| Ultrasonic imaging technology is widely applied in many fields like biomedicine,military defense as well as robot tactile representation system.Ultrasonic transducer is an essential component to realize acoustic and electrical signal conversion.Compared with traditional piezoelectric ultrasonic transducers,CMUT(Capacitive Micro-machined Ultrasonic Transducer)based on MEMS(Micro Electro-Mechanical System)technology have the advantages of wide frequency bandwidth,good dielectric impedance matching,easy arraying,and high compatibility with IC processes.It is expected to complement the piezoelectric transducers and become a mainstream products.Therefore,the basic research on the theory and application of CMUT has important research value and practical significance.The research on CMUT started late in China,and has not formed a complete technology accumulation.Besides,the reliability of MEMS process is poor,because there is not any technical standardization on it;the equipment parameters used by various research institutions are significantly different,and reference value of them is limited.Therefore,the research progress on CMUT is slow.In order to improve the reliability of CMUT and promote the research process,a CMOS-MEMS process is proposed.The main research contents are as follows:(1)A mechanical model of the micro actuator is established,and its film deformation and pull-in displacement are analyzed.In order to adapt to the working state of air coupling,it is necessary to reduce the characteristic frequency of CMUT.Thus,A kind of CMUT with three cantilevered beams supporting membrane is proposed,which effectively reduces the characteristic frequency in a limited area.Under CMOS-MEMS technology,the material of each layer of the membrane is determined.By studying the directivity of the transducer(array),design targets such as the shape of the transducer,the size of the array,and the gap of the array elements are determined.(2)A finite element simulation model of the CMUT element was established under COMSOL Multiphysics.Through frequency response analysis,it is determined that the CMUT is a three-cantilever-type circular membrane,which own a characteristic frequency of 200 kHz.Through steady state analysis,it is determined that its pull-in voltage is less than 20 V.Through simulation analysis of inherent modal and pull-in voltage,the first-order characteristic frequency and pull-in displacement is verified.What’s more,a real-time app for parameter adjustment which is suitable for team collaboration,improves simulation efficiency a lot.In addition,this app has been passed the web test.(3)Field II and its extra toolkit are applied to analyze the ultrasonic field of CMUT array.The self-receiving ultrasonic echo of the transducer is used to determine the radius of the central hole of the membrane.The ultrasonic field of CMUT array is analyzed for focusing and deflection and the amplitude apodization function is applied for sidelobe suppression.Aiming at the improvement of image SNR,based on the ZYNQ-7010 platform,a multi-channel phase-controlled transmission circuit system is designed,with a minimum delay resolution of 2.5ns.(4)A tape-out project for CMUT with 0.18 um CMOS standard process has been finished in GlobalFoundries.Then,MEMS process have been applied to the tape-out chip.After that,this chip is connected to a test PCB through wire bonding.The impedance analysis and the morphology of CMUT are tested.According to the test results,the failure type of this device is analyzed and inferred.Finally,fatigue damage is determined under the finite element simulation tool. |
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