Equivalent Circuit Modeling And Optimization Of Capacitive Micromachined Ultrasonic Transducer

With the development of microelectromechanical system(MEMS),micromachined ultrasonic transducer has wide application in the fields of medical ultrasound imaging,nondestructive testing and aeronautics and astronautics.Compared with PMUT,CMUT possesses advantages such as having wide bandwidth,simple fabrication,impedance matching between the transducer membrane and the air or the fluid of interest.Therefore,the study of CMUT has important scientific value.For the application in ultrasound imaging,CMUT element and array are studied deeply in the dissertation,which covers the aspect including nonlinear equivalent circuit model,simulation and optimization of CMUT array and characteristic testing.The major research works of this paper can be summarized as follows:(1)A nonlinear circuit model for single capacitive micromachined ultrasonic transducers is proposed in coupled Electro-Mechano-Acoustic domains.The influence of atmospheric pressure on collapse voltage is analyzed.The nonlinear circuit model is used for CMUT cells in element to design three types of elements whose cells are arranged in 2×2,3×3 and 4×4 square array.And the optimal pitch of cells in the element is obtained.(2)The radiation impedance of CMUTs is accurately represented using an RLC circuit in the model.DC sweep,AC Analysis and transient are operated in SPICE simulation.Collapse voltage and resonant frequency are obtained.And the influence of spring softening coefficient on the resonant frequency is discussed.(3)Resonant frequency of ellipse and circular membrane is calculated by Rayleigh–Ritz method and solving vibration equation.The results of two methods are verified by Finite Element Method.CMUT arrays of ellipse and circular membrane have been designed.Directional sound field is calculated by Field II simulation to optimize the pitch of elements,membrane size and array arrangement.Finally,2D ultrasonic image is generated.(4)According to current process level,layout has been designed and CMUT arrays has been fabricated with silicon surface micro-fabrication technology.The device is fixed on printed circuit board to experiment.(5)Preliminary tests of the device is conducted by impedance analyzer and Doppler Vibrometer.The tests display the frequency response of impedance and membrane vibration displacement amplitude.And the parasitic capacitance is discussed.Finally,the receiver and transmitter circuits are designed and simulated.