Research Of Electromechanical Coupling Characteristics And Key Technologies In Microaccelerometer

Electrostatic driving and sensing technology is broadly applied in MEMS (Micro-Electro-Mechanical Systems) transducers and sensors, due to its advantages of high sensitivity, high speed of response, high precise, easy fabrication and easy integration with CMOS (Complementary Metal Oxide Semiconductor) technology. The parallel plates capacitors with straight line outline in micro-electro-mechanical-systems and its characteristics of application in capacitive micro accelerometer and resonant micro accelerometer are studied, and three types of macro-model are extracted in common spring-mass-capacitor structure:one with single spring and single capacitor, one with double springs and single capacitor, and one with single spring and double capacitor. The structure form, error model, electromechanical coupling characteristics and apllication technology in micro acceleroemter of those models are studed in paper, which contain subsequent aspects:1. The macro models of spring-mass-capacitor are extracted according to the application traits of MEMS parallel plate capacitors, and the concrete forms of spring, mass and capacitor in MEMS are analyzed. The concentration of research is on the error dependence of stiffness of two typical micro-spring structure builded as folded beams. It is concluded that the one with inner fold and discontinuous truss has smaller sensitivity under positive error, and also has the advantages of simple process and easily expanding under considering the characteristics of deep etching when used as a supporting structure of movable sensing mass block.2. The electromechanical coupling characteristics including effects of electrostatic stiffness and pull-in of macro model are analyzed. The generation condition and analytical expressions of electrostatic stiffness under open loop and closed loop, and the coupling characteristics of displacement stiffness in double elastic supporting structure are studied, respectively. It is concluded that the pull-in phenomenon of macro model occurs under step voltage when the ratio of stiffness is less than 1/4, and finally, the dependence of pull-in voltage on gap error in double capacitor model is analyzed, the theoretical results have the relative error less than 5% compared with simulating results.3. The electromechanical coupling characteristics and 'stiffen' effects and 'soften' effects of displacement stiffness in balanced-force micro accelerometer under linear feedback voltage is studied by combining with the closed loop stiffness of double capacitor model. The studies focus on the influence of the gap asymmetric error and circuit zero bias on the electromechanical coupling characteristics and zero bias of accelerometer output, which indicate that the influence of the former part is much lager than later, based on this conclusion, a prediction theory of average gap asymmetric error from process line is established by combining analytical method and reverse calibration, and the results are coincident with the experience value. The prediction technology can meet the requirements in practical engineering by apply the theoretical results of electromechanical coupling into engineering based on the application of the characteristics of mature device and processing.4. The theoretical analyzing method of combining electrostatic stiffness and resonant theory is proposed and a resonant accelerometer structure based on the change of electrostatic stiffness is designed according to the coupling theory between mechanical stiffness and electrostatic stiffness. The whole structure design process consist of theory of combining resonant principle and electrostatic stiffness, design of resonator with electrostatic stiffness and its frequency robustness, the analytic relation between input and output, design of performance parameters and determination of layout dimension. The condition of robustness of frequency is considered in determining the parameters of sensing structure by the combination theory of electrostatic stiffness and resonant theory, which makes the sensitivity of fabricating prototype close to the design value. The design performance is realized only by twice plate-making and processing, which not only proves the rightness of theory results, also clarifies the validity of theoretical design, reducing the dependence of performance on process and promoting the efficiency of structural design.5. The structure of sensor is fabricated by DRIE (Deep Reaction Ion Etching) technology and silicon bonding technology, and finally, the experiment on centrifuge is carried to verify the feasibility and validity of theories.The macro model method is selected in this paper to analyze the electromechanical coupling characteristics of MEMS parallel plate capacitor, which simplified the analyzing process of complicated systems, made the whole process of derivation more definitude and explicit, it is concluded that the coupling effects of mechanical field and electrostatic field can be used to cultivate new methods of MEMS design and analysis, such as ruboust design based on compensation of quantity in circuit, method of error prediction based on results of system tests and micro resonator based on electrostatic stiffness.