Study On Multifrequency Tranducing And Novel Multiparameter Sensing Mechanisms Of PMUT And Their Applications

In recent years,with the rapid development of Micro-Electro-Mechanical Systems?MEMS?technology and Internet of Things?IoT?technology,micromachined ultrasonic transducers have stood out among all kinds of ultrasonic devices,playing an increasingly important role in the social development,national economy and defense security.Piezoelectric micromachined ultrasonic transducer?pMUT?becomes a new and highly promising ultrasonic transducer due to overcome many defects of the traditional large-size piezoelectric ultrasonic transducers and capacitive micromachined ultrasonic transducer?cMUT?,offering advantages of flexible structure design,simple fabrication process,no bias voltage,high transmitting sensitivity,good reliability,easy acoustic impedance matching,miniaturized size,easy to array and be integrated,etc.Till now,research institutions all over the world are mainly engaged in improving the electromechanical coupling coefficient,sensitivity,working frequency,bandwidth and other properties of pMUT devices,but the single working frequency band and small frequency bandwidth always make the pMUT-based ultrasonic imaging systems fail to combine both large detection range and high imaging resolution.In addition,since it is extremely difficult for pMUTs to offer both high electromechanical coupling coefficient and high Q value,almost all pMUT-based researches are just focused on ultrasound transducing characteristics,not on the sensing characteristics and ralated applications.Therefore,funded by the National Key Research and Development Program of China and the China Scholarship Council,this paper theoretically and experimentally investigates the multiband and broadband ultrasound transducing,multiparameter sensing characteristics and their applications.The main work of this dissertation is summarized as follows:1)Based on Kirchhoff plate theory and piezoelectric constitutive equations,the equation of motion and the modal frequencies of rectangular pMUT are derived.Starting from the energy analysis,the equivalent mechanical stiffness is derived and the electromechanical equivalent circuit model of pMUT is established.Compared with the COMSOL finite element simulation results,it is found that the established theoretical analysis model can accurately predict the vibration deflections and modal frequencies of pMUTs.Furthermore,the PZT thin film deposition and etching technology is also studied.The integrated microfabrication technology of pMUT based on the MPB-PZT thin film is designed.The influence of electric field polarization conditions on the performance of the MPB-PZT piezoelectric film is analyzed,the test results show that the high-voltage single pulse polarization can effectively improve the piezoelectric constant and reduce dielectric constant and dielectric loss.2)A single-element piezoelectric-capacitive multifrequency ultrasonic transducer consisting of a piezoelectric film and a groove dielectric film based cMUT is presented,caplable of operating at low-frequency cMUT mode and high-frequency pMUT mode.The differential equation of motion and the electromechanical equivalent circuit models at two different operation modes is established for multifrequency ultrasonic transducer.The deflection function of the uneven laminated membrane is theoretically analyzed and made comparison with COMSOL finite element simulation results,providing less than1%deviation for predicting deflection waveform.By changing the size ratio of the piezoelectric and dielectric films,a high-to-low frequency ratio of 2 to more than 20 can be achieved without any harmonics.What's more,the high-frequency band and low-frequency band can be optimized independently through the structure design.Compared with high-frequency harmonic operation of the traditional multi-frequency ultrasonic transducers,the designed transducer can theoretically provide at least one order of magnitude higher sound pressure intenity at high-frequency non-harmonic operation.3)A broadband piezoelectric multifrequency ultrasonic transducer based on rectangular piezoelectric thin film is presented.By analyzing the bending modes of rectangular and square pMUTs,the modulation effect of the length-to-width ratio of rectangular film on the modal shapes and frequencies of the pMUT is theoretically investigated.The multi-band and broadband operation mechanism of rectangular pMUT is proposed,which has been verified by COMSOL finite element simulations.Three multi-frequency ultrasonic transducers with small,medium and large rectangular length-to-width ratio has been fabricated using MEMS processing technology.The in-air displacement sensitivity and impedance characteristics of three transducers are tested and underwater ultrasound transmitting experiments are also completed.The test results show that the multi-frequency ultrasonic transducer with a medium rectangular length-to-width ratio can show a multifrequency property with a smoother response curve and a broader frequency bandwidth.The-6 dB bandwidth of the first two frequency bands for the optimized multi-frequency ultrasonic transducer without acoustic impedance matching layer are 112%@38%@0.615 MHz and 1.63 MHz,respectively,which are higher than all previously reported piezoelectric multifrequency ultrasonic transducers and are promisingly used in ultrasonic detection,diagnosis,treatment and imaging.4)A self-powered multi-parameter sensing flexible system based on pMUT linear array and triboelectric nanogenerator?TENG?is created,which is composed of a pMUT humidity sensor,a pMUT temperature sensor,a TENG energy harvester and a TENG based CO₂ gas sensor.Based on pure graphene oxide?GO?and pure polyethyleneimine?PEI?solution,a PEI/GO composite thin film is successfully developed.With the functionalization of pure GO film,pure PEI film and the developed PEI/GO composite films with different volume fractions on pMUTs,various humidity sensors are designed.Through the detailed characterization analysis and experimental testing,the sensitivities of GO film and PEI film based pMUT humidity sensors are 719 Hz/%RH and 543 Hz/%RH,respectively,showing remarkable nonlinear responses.The pMUT humidity sensor based on the PEI/GO composite film with 50%?v/v?PEI volume fraction has a sensitivity of up to 748Hz/%RH,exhibiting a nice linearity,improved hysteresis and improved stability.Adopting the unfunctionalized pMUT for temperature sensing,the pMUT linear array partially functionalized with 50%?v/v?PEI/GO composite film is used to design the temperature sensor and humidity sensor,which can compensate the temperature and humidity effects of the whole sensor array.A conventional TENG is designed to harvest ambient vibration energy to power the pMUT array,providing a maximum output power of 7.5 mW.Another TENG is adopted and functionalized with PEI film on its friction surface for CO₂ sensing.The humidity effect of the developed TENG based CO₂ gas sensor can be calibrated using the PEI/GO composite film based pMUT humidity sensor.After humidity calibration,the TENG based CO₂ gas sensor has a measurement error of less than 5%and a detection limit of up to 10000 ppm.