Study On Automotive Radar Of Flexible Capacitive Micromachined Ultrasonic Transducer(CMUT)

In the research of automotive ultrasonic radar,piezoelectric ultrasonic ranging modules are mostly used,and capacitive micromachined ultrasonic transducer(CMUT)is rarely mentioned.CMUT has the characteristics of higher bandwidth,low power,low noise and easier fabrication.Therefore,according to the theoretical guidance and finite element simulation analysis,a large area vacuum-sealed CMUT array based on lamination of metal foils sandwiched polymer is proposed in this thesis,which is designed for low frequency and suitable for automotive radar.Using the process flow suitable for large-area flexible materials,a flexible CMUT sample was produced.The Bode plot was obtained,and the transmitting and receiving pulses of ultrasound were tested.The resonance frequency reaches 3.85k Hz,breaking the record for the lowest frequency of CMUT.The main contents of this thesis are as follows:(1)According to CMUT membrane vibration model,the calculation formulas of displacement and resonance frequency of circular vibrating membrane are obtained.It can be seen that the displacement of the vibrating membrane is positively correlated with the radius of the vibrating membrane and negatively correlated with the deflection.The resonance frequency is positively related to the thickness of the membrane and the elastic modulus of the membrane material,and negatively related to the equivalent area of the capacitor,the density of the material,and the Poisson’s ratio of the material.According to the parallel plate capacitor model,the calculation formulas of CMUT collapse voltage and electromechanical coupling coefficient are obtained.It can be seen that the collapse voltage is positively related to the gap distance of the parallel plate capacitor,and negatively related to the equivalent area of the capacitor.The electromechanical coupling coefficient is negatively related to the gap distance of the parallel plate capacitor.In the equivalent circuit model,the calculation formula of the mechanical impedance of the CMUT is obtained.It can be seen that the mechanical impedance is positively related with the load on the surface of the membrane and negatively related with the vibration velocity.Therefore,a model with thin membrane,high material density,large diameter,low Young’s modulus,and low pressure in the cavity should be designed to meet the requirements of low-frequency vibration.(2)Using the simulation software COMSOL Multiphysics~?,a two-dimensional and three-dimensional finite element model of the CMUT was established.It is obtained that the resonant frequency of the model with copper membrane is 40k Hz,collapse voltage is 74V,and the bandwidth of-3db is 1.8%.The working frequency decreases with the increase of DC bias voltage.Moreover,the working frequency of the thin copper foil model can be lower and the amplitude of the thin copper foil model can be larger under the condition of the same size and the same applied DC bias voltage.-3db bandwidth is slightly better than that of the silicon model.(3)A thin copper foil membrane and a thick aluminum foil substrate were vacuum-sealed laminated,sandwiching with cast-coated photosensitive emulsion as a cavity layer,tourmaline powder doped polyvinyl alcohol(PVA)as an insulation and charge storage layer,and water-based acrylic hot melt adhesive as a lamination adhesion layer.Organic polymers are not easily shattered and suitable for outdoor use.Costs are reduced and low frequency ultrasound is easier to achieve.A flexible design is realized and using metal foil can save the fabrication of upper and lower electrodes.The entire process can be completed in just two and a half hours.It is easier to realize a large-area array and the material is cheap,which can greatly reduce the cost.To understand the device performance,a Bode plot was obtained to find its resonant frequencies at 3.85k Hz and 100k Hz,and the transmitting and receiving pulses of ultrasound were tested at 5cm and 10cm distances.It demonstrated that the proposed large area flexible CMUT array was successful fabricated with the designed parameters to achieve part of the target performance with a lower than expected resonant frequency but reaching a lowest reported record in the CMUT device.The flexible structural design and process flow of CMUT proposed in this thesis provide theoretical guidance and practical application basis for the subsequent flexible and low-frequency design and mass production of CMUT.