Design Of Capacitive Micromachined Ultrasonic Transducer And Research On Encapsulation Technology

The invention of capacitance mechanical ultrasonic transducer(CMUT)based on MEMS micro processing technology has get the attention of many researchers,because it has the advantage of simple structure,low noise,high electromechanical coupling coefficient,high resolution,high sensitivity,wide bandwidth and impedance matching with the media.It has a wide application value and foreground in the field of seabed resource exploration,underwater topography detection,and medical imaging,and so on.Therefore,the theoretical analysis and basic application research of CMUT has important research value.The main research content is as follows:(1)Three model of CMUT are established according to the content of different analysis.In the vibration model,the vibration displacement equation of the circular vibrating film is deduced.The parameters of CMUT collapse voltage,electromechanical coupling coefficient,receiving sensitivity and sound pressure are deduced by the first-order lumped model.In the equivalent circuit model,Capacitance,mechanical impedance and other parameters were analyzed in detail.(2)On the basis of theoretical analysis,the CMUT finite element model is established using COMSOL Multiphysics.The thickness and radius parameters of CMUT cell are determined by the characteristic frequency analysis.The electrode size were determined by analying the relationship between electrode thickness,electrode radius and collapse voltage in electromechanical coupling model.In the static analysis,the membrane displacement and stress changes under standard pressure are studied and then the CMUT cavity height is calculated.The correctness of the designed CMUT is verified by modal analysis and harmonic response analysis.(3)Ddirectivity is one of the important characteristics of CMUT in the far field.Thedirectivity are calculated of CMUT cell,CMUT element and CMUT array.The number of CMUT cell and CMUT elements space are determined based on the influence of CMUT structure parameter on the directivity of CMUT array.The sound pressure distribution of CMUT cell,CMUT element and CMUT array is deduced.(4)The electrostatic force plays an irreplaceable role in the CMUT parallel plate capacitance model,but the electrostatic force is nonlinear.In order to eliminate and suppress the non-linearity of the CMUT,three methods are applied.1)Removed the DC bias voltage method is used to eliminate the harmonic component.2)By changing the excitation signal type,the bipolar pulse is selected according to the spectrum suppression effect.3)the linear and non-linear compensation method is applied according to the formula for eliminating the second harmonic component.(5)According to the actual application,this dissertation studies the CMUT chip package and design the CMUT encapsulation structure.The material selection,structural parameters,stress size and sound transmission coefficient of CMUT shell package are analyzed in detail according to the acoustic theory,and the CMUT watertight package is realized at last.The CMUT experimental system is designed to test the emission performance,receiver sensitivity and bandwidth performance of the designed CMUT.The test results show that the designed CMUT array can realize underwater imaging.