Study On Key Techniques Of Capacitive Micromachined Ultrasonic Transducers Featuring Various Membrane Geometries

As one of the most active and vibrant acoustic techniques,ultrasonic technique has been widely used in national defense,biology,medicine,aerospace,etc.Ultrasonic transducers are crucial components to achieve the conversion between acoustic energy and electric energy.Compared to traditional transducers,Capacitive Micromachined Ultrasonic Transducer(CMUT)based on micro-electro-mechanical systems(MEMS)technology has great advantages including low acoustic impedance,small size,high sensitivity,low noise,wide bandwidth,high electro-mechanical coupling coefficient,and wide operating temperature range.Therefore,CMUT has become a promising candidate for a wide range of advanced applications such as nondesctructive testing,ultrasonic driving,medical diagnostic imaging,ultrasonic therapy,and cellular control.For CMUT ultrasonic imaging application,this dissertation focuses on the key techniques of CMUT featuring various membrane geometries,which covers the deflection characterization,the frequency characterization,the acoustic characterization,the manufacture and measurements.(1)Aimming at the fully-covered flat circular CMUT featuring sealed cavity,two improved analytical models under uniform mechanical load and nonlinear electro-mechanical coupling load are proposed.These models consider the effects of the top electrode,the fixed boundary conditions,and the pressure inside the sealed cavity,which would be accurate enough for designing while avoiding a massive electromechanical Finite Element Modeling(FEM).Compared to the single-layer plate model,they offer more rapid and accurate predictions for CMUT modeling and designing.(2)Focused on the multilayer geometry of CMUT membrane,an improved analytical deflection and frequency model including residual stress compensation is proposed.This model directly points out the influence of the multilayer membrane geometry on CMUT characterization.Results show that this model agrees well with both the FEM and experiment results.Compared to traditional single-layer plate models,the proposed model can efficiently improve the accuracy.(3)In order to improve the area efficiency and imaging resolution,the ellipitical CMUT membrane is proposed.The analytical frequency model of ellipitical CMUT has been built up.Comparisons between the CMUTs with the elliptical membrane and other regular shaped membranes show that the elliptical CMUT has higher central frequency.Besides,the directivity of the ellipitical CMUT array has smaller mainlobe width and lower sidelobe amplitude.(4)In order to broaden the CMUT's applicable pressure range,the vented CMUT membrane is proposed.Comparisons between the vented and traditional flat CMUTs show that under the same DC bias,the vented CMUT has smaller average deflection,lower pull-in voltages,and wider applicable pressure range.Besides,flexible frequency design can be obtained by suitable parameter adjustment of vented CMUTs.(5)Acoustic distribution characteristics of CMUTs with different membrane geometries have been studied.Directivity characterization and influential parameters have also been discussed and analyzed.In order to reduce the sidelobe amplitude,various optimization methods have been used for the uniform linear array and uniform rectangular array with effective optimization results.(6)Manufacturing processes for specific CMUT membrane geometries have been designed and discussed.Our designed CMUTs have been fabricated and tested.Mechanical,electrical,and acoustical experiment setups for CMUTs are established.Deflection,frequency,and acoustic performances of CMUTs have been obtained.FEM and experimental results validate the proposed models in this dissertation.