Capacitive micromachined ultrasonic transducer (CMUT) as a chemical sensor for DMMP detection

We present a chemical sensor based on a capacitive micromachined ultrasonic transducer (CMUT) configured as a resonant mass sensor with a chemically selective polymer coating. The sensing unit of the CMUT consists of 100s to 1000s of resonators connected in parallel and acts as a single resonator. The high resonant frequency (18.2 MHz) and the small mass (296 pg) enable the CMUT to have a good mass sensitivity of 130 zg/Hz/μm2. We functionalized the CMUT with polyisobutylene (PIB) as the sorbent film, which targets dimethyl methylphosphonate (DMMP), a simulant for nerve agent, sarin. An oscillator circuit was populated to trace the resonant frequency of the CMUT with a fast response time and low noise. We characterized the noise performance of the sensor system and identified the optimal gate time of the read-out frequency counter. Based on the noise measurement, the calculated limit of detection (LOD) of mass loading is 0.192 ag/μm2 (3-σ confidence level). Chemical experiments were performed on the CMUT sensor with several analytes including DMMP, water, and ethanol. The limit of detection of the CMUT chemical sensor to DMMP vapor was measured as 56 ppb (3-σ confidence level).