Research On Touch Mode Capacitance MEMS Device With Double Cavities Applied On Vibration Wave Detection

Capacitive MEMS sensors are very popular MEMS structures.Capacitive MEMS sensors are widely used due to their simple structure,easy processing,low cost,low power consumption and high sensitivity.However,although the typical vacuum-sealed capacitive MEMS sensor has lower temperature sensitivity and higher pressure sensitivity,it has great limitations.For example,the vacuum-sealed cavity must maintain a high vacuum of the cavity.The connection transmission and air tightness of the electric signal between the cavity of the vacuum-sealed cavity and the outside of the cavity.Compared with the vacuum-sealed capacitive MEMS sensor,the manufacturing process of the gas-sealed capacitive MEMS sensor is relatively simpler and easier.However,because the cavity is a compressible gas,the pressure sensitivity of the gas-sealed capacitive MEMS sensor is low,and the gas in the chamber is easily thermally expanded,resulting in a large thermal drift.Therefore,this paper studies a new type of capacitive MEMS sensor to reduce the thermal drift of the sensor and improve its sensitivity.This paper proposes a touch mode capacitive MEMS sensor with double cavities applied to vibration wave detection.The double cavities and small gap structure of the device reduce the influence of gas thermal expansion on the sensitivity of the sensor,thereby improving the pressure sensing ability of the sensor.The top and bottom cavities of the sensor have the same pressure,but the volume of the top cavity is much larger than that of the bottom cavity.Due to the fluid retention effect,when the vibrating membrane of the top cavity bends downwards,the fluid in the cavity moves downwards,resulting in the amplification effect of the diaphragm vibration amplitude of the bottom cavity,to improve the sensitivity of the capacitor.In addition,when the top electrode plate is in touch with the bottom electrode plate,the amount of change in capacitance will be increased,and it will act as a protection device for over-range loads.First,the structure of the touch mode capacitive MEMS sensor with double cavities is designed and its working principle is analyzed,and the relationship between the capacitance change and sensitivity of the capacitive sensor,and the diaphragm deflection of the sensor is deduced and analyzed through theoretical formulas.Then,use the COMSOL Multiphysics simulation software to establish a two-dimensional model of the sensor's capacitive plate structure,and perform simulation analysis to verify that the influence of the dielectric constant change on the capacitance change when the top and bottom plates are in touch,which is consistent with the theoretical analysis result.Then,a three-dimensional model is established for the diaphragm structure of the sensor.By simulating the Young's modulus,size and other parameters of the diaphragm,the theoretical foundation was laid for the experimental preparation,testing and material selection,and the feasibility of the sensor design model was verified.Finally,the processing steps of the touch mode capacitive MEMS sensor with double cavities are planned,that is,the process flow of photolithography,etching,bonding and magnetron sputtering,and the sensor model is made,and it is passed through an impedance analyzer and a pressure gauge.Perform experimental testing and data processing.First,made a comparison of sensor models with different top cavity diameters,the analysis results show that the double cavities structure reduces the influence of the thermal expansion of the gas and the pressure in the cavity,and enhances the pressure sensing capability of the sensor.As the diameter of the top cavity increases,the capacitance change of the sensor also increases.The pressure required for the touch between the diaphragm and the bottom electrode plate decreases,and the dielectric constant changes,which increases the capacitance change of touch mode capacitive MEMS sensor with double cavities.The experimental results show the sensitivity of the capacitive sensor in the working range which is 0.3-0.7 N is 1003210.27 ppm/kPa,and the top and bottom electrode plates touch protection devices will not be damaged by the over-range load in harsh environments.Second,compared the sensor designed in this paper with the traditional air-sealed capacitive pressure sensor,the sensitivity of the sensor designed in this paper is significantly higher than that of the traditional air-sealed capacitive pressure sensor.Third,when the diaphragm is made of folded structure and no folded structure,the folded diaphragm of the sensor is more conducive to detecting micro signals.Fourth,the sensor is tested for hysteresis error through forward pressurization and reverse decompression.The maximum hysteresis error of the sensor is 0.15 pF.Fifth,compare the sensitivity of the capacitive sensor proposed in this article with the sensitivity of the sensors involved in existing work.The capacitive sensor has a higher sensitivity.