Research On Capacitive Micromachined Ultrasonic Transducer

Capacitive Micromachining Ultrasonic Transducers(CMUT)are Microelectro Mechanical Systems(MEMS)sensors commonly used for ultrasonic positioning,ultrasonic imaging,gas detection,particle manipulation,etc.Compared to Piezoelectric Micromachined Ultrasonic Transducers(PMUT),CMUT in bandwidth,sensitivity,resolution,medium impedance matching,cost,and process difficulty all have advantages,and have been widely valued by the society and scientific research personnel.In order to be able to mass produce CMUT,this paper studies its working principle,simulation method,fabrication process and test method.The main contents include the following:1.Introduce the basic working principle of CMUT,and take the gas sensing CMUT as an example.After simplifying the model,adjust the parameters such as the diameter of the circular unit,the thickness of the vibrating film and the electrode spacing to obtain the specific collapse voltage and sensitivity requirements;2.The 3D finite element model is established by using the multiphysics simulation software COMSOL.And the capacitor structure designed based on the simplified model is further optimized by considering the viscous acoustics,the multilayer film structure and the influence of the cavity wall structure.Multi-physics module,electrostatic module and circuit module are combined with mechanical module.And external electrical signal excitation is applied to the surface of the vibrating film to obtain more realistic performance results.The theoretical analysis results and simulation results are used to fine-tune the CMUT size parameters;3.For the process steps with lots of factors,the control factor level affecting the output of quality characteristics is adjusted innovatively combined with Taguchi experimental design theory,so that the output is close to the target value and the signal-to-noise ratio is relatively high.For the designed CMUT structure,the key processes such as thin film deposition,metal stripping,dry etching,and wet etching used in the device implementation process are adjusted;4.The corresponding process plan is formulated and the CMUT layout is innovatively improved to improve the antifouling and stability of the device on the results of theoretical analysis and simulation calculation considering the MEMS fabrication conditions that nano research institute can provide,.The L-Edit drawing software is used to convert the 3D structure into 2D graphics.After the pattern is transferred to the mask,the MEMS surface processing technology is used in the ultra-clean room to make the actual production and the CMUT stability and antifouling are confirmed;5.The surface of the sample was observed and measured by optical microscope,step meter and digital holographic microscope.After stealth laser dicing,the side of the sample was observed and measured by electronic scanning microscope to verify the physical integrity of CMUT.The electrical properties of the sample were tested by probe station test system,AC small signal test system and impedance analyzer.It was verified that the fabricated capacitor structure has capacitance collapse characteristics,and the capacitor upper electrode has mechanical resonance characteristics.