Structural Design And Simulation Of A Novel Capacitive Micromachined Ul Trasonic Transducers

Ultrasound imaging is one of the main methods of modern medical imaging.Compared with other medical imaging technologies,ultrasound imaging technology has a strong ability to distinguish human soft tissues and has no ionizing radiation,so it is widely used.However,the low resolution of ultrasound image limits its application.In recent years,the capacitive micromachined ultrasonic transducer(CMUT)based on MEMS technology has the characteristics of high sensitivity,no matching layer,easy fabriacting,wide frequency bandwidth and so on.It becomes the hot spot of research on medical imaging.Nevertheless,the clarity of ultrasound image is still the barrier to the application of ultrasound image.Therefore,it is very important to improve the resolution of medical ultrasound imaging and expand its application.In this paper,a new type of capacitive micromachined ultrasonic transducer is proposed.By increasing the frequency bandwidth of the transducer,the resolution of ultrasonic image can be improved.It can be applied to medical ultrasound imaging and therapy at the same time,and it can also be applied to multi frequency requirements.Firstly,the working principle of CMUT is analyzed.Through the first-order lumped model,the collapse voltage of CMUT is analyzed.Based on the Mason equivalent circuit model,effective capacitance and the mechanical impedance of the membrane are analyzed.The formulas of the deflection and resonance frequency of the square diaphragm used in the new CMUT are deduced,which lays a theoretical foundation for the design of the new CMUT.The three-dimensional model of the square diaphragm is established by using the finite element simulation,and the simulation is carried out to verify the validity of the theoretical derivation formula applied to the new CMUT square diaphragm.Then,the gain formula of the transducer is built and studied.Based on the model,the factors that affect the performance of CMUT in transmitting and receiving modes are analyzed.In the emission mode,the effects of the side length of membrane,gap height,resonance frequency and other factors on CMUT's bandwidth and substrate thickness on pressure bandwidth product are analyzed.It can be seen from the analysis that in the emission mode,with the increase of the side length of membrane,the pressure bandwidth product of CMUT increases at the initial stage,and then,the pressure bandwidth product tends to a stable value,but the bandwidth is very small.Moreover,in the transmit mode,the bandwidth is independent of the gap height.Therefore,if a higher emission sound pressure is needed in the emission mode,the effect of gap height on the bandwidth can be ignored.In the receiving mode,the effects of the side length of membrane,electrode position,dielectric impedance and gap height on the c MUT bandwidth are analyzed.It can be concluded that the acoustic impedance,the side length and the thickness of the membrane will affect the gain bandwidth product and bandwidth.At the same time,the influence of gap height on the gain bandwidth product and bandwidth is also analyzed.When the side length of the membrane increases,the bandwidth will decrease and the stransducer gain will increase.Based on the analysis of CMUT performance above,the high-frequency and low-frequency structural parameters of the new CMUT are designed.The new structure of CMUT is designed and simulated,including the natural frequency of CMUT.In addition,the electromechanical coupling of the new CMUT is simulated.By comparing the performance of our new CMUT to traditional one,it can be concluded that the new CMUT has wider bandwidth and can improve the resolution of ultrasonic image.Finally,the new CMUT process flow is designed,and the key process is analyzed.