Capacitive silicon micro-electromechanical resonator for enhanced photoacoustic spectroscopy

Abstract

Photoacoustic spectroscopy (PAS) has been increasingly applied to detect gas traces in many applications. Gas absorption is detected through the excitation of a mechanical transducer, actuated by the acoustic pressure generated after optical absorption. PAS is potentially the best method to achieve some selective, sensitive, compact, and reliable sensors. However, the main limitation comes from the use of some mechanical transducers which are not specifically designed for this application. Great interest for realizing efficient devices with specific characteristics led us to study microelectromechanical systems (MEMS). Silicon is the core material of this technology. It offers high performances in terms of quality factor and residual stress and is an attractive alternative to conventional acoustic transducers. MEMS are widely used as transducers, and electrostatic transduction is a well-established method. In this work, we describe mechanical resonators fabricated on a silicon-on-insulator (SOI) wafer to be used as acoustic transducers in PAS. The performances of the developed devices are strictly linked to their mechanical properties and viscous damping. Their sensitivity is evaluated through an experimental setup; we achieved to detect methane and ethylene using a distributed feedback (DFB) laser diode and a DFB-QCL (Quantum Cascade Laser) emitting at 1.6 μm and 11 μm, respectively. By demonstrating stable and reproducible detection, this work opens the way to a concept of compact gas sensors based on tunable diode laser absorption spectroscopy and capacitive silicon microelectromechanical resonators.