A low-power gas sensor for environmental monitoring using a capacitive micromachined ultrasonic transducer

We present a low-power gas sensor design based on a capacitive micromachined ultrasonic transducer (CMUT), for use on self-powered wearable platforms. Earlier a CMUT-based sensor, with 70-mW power consumption operating at 50 MHz, achieved ppt-level detection limit for chemical warfare agents. In this work we present a sensor operating at 4.33 MHz and consuming 0.77 mW for environmental monitoring. The sensor comprises a polymer-functionalized CMUT resonator in the feedback loop of a Colpitts oscillator. We fabricated the CMUT resonators using a novel process based on anodic bonding. The cavities and bottom electrodes are formed on a borosilicate glass wafer. The device layer of an SOI wafer bonded on glass forms the vibrating plate on top of vacuum-sealed cavities. This fabrication approach reduces process complexity and helps minimize parasitic components. CMUTs with center frequencies in the 3-50 MHz range with Q-factors as high as ~400 have successfully been fabricated. We used a 4.52-MHz device (Q=180) coated with a thin layer of polyisobutylene (PIB) for sensor demonstration.