Research On Flexural And Longitudinal Mode MEMS Piezoelectric Acoustic Transducers And Their Sensing Applications

Recently,with the rise of Internet of things（Io T）,wearable devices and smartphones,the main drivers for acoustic sensors are miniaturization and integration.Motivated by the development of microelectronics manufacturing technology and thin film piezoelectric material,micro-electromechanical（MEMS）piezoelectric acoustic devices get more and more attention in the application field of sensing.They are micro acoustic transducers based on piezoelectric effect to realize acoustic to electric conversion.Based on the main electro-acoustic conversion effect,the physical and chemical changes of the measured substance can be reflected by monitoring the acoustic output signal of the piezoelectric transducers.The MEMS piezoelectric acoustic devices have obvious merits such as small size,low power consumption,low cost,and CMOS-compatibility.They have been widely applied for molecular mass detection,pressure detection,distance and velocity sensing,acceleration and intertia detection,and gas detection by adpoting different vibration mode to satisfy the performance requirement.In this thesis,we main study two common vibration modes of MEMS piezoelectric acoustic devices:flexural mode（FM）and longitudinal mode（LM）.According to the performance of acoustic waves from the two modes,we develop novel sensors which can be used in the wearable devices and smartphones:MEMS acoustic gyroscope and dual-mode VOC gas sensor.The main achievements of the study are as follows:1.The working principles and design rules of the piezoelectric acoustic devices on flexural mode and longitudinal mode have been systematically studied.Theoretical analysis,lumped element modeling and finite element method（FEM）models are used to deeply study the impacts of vital variables,including membrane and electrodes shapes,electrodes coverage ratios,supporting layer materials,layer thickness and residual stresses,on the vibration modes,resonant frequency,effective electro-mechanical coupling factor k²_effand bandwidth are discussed.Optimum variables are obtained.2.A novel MEMS acoustic gyroscope is proposed based on the flexural-mode piezoelectric acoustic devices.The acoustic devices here are used as acoustic transmitter and receivers.The unique operating mechanism is based on the“acoustic version”of the Sagnac effect in fiber-optic gyros.The device measures the phase difference between two sound waves traveling in opposite directions,and correlates the signal to the angular velocity of the hosting frame.1D,2D and 3D FEM models are established to discuss the working principle and structure parameters of the acoustic gyroscope.An emulation test with a hula-hoop is adopted to evaluate the feasibility and performance of the acoustic gyroscope.A series of design rules and process flows have been systematically evaluated and optimized,and the first“acoustic Sagnac”gyroscope in a 5×5 mm² chip through a 9-mask process is built.It has a good potential to outperform other device types（fiber-optic,vibratory and BAW）in certain aspects.3.We studied the longitudinal-mode piezoelectric acoustic devices in gas sensing applications,and presented a novel concentration-independent VOC（voltaic organic compounds）sensor.Firstly,the inertial mass load effect of the piezoelectric acoustic devices is studied,which is the theoretical basis for its application in gas sensing.Then,we studied the longitudinal-mode piezoelectric acoustic gas sensors.Compared to other mode acoustic gas sensors,the longitudinal-mode demonstrates considerably lower detection limit and higher sensitivity.Finally,we creatively combine the acoustic devices with 2D material graphene to form a dual-mode sensor,enabling the acquisition of mass attachments and charge transfer from the same gas adsorption event simultaneously and independently.The two responses are monitored by a Lab VIEW program and form a unique electrical-mechanical trace of each gas species as the concentration varies.The electrical vs mechanical trace combined the two signals can realize target recognition and quantification at the same time.In this work,this sensing system has been tested for several VOCs,and it shows a great potential as a portable detection for VOCs discrimination with concentration-independent.