone resonant MEMS based on capacitive transduction mechanism. This sensor, specifically designed for acoustic sensing purposes, can be an efficient transducer for sound wave detection able to advantageously replace a QTF. Capacitive transduction allows reaching high sensitivity of the sensor while choice of silicon is favorable in design flexibility, fabrication maturity, stability and further integration with CMOS electronics. We have developed and fabricated various resonator designs on silicon. Specific designs were created to sensor voltage output using an analytic model developed by our group [2]. Photoacoustic measurement was performed on calibrated mixtures of methane using commercial Eblana distributed feedback laser laser emitting at 1.63 μm. We achieved a reproducible limit of detection on methane: 1000ppmv in 5s for 2f detection and 700ppm in 5s for 1f detection (figure 1) (resonator resonance frequency 22.65 kHz and Q-factor of 250). Then, we compared the experimental results with standard QTF in off-beam configuration for which the limit of detection: 30ppmv in 5s for 2f detection and 25 ppm in 5s for 1f detection. Thus, the difference of detection limit between QTF and MEMS amounts factor 28 for 1f detection and 33 for 2f detection. [1] Kosterev, A. A., Bakhirkin, Y. A., Curl, R. F., & Tittel, F. K. (2002). Quartz-enhanced photoacoustic spectroscopy. Optics letters, 27(21), 1902-1904. [2] Trzpil, Wioletta, et al. "Analytic Optimization of Cantilevers for Photoacoustic Gas Sensor with Capacitive Transduction." Sensors 21.4 (2021): 1489.