Development of a highly sensitive humidity sensor based on the capacitive micromachined ultrasonic transducer

Abstract In this paper, we present a highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based resonant gravimetric humidity sensor using graphene oxide as the sensing material. The CMUT transducer with a diameter of 2 mm and an operating frequency of 12 MHz was fabricated by the direct wafer bonding technique. To validate the design, both the laser vibrometer and the vector network analyzer were adopted to characterize the resonant properties of the fabricated device. A graphene oxide thin film was then coated as the water vapor absorbing layer on the resonating membranes of the CMUT. The humidity sensing performance of the CMUT humidity sensor was evaluated through the electrical impedance analysis over a relative humidity (RH) range of 22.5% to 93.6%. Frequency shifts caused by the added mass of the absorbed water molecules was observed in the measurements. The CMUT humidity sensor exhibited a sensitivity of up to 11.5 kHz/%RH in the tested humidity range and showed a good repeatability and a rapid response/recovery. Benefit from the microelectromechanical (MEMS) technology and the water-sensitive graphene oxide, the developed CMUT humidity sensor is expected to be a strong candidate for miniaturized, highly sensitive, and reliable humidity sensors.

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