Design And Fabrication Of Capacitive Micromachined Ultrasonic Transducer 2D Array Devices

With the development of ultrasonic medical testing technology,real-time 3D ultrasonic imaging technology has become a new trend of medical ultrasonic imaging.Because the 2D ultrasound imaging technology can only get planar image,which can not display the 3D structure and undetermine the state of the biological tissue in real time.The real-time 3D ultrasound imaging can reflect the distribution of the biological tissue in a comprehensive and three-dimensional way,which is conducive to identify complex biological tissues and get more valuable information.The 2D array of ultrasonic transducer is the core component of real-time 3D ultrasound imaging and whose development has practical value and significance.In this paper,a 2D array of capacitive micromachined ultrasonic transducer(CMUT)for 3D ultrasound imaging has been designed.COMSOL Mutiphysics establishes the CMUT cell model and obtains the key parameters of CMUT cell.According to the CMUT cell parameters and sound beam directivity analysis,a 16×16 CMUT 2D array is designed and fabricated by using Si-SOI bonding technology.At the same time,through silicon vias(TSV)interconnection technology is proposed to lead out the inner upper electrode of CMUT 2D array,which lays the foundation for the subsequent research of CMUT 2D array.The specific research contents are as follows:(1)Firstly,the basic structure of CMUT cell is determined,and the working principle of CMUT is analyzed.Then,the theoretical derivation and analysis of the vibration of CMUT film are carried out,the relationship between the resonance frequency of CMUT and the size of the film,the deformation of the vibrating film and the height of the cavity as well as the relationship between the collapse voltage and the cavity height are obtained.The structure size and performance parameters of CMUT are determined.(2)Through the modal analysis,static analysis and electromechanical coupling analysis of CMUT cell by using COMSOL finite element simulation software,the simulation results of CMUT are obtained,which are basically consistent with the theoretical calculation results.According to the directivity function of CMUT 2D array,the relationship between CMUT 2D array and the number,spacing and size of array elements is drawn by using MATLAB software,and the CMUT 2D array with 16×16 array elements and spacing of 0.7λand center frequency of 1MHz is designed.(3)The mask layout and process flow of CMUT 2D array are designed based on Si-SOI bonding technology,and the processing,packaging and performance test of CMUT 2D array are completed.Firstly,the CMUT cell is tested by C-V test,vibration test and consistency test.The average capacitance is 26.3pF and the standard deviation is 4.27 pF.Then,the CMUT 2D array is tested by transmitting performance and receiving performance.It is verified that it has good transceiver performance.Finally,the bandwidth of CMUT is tested and the relative bandwidth of-6dB is 85.7%.It has the advantage of wide band and can meet the requirement of high resolution imaging.(4)In order to solve the problem of the top electrode lead in the CMUT 2D array,the through silicon vias(TSV)interconnection technology is proposed to lead the top electrode of the CMUT 2D array to the back of the device.Then it is packaged on PCB by bump flip chip bonding technology.In this paper,a fabrication method of electroless through silicon vias metal interconnects for CMUT 2D array is designed,fabricated and tested,and its feasibility is verified by experiments.