Piezoelectric Micromechanical Ultrasonic Transducers For Wearable Devices

With the rapid development of the Internet of Things technology in recent years,wearable devices have attracted wide attention.Ultrasound has the advantages of biosafety,information security and working in a complex environment.Microelectromechanical system sensors have the advantages of small volume,light weight and low power consumption.Piezoelectric micromachined ultrasonic transducer(PMUT)combined with these advantages has huge potential in the field of wearable application.Eye blinking,eye movement and heart rate are important physiological activities of the human body,which are closely related to the human physiology and psychological state.Towards wearables,this work studies PMUT and its arrays,and demonstrates the relevant applications of human characteristic signal detection.Details are as following:(1)Structural parameters and properties of PMUT elements and PMUT arrays were studied,providing a theoretical,design and manufacturing foundation for latter work.A cavity-type structure was used and the first-order vibrational mode was excited using a molybdenum electrode thickening structure.The equivalent circuit model and finite element multi-physical field coupling model were established to analyze the influence of structural parameters on resonance frequency and performance.The optimal ratio of emission and receiving performance were determined and the effect of the confined cavity on the frequency was also considered.The effects of different parameters on the sound field of the PMUT array were analyzed by simulation.Manufacturing processes were established and considering the effect of the etching deviation and the effective plasma concentration in the process.Virtual cavities were added to improve the consistency of the cavity depth.(2)Eye blinking detection application using PMUT arrays was first proposed.The eye blinking states were tested based on the ultrasound time of flight(TOF)principle,and the low-frequency PMUT array suitable for eye blinking detection was developed.Then we integrated the array with the glasses to complete the detection of four different states-single eye open and closed,two eyes open and closed,and both eyes open and closed.The eye blinking tests of different volunteers were performed,and the volunteer blink states were tested in real time within 1 minute.(3)Eye tracking detection application using PMUT arrays was first proposed.Eye rotation was detected based on the principle of ultrasound TOF and two PMUT arrays suitable for eye tracking with different frequencies were designed.Based on the characterization of the eye model,the PMUT arrays were integrated with the glasses;then eye rotation was detected by a single array and eye steering was detected by two PMUT arrays.Finally,the eye movement specificities of different volunteers were obtained.(4)Fingertip heart rate detection application was performed using the PMUT.The expansion and rebound of blood vessels were detected based on the principle of ultrasound TOF and a high-frequency PMUT array suitable for heart rate detection was designed.Then fingertip heart rate and neck vascular pulse were detected in different volunteers.In accordance with the existing commercial electronic stethoscope,the test results show good agreement.