Research On Air Coupled Capacitive Micromachined Ultrasonic Transducer Array Based On SOI

Ultrasound technology is widely used in the military field and people's life.However,with its application in recent years,the drawbacks of the ultrasonic transducer have become increasingly prominent.Capacitive Micro-machined Ultrasonic Transducer(CMUT)based on micro-electro-mechanical system technology,has many advantages over traditional piezoelectric ceramic transducers in terms of device cost,sensitivity,integration and acoustic impedance matching,which cause great concern in academic and engineering fields.Especially,the supernatant medical imaging instruments have been imported for a long time in our country,and the key problem is the lagging development of ultrasound probe.Therefore,the research on CMUT has a long way to go.Firstly,we investigate the progress of ultrasonic transducers at home and abroad,which can provide reference of research directions and methods.The basic structure of CMUT,working principle and derivation of characteristic parameter formula are studied theoretically.Especially,the theory of collapse voltage and the mathematical model of sound pressure of circular vibration thin plate,as well as the directivity function of two-dimensional uniform plane array are deeply studied.To sum up,an air-coupled two-dimensional CMUT array based on SOI(Silicon-On-Insulator)silicon wafer is proposed,which is realized by micro-fabrication technology of micro-machined surface of MEMS.Based on the theoretical analysis results,the relationship between the natural frequency of the device and the radius of the vibration plate is plotted and analyzed,the influence of thickness and air gap on the collapse voltage is analyzed,which provide a theoretical basis for optimizing the parameters.Considering all these factors,the CMUT cell model of circular vibrating thin plate structure supported by cross-shaped cantilever beam is constructed using COMSOL.Then the static compressive stress analysis,displacement analysis and modal analysis under the coupling of structural mechanics and electrostatics are carried out.The stress nephogram of the vibrating plate under top electrodes and under bias voltage are obtained respectively.Vibration modes corresponding to different frequencies are obtained.Field II is used to complete the array model building,the simulation of axial ultrasonic intensity,acoustic pressure,and scattered echo signal response.According to the simulation results,the effects of different unit spacing and number on the axial acoustic intensity and acoustic pressure of the array,as well as the effects of different unit size on the axial acoustic intensity and acoustic pressure of the array are obtained.Then the appropriate SOI wafer parameters and the micro-fabrication process of MEMS are selected based on the research,analysis and simulation results,and CMUT array technology is completed in the micro-nano ultra-clean room.Scanning electron microscopy(SEM)was used to check the effect of the process.The test system is built to complete the low frequency coupling power-on test,and the response characteristics of the device under different excitation signals are analyzed.The input impedance of CMUT is obtained by impedance analyzer,and the displacement curves of vibration plate under different bias voltage are obtained by Doppler vibrometer.The test results show that the device structure can be released correctly by micro-fabrication technology,and the intrinsic characteristic parameters of the device basically meet the design requirements.Compared with the existing CMUT technology,the air-coupled CMUT device has simple structure and simplified manufacturing process,which is conducive to integration with the back-end processing circuit,and lays a foundation for the future design of the ultrasonic transducer chip and the research of the ultrasonic imaging system.