Design And Application Of AlN Thin Film Based MEMS Piezoelectric Ultrasonic Transducers

This thesis focuses on the design and applications of air coupled Piezoelectric Micro-machined Ultrasonic Transducer(PMUT)with Aluminum Nitride piezoelectric thin film.After introducing piezoelectric materials comparison,dimensions selection,device fabrication and electromechanical characterization of PMUT,three applications of PMUTs are studied.Firstly,Doppler ultrasound measurements based on the designed PMUT are presented in this paper.Aiming at solving the problems of small bandwidth and high sampling rate in traditional ultrasonic Doppler measurement methods,an ultrasonic Doppler measurement method based on envelope method is proposed.The frequency and time information of echo signal is successfully extracted by this method and the test results agree with the theoretical model.Using the response model under electric excitation,the PMUT can be used as a mechanic sensor to measure the frequency shift induced by external mass or pressure.Using the response model under acoustic excitation,the PMUT can be used as acoustic sensor to measure the motion velocity through Doppler effect of ultrasound.Then,the PMUT is used for ultrasonic ranging.A ranging experiment based on PMUT unit is first implemented,then a thinner PMUT array with larger transmission power is designed and fabricated.The maximum measurement distance of the PMUT array is about 50 cm with the measurement error at the maximum range is about 3 mm,and the power consumption is less than 200 μW.To solve the problem of long vibration attenuation time of high Q ultrasonic transducer fabricated by MEMS technology,a vibration suppression method based on phase-delay is proposed.Firstly,the feasibility of this method is demonstrated theoretically,and then the theoretical analysis is verified by the tests in air and underwater domains.The experimental results of the method agree well with the theoretical calculation.Finally,the improvement of effective acoustic emission time and the reduction of ring-down time of PMUT after oscillation suppression are analyzed.Finally,the PMUT device is performed as a membrane resonator to operate mechanical logic gates.The traditional mechanical switches are mainly RF contact switches and optomechanical resonant switches.Thin film resonators have great potential for optical switches and acoustic sensors,and the switching speed increases rapidly with the miniaturization of devices,which makes it possible to realize high-speed resonant mechanical switches.Based on the idea of modal modulation,a flower shape PMUT is designed and fabricated,which is able to fulfill four 2-bit logic gates:NOT,OR,XOR and XNOR.The mechanical logic actuator can not only use voltage as output,but also use vibration displacement as output.Such a result paves the way for micromechanical acoustic communications and digital resonant sensor applications.