A high-performance CMUT humidity sensor based on cellulose nanocrystal sensing film

This work presents a high-performance capacitive micromachined ultrasonic transducer (CMUT) based humidity sensor for gravimetric detection of water molecules. A hexagon CMUT with a side length of 1mm was designed and micro-fabricated by the nitride-to-oxide wafer bonding process. The device was composed of 2269 parallel connected microscale resonator units that acted as a single resonator. Biodegradable and renewable cellulose nanocrystal (CNC) was employed as the humidity sensing material to functionalize the CMUT due to its unique properties such as large surface area and abundance of hydroxyl groups. These properties of CNCs were characterized and verified by the transmission electron microscopy and Fourier transform infrared spectroscopy. To evaluate the performance of the CMUT humidity sensor, sensing measurements were conducted over a relative humidity (RH) range of 11%-94%. The sensor exhibited high sensitivity (up to 2 kHz/%RH), rapid response and recovery (7 s/2 s), low hysteresis (2.5 %RH), and excellent repeatability and long-term stability. This work demonstrates that the combination of CMUT and CNC is a promising candidate as a high-performance miniaturized humidity sensing platform.