Development of a Highly Sensitive Humidity Sensor Based on a Piezoelectric Micromachined Ultrasonic Transducer Array Functionalized with Graphene Oxide Thin Film.

Changhe Sun,Qiongfeng Shi,Mahmut Yazici,Yufei Liu,Chengkuo Lee

doi:10.3390/s18124352

Changhe Sun, Qiongfeng Shi + Show 3 more

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https://doi.org/10.3390/s18124352

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Abstract

A novel relative humidity sensor that is based on a linear piezoelectric micromachined ultrasonic transducer (pMUT) array was proposed and microfabricated for high sensitivity, fast response, and good stability. The humidity-sensitive graphene oxide (GO) film was deposited on the pMUT array with a facile drop-casting method and characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and Fourier transform infrared spectrum (FTIR). With the humidity level ranging from 10% to 90% RH, the sensor exhibited a high sensitivity of 719 Hz/% RH and an extremely high relative sensitivity of 271.1 ppm/% RH. The humidity-sensing results also showed good short-term repeatability and long-term stability, fast response and recovery, and low hysteresis. Moreover, the temperature coefficient of frequency (TCF) of the present humidity sensor was investigated and it could be easily compensated owing to the pMUT array structure design. This work confirmed that the GO functionalized pMUT is an excellent candidate in humidity detection and it may enable many potential applications, such as ultrasensitive mass detection and simultaneous detection of multiple parameters.

Highlights

Miniaturized humidity sensors enabled through micro-/nano-fabrication technologies have gained extensive attention due to distinctive advantages of small size, low cost, fast response, high sensitivity, and high stability
The piezoelectric micromachined ultrasonic transducer (pMUT) based humidity sensor contains a pMUT linear array that was fabricated on a released silicon-on-insulator (SOI) wafer and a graphene oxide (GO) thin film deposited on the pMUT array
The finite element method (FEM) simulation that was based on COMSOL Multiphysics v5.2a was first employed to study the resonant characteristics of the pMUT

Summary

Introduction

Miniaturized humidity sensors enabled through micro-/nano-fabrication technologies have gained extensive attention due to distinctive advantages of small size, low cost, fast response, high sensitivity, and high stability These micromachined humidity sensors have great potential for portable electronic systems and they play crucial roles in a wide range of applications, such as environmental monitoring, industrial process control, agricultural production, and medical treatments [1,2]. Benefitting from the extremely high mass sensitivity at the zeptogram (zg) scale, the cMUT-based chemical sensor with a selectively sensitive layer exbibits an excellent sensing performance towards water vapor [12] Such good performance relies heavily on very narrow cavity (~40 nm) and high bias voltage (~50 V), which largely increases the process difficulty and fabrication cost and further limits its application fields. The humidity sensor that is based on the pMUT has not been investigated yet to our best knowledge

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