| Dual-frequency ultrasonic transducers have broad application prospects in parametric array technology,ultrasonic harmonic imaging and some emerging biomedical fields.Dual-frequency capacitive micromachined ultrasonic Transducer(CMUT)has become a promising candidate for these applications due to its great advantages such as flexible frequency control,simple processing technology,good array consistency and easy circuit integration,while the dual-frequency CMUT has been mainly designed for the application of ultrasonic imaging.For that CMUT has low acoustic impedance and needs no matching layer,it can be more easily used in air than traditional piezoelectric ultrasonic transducer.However,the researches of CMUT in air have been relatively weak,and the problem of narrow frequency exists in most of these researches.In view of these problems,combined with the unique advantages of CMUT in manufacturing dual-frequency ultrasonic transducers,this dissertation focuses on the structural design,processing technology of air-coupled dual-frequency CMUT as well as its applications in pressure detection,humidity sensing,air-coupled imaging and air parameter arrays.Specific research contents are as follows:1.Theoretical analysis,structure and array design of CMUT.According to the thin plate theory,lumped parameter model and equivalent circuit model of CMUT,the static characteristic,frequency characteristic,electrical characteristic and vibration characteristic of CMUT cell were analyzed and the variation rules of the performance parameters of air-coupled CMUT were obtained.All of these provided theoretical guidance for the structural design of CMUT.The finite element model of CMUT cell with multi-physical field coupling was established,and the structural parameters of CMUT cell with two frequencies were obtained by simulating the mechanical,electrical and acoustic characteristics of CMUT.In order to optimize the CMUT array,Field II was adopted to simulate and analyze the emitted acoustic beam of two arrays(hexagon array and square array).Finally,the configuration of CMUT arrays for processing and acoustic testing was determined.2.The processing technology and performance test of dual-frequency CMUT.The process flow of dual-frequency CMUT was designed,and CMUT arrays with different structural parameters and different frequencies were fabricated on the same wafer.The deflection,resonance frequency and vibration mode of CMUT cells were tested and the results verified the theoretical analysis and the finite element model.After optimizing the drive circuit of CMUT,the transmitting and receiving performance of two CMUT arrays were tested and the best electrical excitation parameters were obtained.3.Applications of dual-frequency CMUT in pressure detection and humidity sensing.Based on the small deflection theory and large deflection theory of CMUT membrane,the frequency characteristics of CMUT under the combined action of the external pressure and the static electric force were analyzed.And then a dual-frequency CMUT array with two staggered frequency elements was proposed to realize the detection of two different pressure ranges with high sensitivity and linearity.In addition,considering that CMUT is vulnerable to environmental pressure in humidity sensing in some cases,for the first time,a differential method was proposed to process the measured values of dual-frequency CMUT elements to compensate the influence of pressure in humidity sensing,which improves the accuracy of humidity detection by CMUT.4.Applications of dual-frequency CMUT in air-coupled imaging and air parametric array.Two methods of adjusting the resonant frequency of the CMUT by changing the DC voltage to widen the frequency band and optimizing the array arrangement to improve the resolution of ultrasonic imaging were proposed.And two air-coupled ultrasonic imaging systems,penetration detection system and echo detection system,were built to preliminarily verify the improvements of these two methods in air-coupled CMUT ultrasonic imaging.The ultrasonic imaging results of penetration detection showed that the imaging effect was related to the array arrangement of CMUT.The experimental results of echo detection ultrasound imaging showed that the frequency band of CMUT could be widened by changing the DC voltage,and good ultrasonic imaging results could be obtained at different frequencies.In addition,a method of generating differential frequency acoustic waves by applying two excitation frequencies to the corresponding CMUT elements was proposed for the first time to solve the problem of sound energy weakening caused by the deviation of excitation frequency from the resonant frequency of CMUT in the existing research.By exciting the dual-frequency CMUT array to transmit dual-frequency ultrasounds,we realized preliminary detection of the difference frequency acoustic signals in air. |
