Research And Application Of Humidity Sensor Based On MEMS Aluminum Nitride Piezoelectric Resonator

Humidity sensors play an important role in fields such as industry and agriculture.In the context of the Internet of Things,miniaturized and integrated MEMS humidity sensors have gradually become the trend.Compared with other types of MEMS humidity sensors,MEMS resonant humidity sensors have the advantages of high sensitivity,low power consumption,and quasi-digital output.Besides,they can measure both relative humidity and absolute humidity.The research work in this paper mainly focuses on humidity sensors based on MEMS aluminum nitride piezoelectric resonators.Aluminum nitride is used as piezoelectric material,and piezoelectric microcantilever resonators are designed and fabricated by Piezo MUMPs process.Piezoelectric microcantilever has the advantages of high resolution,wide dynamic range and high relative sensitivity.Graphene oxide is used as humidity sensing material and coated on the surface of device,then piezoelectric microcantilever relative humidity sensor is successfully prepared.The test is performed on the humidity sensor by the measurement setup.The test results show that the sensor has high sensitivity,fast response,good repeatability and long-term stability.Based on the equivalent circuit model of the piezoelectric resonator and principle of selfexcited oscillation,a closed-loop frequency tracking circuit for the piezoelectric resonator is designed.The closed-loop circuit is composed of resonator,preamplifier circuit,low-pass filter,automatic gain control module and phase shifter.Each circuit module is established and tested to realize corresponding function.By combining the humidity sensor with frequency tracking circuit,a closed-loop humidity measurement system is constructed.The closed-loop test results match well with the open-loop test results,indicating that the closed-loop system can track the resonator frequency in real time.Then the sensor is integrated with mask to monitor breathing.The test results show that the sensor can not only monitor human respiration,but can also successfully track respiratory signals of rats which are weak and fast.Finally,a dew point sensor based on a circular piezoelectric resonator is proposed.The operating frequency of the resonator is megahertz level,which improves the sensitivity and anti-interference ability of the sensor.Due to the linear negative TCF coefficient and mass loading effect,which cause resonance frequency of resonator to increase as it cools down and then decrease when the temperature reaches the dew point temperature,detection and identification of the dew point can be realized.The measurement setup consisting of dew point generation system,sensor head and measurement unit is built.The test results show that the dew point sensor is highly accurate.