Study On Charge Effect Of Capacitive Micromachined Ultrasonic Transducers

Micromachined ultrasonic transducers are widely used,especially in the fields of medical imaging and industrial nondestructive detection.So far,the application of piezoelectric micromechanical ultrasonic transducers has occupied a dominant position.In the past two decades,the research on capacitive micromachined ultrasonic transducer(CMUT)has become more and more in-depth.Because CMUT has many advantages,it has broad application prospects and has a tendency to prevail.However,although CMUT has many advantages in ultrasound imaging,there is a problem of charge effect during application.Charge effect will affect the normal operation including long-term reliability and the life of CMUT,which is a major disadvantage of CMUT and directly hinders the commercialization of CMUT.If this problem is not solved,CMUT cannot be applied.The charge effect of CMUT is essentially the problem of dielectric charging.This paper reviews and analyzes the history and development of charge effect research and its specific impact on the performance of CMUT.Based on the analysis of several mechanisms and characteristics of charge transport in the dielectric layer and the conduction mechanism of CMUT,a model with non-contact charging and contact charging is established.By analyzing the conduction mechanism of dielectric charging,the influencing factors and rules of dielectric charging of dielectric charging are found out.Several factors affecting dielectric charging are analyzed,including thickness of membrane and insulating layer,height of cavity,materials and temperature of membrane and insulating layer.Through the study of the mechanism of dielectric charging combined with the parameters and the values of the factors affecting the dielectric charging,MATLAB was used to perform parameter assignment analysis.By analyzing the previous six factors and the influence of materials of CMUT membrane and insulating layer on the current density,the charging effects are compared,analyzed and discussed.The results show that the silicon dioxide has a much smaller charging effect than the silicon nitride.And the membrane is selected from 1?m to 2?m in thickness,the insulating layer is selected as 0.15?m in thickness,the radius is selected by 20?m,and the working environment is kept at 300 K to reduce the charging effect to a certain extent.In addition,based on the previous modeling analysis,the methods and measures for reducing dielectric charging are proposed such as decreasing the coverage area of the insulating layer,and reducing the surface roughness of the insulating layer.In order to reduce the surface roughness,a double SOI wafer bonded CMUT was designed.Experiments show that the voltage offset caused by charging is in the range of 2±1 V,the charge effect is greatly reduced,and the CMUT works well.In addition,this paper analyzes the principles of CMUT's transmitting mode and receiving mode.Based on this,the collapse and non-collapse modes of CMUT are compared.Then the plate capacitance model and equivalent circuit model are established,and the displacement,collapse voltage and electromechanical conversion efficiency of CMUT are analyzed and discussed.Finally,the conclusions of this paper are applied to improve the micromachining process of the CMUT.The advantages and disadvantages of traditional CMUT sacrificial layer release method and wafer bonding are compared in process control,yield,uniformity and micromachining process.A process method for reducing the area of the insulating layer and improving the smoothness of the membrane and substrate are proposed.This improved process potentially reduces parasitic capacitance and greatly reduces the charge effect.