Characteristic Study On A MEMS Torsional Micro-actuator With Electrostatic Actuation

Many micro-electro-mechanical systems (MEMS) devices and components,including sensors and actuators, have been developed as this technology is maturingrapidly. Several actuation methods have been presented, such as electromagnetic,electrostatic, piezoelectric and electrothermal. However, electrostatic actuation is themost widely utilized method for the design of MEMS. In this thesis the characteristicsanalysis and study are investigated for a MEMS torsional micro-actuator with electrostaticactuation. Based on the micro-mirror model, the static characteristics, dynamiccharacteristics and measurement method are proposed, which are developed that allowedto give insight into the mechanism and performance for micro-actuator.First, the analysis model of the torsional micro-actuator is proposed. Modal analysiswas performed using the finite element modeling (FEM) method. For studying themovement of the torsional micro-mirror, the fundamental and fourth modes only need tobe considered, which are the movement modes for twisting and bending. Furthermore, theanalysis methods are given for the two modes, and the comparison results are discussedunder the actuating voltage. Therefore the torsion beam twist but not bend when themirror moves down. This is also reason why a design using a micro-mirror based ontorsional movement can reduce the actuating voltage considerably compared to a designbased on linear motion. The above conclusion is the foundation for the further researchwork on the micro-actuator.When an actuating voltage is applied between the cantilever beam and the lowerelectrode, an electrostatic force results, however, for many MEMS devices there is theelectrostatic pull-in phenomena, which is basically due to the nonlinear characteristics ofelectrostatic forces. These are always attractive as they depend on the square of theapplied voltage, and the inverse of the distance between the electrodes squared. Themechanical restoring force of the microbeam cannot keep up with the electrostatic forcefor higher actuating voltages, which can lead to the upper electrode collapsing into thelower electrode;the so-called 'pull-in' phenomenon. For the micro-actuator modal of thenormal structure, the method and relative formulae are presented for analyzing theelectrostatic pull-in phenomena, the "stable domain" and "unstable domain" of the deviceare discussed. For studying the static characteristics of the pull-in phenomena, the inertiaand damping can be neglected. The cantilever beam is driven by the actions of theelectrostatic actuation and elastic restoring force. We can get that the pull-in deflectiveangle is 44.04% of the maximal deflective angle, and the pull-in angle is independent ofthe pull-in voltage or the spring constant, which is a constant of the structure, and this isdifferent for the pull-in voltage. Then we obtain the analytical method of optimization anddesign for the device by discussing the relationship between the geometrical structuralparameters and pull-in voltage. Furthermore, we have developed a computer aided designsoftware on the micro-actuator, which can give the simulation and analysis on actuatingvoltage, switching time, gravity distortion and intensity condition etc for the differentstructures, and these results are instructive and helpful for design, analysis and fabricationof the micro-actuator.Due to the dimensional constraints of MEMS devices, the air damping is animportant parameter for their performance, design and control. In this thesis, somepossible damping effects for the micro-actuator are investigated, the mechanism andstudy method are described for the couette flow damping and squeeze film damping.Furthermore, the theory of torsion dynamics and the solution of the dynamic equation arepresented. The analysis of the dynamic pull-in phenomena additionally has to take intoaccount the inertial and damping effects and the influence of external acceleration, whichmay significantly change the pull-in voltage threshold. Without considering the airsqueeze film damping, the pull-in voltage is smaller and the pull-in angle is larger thanthe static analytical results in Switch-Off process;and the cantilever beam can oscillateperiodically in Switch-On process when the mechanical loss is neglected. Withconsidering the air squeeze film damping, the analytical results of pull-in phenomena aresimilar to the static analytical results in Switch-Off process;the cantilever beam can stopat the end of the movement in Switch-On process. At the same time, the switching timehas an obvious increase. Therefore, the air squeeze film damping plays a very importantrole on the performance of the micro-actuator.Finally, the basal techniques of the bulk micromaching are introduced, and this kindof micro-actuator is fabricated in terms of the optimized structural parameters.Furthermore, an optical technique based on CCD camera is described for the measurement ofthe actuating voltage and switching time of the device. The results obtained bymeasurements on the micro-mirror are discussed, and it is shown that the experimentalresults in the air environment are in good agreement with the theoretical analysis of theswitching time with considering the air damping. In the end part of this dissertation, wepropose some idea about the further theoretical and experimental research work.Some novelties of this thesis are as follows:1. The analyzed modal method for the torsional MEMS electrostaticmicro-actuator. We obtain the possible movement modes by the finite element modeling(FEM). The analysis methods are given for the two modes, and the comparison results arediscussed under the actuating voltage. Therefore the torsion beam twist but not bendwhen the mirror moves down. This method is not very complicated, but clear, graspableand reasonable, and we can give insight into the characteristic of the micro-actuator.2. Compared analysis of the static and dynamic pull-in phenomena. In this thesis,presented. The analysis of the dynamic pull-in phenomena additionally has to take intoaccount the inertial and damping effects and the influence of external acceleration, whichmay significantly change the pull-in voltage threshold. Without considering the airsqueeze film damping, the pull-in voltage is smaller and the pull-in angle is larger thanthe static analytical results in Switch-Off process;and the cantilever beam can oscillateperiodically in Switch-On process when the mechanical loss is neglected. Withconsidering the air squeeze film damping, the analytical results of pull-in phenomena aresimilar to the static analytical results in Switch-Off process;the cantilever beam can stopat the end of the movement in Switch-On process. At the same time, the switching timehas an obvious increase. Therefore, the air squeeze film damping plays a very importantrole on the performance of the micro-actuator.Finally, the basal techniques of the bulk micromaching are introduced, and this kindof micro-actuator is fabricated in terms of the optimized structural parameters.Furthermore, an optical technique based on CCD camera is described for the measurement ofthe actuating voltage and switching time of the device. The results obtained bymeasurements on the micro-mirror are discussed, and it is shown that the experimentalresults in the air environment are in good agreement with the theoretical analysis of theswitching time with considering the air damping. In the end part of this dissertation, wepropose some idea about the further theoretical and experimental research work.Some novelties of this thesis are as follows:1. The analyzed modal method for the torsional MEMS electrostaticmicro-actuator. We obtain the possible movement modes by the finite element modeling(FEM). The analysis methods are given for the two modes, and the comparison results arediscussed under the actuating voltage. Therefore the torsion beam twist but not bendwhen the mirror moves down. This method is not very complicated, but clear, graspableand reasonable, and we can give insight into the characteristic of the micro-actuator.2. Compared analysis of the static and dynamic pull-in phenomena. In this thesis,...