Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing

Abstract The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To do such measurements, CMUTs were especially designed to work out of resonance, as a microphone. The chosen membrane size is 32x32 µm2 and gap height 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequency. The transducer impulse response was characterised, and a -6 dB relative fractional frequency bandwidth (FBW) higher than 100 % was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6×8 mm2. To validate the general principle of the proposed technique, two binary gas mixtures of CO2/N2 and H2/N2, with varying concentration, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3 % for CO2 in N2 and 0.15 % for H2 in N2. Indexterms: Capacitive micromachined ultrasonic transducers, gas sensors, air-coupled transducers, time-of-flight.