Inkjet-printed CMUT humidity sensors with high sensitivity and low hysteresis

Abstract

Capacitive micromachined ultrasonic transducer (CMUT) is a proven MEMS based gravimetric sensor featuring high mass sensitivity. It can be highly sensitive for relative humidity (RH) measurement when drop-casted with moisture-sensitive graphene oxide (GO) film. However, high hysteresis appears to be a critical problem for the sensor partially due to the non-uniformity of the sensing film. Here, we present miniaturized CMUTs that are functionalized by the inkjet-printed GO film and exhibit high sensitivity and meanwhile low hysteresis. Hexagon CMUTs with a side length of 500 μm were designed and fabricated by the nitride-to-oxide wafer bonding process. GO inks were prepared using a mixture of DI water and ethanol as the solvent. A square pattern formed by a 2D array of ink droplets was inkjet printed on the CMUT membranes. GO films with different thicknesses were deposited on three CMUTs. The humidity sensing performance of the sensors was evaluated by measuring the resonance frequency shifts induced by the added mass of adsorbed water molecules. The sensors delivered high RH sensitivity of up to 3.35 kHz/%RH, short response/recovery time down to 8 s/2 s, and hysteresis as low as 2.6 %RH. This work demonstrates that the inkjet printing technique is an excellent tool to functionalize CMUT with the advantages of minimized material consumption, batch-production capability, and exceptional control over the film thickness and quality. The CMUT printed with GO film has also shown to be a promising candidate as a miniaturized humidity sensing platform.