Study On Air-coupled Capacitive Micromachined Ultrasonic Transducers

Air-coupled ultrasonic testing technology doesn't require water immersion or coupling agent and has the characteristics of non-contact and completely lossless.It can also achieve fast on-line scanning and a wide range of applications.All these features makes air-coupled ultrasonic testing technology has great advantages in non-destructive testing and other fields.Air-coupled ultrasonic transducers are crucial components to achieve the conversion between acoustic energy and electric energy.Air-coupled capacitive micromachined ultrasonic transducer(CMUT)based on micro-electro-mechanical systems(MEMS)technology has the advantages of low acoustic impedance,high sensitivity,wide bandwidth,small size.CMUTs can easily be made into array and integrated with related circuits.Therefore,the study on air-coupled CMUT has important scientific value.For air-coupled ultrasonic testing application,study on air-coupled CMUT is carried out in this dissertation,including frequency characteristics,finite element analysis of different membrane geometries,sound characteristics simulation and optimization design of CMUT array,the manufacture and measurements.The major research contents can be summarized as follows:(1)CMUTs featuring different membrane geometries are designed.The analytical frequency models of CMUT with circular membrane and CMUT with square membrane are deduced according to the vibration equation.The analytical frequency model of CMUT with elliptical membrane based on Ritz method has been built up.(2)The finite element simulation analysis models have been established.The analytical frequency models are validated and the influence of electrodes on the CMUT frequency characteristics is analyzed.The influence of the holes through membrane and DC bias voltage on the frequency characteristic of CMUT is simulated and the pull-in voltage and spring softening effect curve of CMUT are obtained.The frequency of perforated membrane can be flexibly designed by adjusting parameters of the holes through membrane.(3)The directivity of CMUT featuring different membrane geometries has been analyzed.The acoustic field analysis of 1-D CMUT array and 2-D CMUT array have been carried out using Field II software.The general optimization method of CMUT array and a kind of optimization design method of sparse array are obtained.The characters of CMUT array with circular membrane and CMUT array with elliptical membrane are compared and analyzed.(4)According to the structure of CMUTs designed in this paper,the surface micromachining process is designed to manufacture the transducers.The mechanical,electrical and acoustical properties of our CMUTs have been tested and analyzed.The residual stress,impedance,frequency characteristics,displacement of the vibration membrane and some acoustic performances of CMUTs have been obtained.The experimental results validate the rationality of the design in this dissertation.