| Ultrasound tomography is a new ultrasound imaging technology,which greatly breaks the limit of traditional ultrasound imaging and provides a new and effective screening method for its application in breast screening.Ultrasonic transducer is the key device to realize ultrasonic imaging.As a kind of ultrasonic transducer with simple structure,high sensitivity and high resolution,the capacitive micromachanical ultrasonic transducer(CMUT)has more extensive application prospects in the fields of nondestructive testing and medical imaging due to its advantages that traditional piezoelectric transducers do not have,such as frequency bandwidth,easy integration,miniaturization,etc.With the high requirements of the breast ultrasound CT imaging system for the detection environment,the watertight packaging of the capacitive micromechanical ultrasonic transducer has become an important link in the design and research of the ultrasonic transducer.To solve this problem,based on the CMUT array elements developed by the research group in the early stage,this paper analyzes the impedance characteristics,vibration characteristics and acoustic characteristics of CMUT,explores the broadband packaging scheme of CMUT,and studies the impact of this scheme on the performance of CMUT.The main research contents of this paper are summarized as follows:Firstly,based on the working principle of CMUT,a parallel plate capacitance model is established to analyze the characteristics of CMUT film under static bias voltage,and the collapse voltage of CMUT is derived from the formula;The distributed parameter system model is used to analyze the vibration characteristics of CMUT film,and the vibration displacement,velocity and natural frequency formulas of CMUT film are derived;Mason equivalent circuit is used to analyze the equivalent circuit model of CMUT,and the mechanical impedance and the mutual radiation impedance between micro-elements are deduced and analyzed by formula;Formulas are derived for the emission sound pressure,reception sensitivity and directivity of CMUT.In addition,based on the basic theory of ultrasonic propagation,the propagation of ultrasonic in different media is analyzed,and the conditions for the acoustic impedance of the matching layer are deduced.Secondly,the acoustic properties of the composite are predicted by the model and analyzed by using the effective medium theory(such as the coherent potential approximation model).On the basis of determining the properties of composite matrix and mixed particles,its parameters are used in the coherent potential approximation model to predict the phase velocity and density of composite,explore the functional relationship between them and the volume fraction of mixed particles,and determine the appropriate volume fraction.Apply COMSOL Multiphysics to establish the CMUT finite element model for modal analysis and harmonic response analysis.Based on this,the CMUT micro-element covering composite material is simulated,and the sound field characteristics of CMUT in the presence of spin-coated air gap are analyzed,which provides an important basis and reference for subsequent practical testing.Then,the preparation process and performance test of 0-3 composite materials were carried out.According to the research results of the characteristics of titanium dioxide powder,the content of PDMS and other factors,the acoustic impedance of the material was controlled.The composite materials with different PDMS/titanium dioxide powder ratio and different thickness were prepared,and the single variable(thickness)was controlled to analyze the effect of titanium dioxide particle concentration on the acoustic impedance and acoustic attenuation of the composite materials;Control a single variable(PDMS/titanium dioxide powder ratio)to analyze the effect of thickness on the sound attenuation of the composite.In addition,the dependence of the composite on the frequency was observed and analyzed,and the research showed that the composite has a broadband matching property.After that,the packaging scheme of CMUT is studied and the performance test is carried out.Build a CMUT test platform to test the impedance characteristics,vibration characteristics and acoustic characteristics of CMUT bare chip in silicon oil medium;To package CMUT,in order to reduce the sound attenuation of 0-3 composite materials and improve the efficient transmission of sound energy,the surface of CMUT film is packaged by spin-coating process to control the thickness of matching layer;In addition,the CMUT chip is waterproofed with irrigation technology to achieve the effect of electrical insulation;The performance of packaged CMUT was tested in water medium,and the influence of this packaging scheme on the characteristics of CMUT was analyzed.The results show that the 0-3 composite material based on PDMS and Ti O2 particles proposed in this paper is stable and feasible as the underwater CMUT matching layer,and this method has the characteristics of adjustable acoustic impedance magnitude and wide range of acoustic impedance gradient.The ratio of particles to matrix has a great impact on the acoustic impedance of the composite.By adjusting the ratio parameters,the 0-3 composite prepared can meet the technical requirements of underwater CMUT broadband and efficient transmission.Some performance parameters of CMUT designed and packaged are better than those of CMUT bare chip,but further research and optimization are needed to achieve better results. |
