Abstract
Gas sensing find tremendous applications in various fields like medicine, air quality, food processing or security and defense. The main challenge in industry is to create an integrated and compact sensor while maintaining its performance and power consumption. Photoacoustic spectroscopy (PAS) gains particular interest in this field due to its excellent selectivity while maintaining compactness. In tunable laser diode absorption spectroscopy (TDLS) the signal is proportional to optical path. Sensitivity in photoacoustic spectroscopy is proportional to the power of the laser, which allows to keep a good sensitivity even with small gas cells. The use of mechanical resonator with high quality factor allows to improve the signal-to-noise ratio and avoid the use of an acoustic chamber [1] as is the case for microphone-based PAS. Micro-electro mechanical systems (MEMS) fabricated in silicon technology remain a reasonable choice to realize a compact and integrated sensor, including laser source and electronics. Cantilever enhanced PAS (CEPAS) [2] offers integrated solution; however, it suffers from the lack of compactness due to optical read out mechanism. Therefore, as a replacement of optical detection we propose a capacitive transduction method, which can be easily integrated, compact and highly sensitive [3]. Nevertheless, to improve capacitive signal it is advantageous to enlarge the capacitor surface which leads to a rise in viscous damping and abbreviates device performance. Undoubtedly, for parameters characterized by opposite trends an optimization based on a theoretical model seems to be a first step towards sensor performance improvement.
Therefore, as a solution, we propose an analytical model for geometry optimization of silicon cantilever for photoacoustic wave collection using capacitance as a transduction mechanism. The study was carried out using silicon cantilever as a model, which brings the opportunity to obtain an analytical solution for all physical parameters. The goal of this research stands maximization of energy harvesting from a photoacoustic wave using cantilever with capacitive transduction mechanisms. To reach this objective we studied the creation of photoacoustic wave in order to find the optimal cantilever geometry for photoacoustic wave energy collection, which includes cantilever resonance frequency related with molecule relaxation time and the relative position of the laser beam and cantilever. In the next step, we optimized energy conversion from photoacoustic force to mechanical movement, thus mechanical sensitivity of the system. This stage focuses on the cantilever mechanical susceptibility enhancement which comprises quality factor, effective mass and favorable resonance frequency. The study for quality factor increasement considers the following losses mechanisms: acoustic, thermoelastic, support and viscous. The last step covers maximization of the capacitive transduction mechanism.
Conducted study gives a solution of cantilever dimensions and frequency for integrated compact gas sensor performance enhancement. Obtained results support the hypothesis that focusing on one aspect, like increasement of quality factor, is insufficient to enhance the performance of the whole device. Finally, it brings valuable insight for further system optimization since it can be extended to more complex structures which is a further perspective.