A material-independent gas sensing concept by coupling MEMS and acoustic resonance

Abstract

Conventional gas sensing technologies primarily rely on specific materials requiring individual sensor development. We propose a material-independent gas sensing concept that excites and detects the acoustic resonance in a cavity with in-house fabricated in-plane MEMS resonators. The acoustic coupling between the MEMS resonators, determined by the gas viscosity and speed of sound in the gas, is unique for each gas and can act as a gas sensor. Instead of measuring gas effects on a sensing element, the proposed method directly captures the gas resonance as a fundamental advantage. We experimentally demonstrate the acoustic coupling concept with and without an acoustic cavity. Efficient excitation of the acoustic resonance requires high displacement and frequency tuning. So, we modeled and designed length-tapered frequency tuning combs for high displacement (7 µm). We extract the cavity’s acoustic frequency (f_acs) and quality factor (Q_acs) as 35.8 kHz and 10, corresponding to 343 m/s speed of sound at room temperature. We prove the functionality and selectivity of the proposed method with semi-controlled gas (CO₂ and O₂) testing. As expected, the acoustic frequency decreases by 4.2 kHz (>10 % of the MEMS resonators), and the acoustic quality factor increases by 14 % with increased CO₂ concentration. We show the selectivity of our sensor with unique outputs to two different CO₂ and O₂ compositions having the same speed of sound.