Design And Development Of A Flexible Capacitive Micromachined Ultrasonic Transducer

Ultrasonic transducer and array probe are the key components of ultrasonic detection and ultrasonic imaging technology,widely used in medical diagnosis and treatment,underwater communication and detection,defect nondestructive testing and other technical fields.Traditional PbTiZrO3-based(PZT)piezoelectric ceramic ultrasonic transducer is a kind of electro-acoustic conversion element which is widely used because of its high electromechanical conversion efficiency.However,the PZT-based piezoelectric ultrasonic transducer has high acoustic impedance which is not easy to match with the acoustic impedance of human soft tissue and water,and there are some defects,such as high mechanical quality factor,narrow bandwidth,high brittleness,low tensile strength,high density element and processing difficulty for ultra-thin high frequency transducer,that constraints the application range of PZT-based transducers.In contrast,the capacitive micromachined ultrasonic transducer(CMUT)has many advantages including wider bandwidth,higher sensitivity,lower noise,easy to fabricate two-dimensional array,and thus make up for the shortcomings of PZT piezoelectric transducers.CMUT is expected to play an important role in many technical fields such as medical testing,wearable equipment,non-destructive testing.In this work,a flexible CMUT device is proposed by means of theoretical analysis and finite element simulation for the application of the surface irregular object.Aiming at the low resonant frequency of CMUT with flexible single-layer resonating membrane,the CMUT with stacked membrane structure is proposed.A series of flexible CMUT samples were developed by surface micromachining,and the properties of the samples were tested and analyzed.The research work mainly includes:Based on the working principle of CMUT,the force coupling characteristic of CMUT is analyzed,and the finite element model of CMUT sound field is established according to the flow-solid coupling equation.The relationship between the structural parameters of CMUT device and its mechanical properties,electrical properties and sound field characteristics is analyzed by finite element simulation.The CMUT devices with flexible single-layer PVC resonating membrane and PVC/Cu stacked membrane structure are designed,and their mechanical and acoustic properties are simulated by finite element simulation.The results show that the relative bandwidth of the CMUT with single-layer PVC membrane is two orders of magnitude higher than that of the CMUT with Si3N4 membrane for the same device size,but the resonant frequency is nearly two orders of magnitude,and the maximum,and the maximum sound pressure is reduced by nearly half.By using a stacked membrane structure combined with PVC and Cu films,the resonant frequency can be increased by an order of magnitude,and the maximum emission sound pressure is increased by nearly 30 dB,which is more conducive to improving the ultrasonic detection resolution of the flexible CMUT.A series of flexible CMUT samples were prepared by simple process,and the CMUT samples with the flexible single-layer membrane and the stacked membrane were tested to analyze the resonant characteristics by impedance analyzer.The test system of ultrasonic emission and reception of CMUT samples was designed and constructed to carry out the ultrasonic emission and reception characteristics of the CMUT samples.The experimental results show that the developed CMUT sample can effectively transmit and receive ultrasonic signals with a resonant frequency of 40 KHz and a relative bandwidth of 52.5%,indicating that this transducer can be used as ultrasonic detectors in air and water.