Design Of Capacitive Micromachined Ultrasonic Transducer For Air-coupled Ultrasonic Nondestructive Testing

Air-coupled ultrasonic testing is an important method in ultrasonic nondestructive testing and has the characteristics of non-contact,non-invasive,fast scanning and no coupling agent.Air-coupled ultrasonic transducer is the key component to improve the air-coupled ultrasonic nondestructive detection capability and achieve multi-field coupling and energy conversion.With the development of micro electro-mechanical system（MEMS）,a new type of capacitive micro-ultrasonic transducer（CMUT）has been widely studied due to its advantages.It has the advantages of easy fabrication of array,batch production,easy integration with circuit,high sensitivity,high electromechanical conversion efficiency.And it is suitable for air-coupled ultrasonic nondestructive applications.In this paper,a CMUT array of 16 elements was designed for the air-coupled ultrasonic nondestructive testing.It was fabricated by SOI（silicon on insulator）wafer bonding process and was studied by modeling and experimentation.The specific research contents are as follows:（1）The resonant frequency theoretical model of CMUT was established according to the circular plate vibration equation.And the CMUT structural parameters CMUT were designed by analyzing the influence of each structural parameter on its performance.The field directivity of the CMUT array was studied,and the number of CMUT array elements and the array pitch were obtained.At last,a16-element linear CMUT array with circular diaphragms was designed.（2）According to the structure of CMUT and its application environment,a finite element simulation model was established.Through this model,the performance of the CMUT was studied by static analysis,frequency analysis and electromechanical coupling analysis.Based on parallel plate capacitance theory,the CMUT transmitting and receiving simulation model was established by MATLAB/SIMULINK.The maximum transmitting sound pressure and receiving sensitivity of CMUT under different DC bias voltage and AC excitation voltage were analyzed,and the optimal excitation conditions are selected for the experiments.（3）According to the structural characteristics of the air-coupled CMUT,an SOI wafer bonding process was designed.Mask map was drawn to complete the air-coupled CMUT processing production and packaging.The mechanical,electrical and acoustic performance tests of the CMUT arrays were carried out,including static topography test,impedance analysis test,resonance characteristic test,array element consistency test,array directivity test and acoustic performance test.The study demonstrated that the CMUT array has good performance of transmitting and receiving,and the Lamb waves of the A₀ mode in the metal plate can be successfully excited and received.