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首页 TWO-AXIS OPTICAL MEMS SCANNER

TWO-AXIS OPTICAL MEMS SCANNER.pdf

TWO-AXIS OPTICAL MEMS SCANNER

xiao_20141220
2010-08-27 0人阅读 举报 0 0 暂无简介

简介:本文档为《TWO-AXIS OPTICAL MEMS SCANNERpdf》,可适用于工程科技领域

TWOAXISOPTICALMEMSSCANNERShoryaAwtar∗,KlintARose∗∗,HoePhongTham∗∗∗∗PrecisionEngineeringResearchGroup,MassachusettsInstituteofTechnology,MA∗∗MechanicalEngineering,StanfordUniversity,CA,∗∗∗PrecisionEngineershoryamitedu,klintstanfordedu,thamhpalummiteduAbstractThispaperpresentsthedesignofatwoaxisopticalMEMSmirrorforuseinrasterscanningapplicationsThedesignisbasedonanovelresonantstructurethatprovidestwoorthogonalanddecoupledrotationsofasinglemirrorTranslationalerrorsofthemirrorareeliminatedbyusingtheprincipleofvirtualpivotincantileverbeamsThefirstfewresonancemodesandfrequenciesoftheproposedstructurearedeterminedusingalumpedmassanalysis,andvalidatedbymeansofcomputationalFEAAppropriatelyplacedPZTelectrodesareusedtoexcitethetwomodesofthestructurethatcorrespondtotherotationsrequiredforscanningAsimplefabricationprocessissuggestedformanufacturingthedeviceKeywords:TwoaxisScanner,DecoupledRotations,VirtualPivotIntroductionandBackgroundTwoaxisbeamsteeringandrasterscanningMEMSmirrorsarerequiredinabroadrangeofapplicationssuchasbiomedicalimaging,scannedbeamdisplaysandimagers,andbarcodereadersWhilesuccessivesingleaxismirrorshavebeenfrequentlyusedtosteerabeamalongmultipleaxes,amorecompactalternativeistouseasingletwoaxesmirrorAlthoughthisoptionsimplifiestheassembly,integrationandpacking,itoftenthisleadstoarelativelycomplexdevicedesignwithlimitedperformanceThebendingandtorsionofasinglecantileveredbladehasbeenemployedtoproducetwoorthogonalrotationsofamirrorfixedattheendoftheblade,thusgeneratingthetwoscanningmodesHowever,alongwithrotation,bendingalsoproducesbladetiptranslation,whichcanalterthealignmentofthelightbeambeingreflectedandaffecttheopticsperformanceFurthermore,thesamestructuralelementisresponsibleforboththeexcitationmodes,bendingandtorsion,thuscouplingthetwoOtherdesignsemploydoublegimbals,whichprovideperfectdecouplingbetweenthetwoscanningaxes,butalsoposeactuationchallengeswhenlargerotationsaredesiredTheexcitationforthesecondaxishastobewiredthroughthelivehingeofthefirstaxis,whichleadstofabricationandreliabilityconcernsGroundmountedactuationschemessuchaselectrostaticpadsrequireveryhighvoltagesforthedesiredmirrorsizeandrangeofrotationsDualaxisMEMSmirrorsthatemployDselfassembledstructuresposeobviouschallengesinfabricationandpackagingInthefollowingsections,wepresentanewtwoaxisMEMSscannerdesignthataddressessomeofthekeyperformanceandfabricationissuesAsimpledesignprovideshighperformancewhilekeepingthefabricationcomplexitylowSpecificationsforthescannerincludeadevicesizeofmmxmm,andamirrorsizeofmmxmmThetwoscanaxesaredecoupled,thescanfrequenciesarewidelyspaced(HzandkHz),andparasitictranslationalerrorsofthemirrorareminimizedThedesignallowslargerangesofrotations(°)whileconsuminglessthanmWpowerProposedDesignTheproposedstructure,illustratedinFig,comprisesofanintermediatestagesupportedbytwocantileverblades,anassemblythatiscompliantinbendingbutstiffintorsionThemirrorisattachedtotheintermediatestagebymeansofabarthatisstiffinbendingbutrelativelycompliantintorsionThus,onescanmodeisproducedbythebendingofthecantileversduringwhichtheintermediatestageandthemirrorbehavelikeasinglerigidbodyalongtheZaxisThesecondscanmodearisesduetothetwistingofthetorsionbarIfthetorsionalstiffnessofthecantileverbladesassemblyislargecomparedtothetorsionbarandtheinertiaoftheintermediatestageismuchgreaterthanthatofthemirror,thetwistingcanbelimitedtoprimarilythetorsionbarThedependenceofthesetwomodesondistinctstructuralelements,namelythecantileversandthetorsionbar,providesthenecessarydecouplingbetweenthetworotationalaxesFiga)DeviceConceptforaAxisScannerb)PrincipleofVirtualPivotThemirrorislocatedwithrespecttotheintermediatestagesuchthatitscenteraxiscoincideswiththevirtualcenterofrotationoftheintermediatestageattachedatthecantileverendThisarrangementeliminatesanytranslationofthemirrorcenter,asillustratedinFigThevirtualcenterofrotationoftheintermediatestagemaybedeterminedanalyticallyandremainsfixedaslongastheloaddisplacementbehaviorremainslinear,dampingissmall,andtheendloads–externalorinertial,varyproportionallyInadynamiccase,forexampleifthecantileverisvibratinginitsfirstnaturalmode,whiletheinertiapropertiesoftheintermediatestageandthefrequencyofvibrationwilldeterminethevirtualpivot,itslocationremainsfixedforsmalldisplacementsTheproposedstructuremaybedesignedsothatthedesiredbendingandtorsioncorrespondtotwodistinctresonancemodes,whichmaybeexclusivelyexcitedbyactuatingthestructureattheappropriatefrequenciesFurthermore,thebendingnaturalfrequencycanbeconsiderablyloweredbyselectingaheavyintermediatestageThisisanimportantadvantagebecauseitisdesirabletohavethetwoscanfrequenciesseparatedbyatleastadecadetoensureoptimalscanningThetorsionfrequencywillstillbehighbecauseofthelowmomentofinertiaofthemirrorascomparedtotheintermediatestageBecauseofclosetoresonanceoperation,largerangesofmotioncanbeobtainedevenforsmallactuationsignalsAfterevaluatingseveralactuationschemes,PZTelectrodesdepositedatthebaseofthetwocantileversarechosenbecauseoftheirrapidtimeresponse,andfabricationeaseandreliabilityinMEMSSystemDynamicsAnalysisFiga)BendingModelb)TorsionModelTheobjectiveofthisexerciseistoanalyticallypredicttheabovedescribedresonancemodesandfrequenciessothatthestructuremaybedesignedtomeetthedesiredscanningrequirementsToachievesimpleclosedformresults,bendingandtorsionofthestructurearedealtwithindependentlyInthebendingandtorsionmodels,illustratedinFig,themassofthespringelements,namelythecantileversandthetorsionbar,areneglectedincomparisontotheintermediatestageandmirrorForthebendingmodel,theintermediatestage,torsionbarandmirrorareassumedtobeasinglerigidbody,withitscenterofmasslocatedatCViscousdampingisseparatelyestimatedusingstandardresultsformotionofbarsandplatesthroughairb,b,b,andbarethedampingcoefficientsassociatedwiththeXtranslationoftheintermediatestage,Yrotationoftheassumedrigidbody,ZrotationoftheintermediateFixedBaseMirrorTorsionBarCantileverBladeIntermediateStageCantileverBladeVirtualPivotAxisXYZxθxθVirtualPivotFxFxxxθLLCLCOmIyykEIL�ΦΦItItKKstage,andZrotationofthemirror,respectivelyTheexcitationsproducedbythePZTelectrodesatthecantileverbaseappearasbladetipdisplacement,bendangle,andtwistangle,givenby,andinininoooxLcVcVcVθφ===,respectivelyUsingthenomenclatureofFigandapplyingNewton'sLawyieldsthefollowingsystemdynamics()ccocccccLLkLmIsmbIbsmkLLkIbbsbkLLbkLbksLLkLmLsbLs⎡⎤⎧⎫⎧⎫⎞⎞⎛⎛−−θ⎨⎬⎨⎬⎢⎥⎜⎟⎜⎟⎝⎝⎠⎠⎩⎭⎩⎭⎣⎦⎡⎤⎞⎞⎛⎛=−−⎜⎟⎜⎟⎢⎝⎝⎠⎠⎣inkLcV⎥⎦()occcccLLbLLkLLLkLImsbbLsmLsbLsx⎡⎤⎡⎤⎞⎞⎞⎞⎛⎛⎛⎛−−θ=−−⎜⎟⎜⎟⎜⎟⎜⎟⎢⎥⎢⎥⎝⎝⎝⎝⎠⎠⎠⎠⎣⎦⎣⎦()()()()()()ttttttinIIsIbIbsIKIKKbbsKbbKbsKKKcV⎡φ=⎤⎦⎣()Thecoefficientofθintheexpression()isthecharacteristicpolynomialofthebendingsystemmodelAnorderfourcharacteristicpolynomialreflectsthatthismodelcapturestwobendingmodesoftherealstructureThecenterofrotationforagivenexcitationfrequencymaybedeterminedfromexpression()Forthedesignedstructuregeometry,thevirtualpivotsarefoundtobelocatedatLandLfromthebaseforthetwobendingmodesThecoefficientofφinexpression()providesthecharacteristicpolynomialforthetorsionmodel,andpredictstwotorsionmodesoftherealstructureFigprovidesaBodeplotoftheabovetransferfunctions,andalsoatablethatcomparesthepredictedmodeswithFEMresultsThederivedmodelaccuratelypredictsfouroutofthefirstsevenmodesofthestructure,includingthetwothatareofinterest:simplebendingofthecantileversandpuretorsionofthetorsionbarModeDescriptionFEM(Hz)Analysis(Hz)SimpleBendingoftheCantileversSimpleTorsionoftheCantileversSecondaryBendingoftheCantileversInplaneBendingoftheTorsionBarInplaneBendingofCantileversandTorsionBarOutofplaneBendingoftheTorsionBarPureTorsionoftheTorsionBarFiga)AnalyticallyPredictedBendingBlueandTorsionRedModesb)ComparisonwithFEMFabricationTheproposedfabricationplanforthisdevicebeginswithanSOIwaferwithahandlewaferthicknessofµmseparatedbyµmofsilicondioxidefromtheµmtoplayerofsiliconThefinalthicknessofthescanningmirrorandcantileverbladesisµm,andthatofthetorsionbarisµm,whichdeterminesthethicknessofthesecondarysiliconontheSOIwaferThebacksideofthewaferismaskedandetcheduptotheoxidelayer,whichactsasanetchstop,tocreatetheintermediatestage(Figa)ThenextstepintheprocessisalargeareaµmDRIEetchonthefrontsideleavingbehindµmofsiliconprotrudingfromthesurfaceforthethickertorsionbar(Figb)ThisraisedfeaturemaycauseimagedistortionduringsubsequentphotolithographybecausethedistancebetweenthemaskandphotoresistwouldbeaminimumofµmThenetresultisanuncertaintyofapproximatelyµminthetorsionbarwidthofµm,whichisacceptableNext,anmsilicondioxidelayerisdepositedtoinsulateelectrodesforthePZTfromthesiliconsubstrateTheoxidelayerisdepositedusingCVDthenetchedinanRIEFrequency(Hz)a)BacksideDRIEEtchb)FrontsideEtchc)OxideinsulationandpatternPZTd)Patterntopelectrodes,leadsandcontactpadse)FinalDRIEetchreleasesmirrorandbladesFigKeyFabricationStepsWiththeinsulationinplace,alayerofphotoresistforaliftoffprocessisspunonandpatterned,leavingopenareasontheoxidefortheelectrodesAÅadhesionlayeroftitaniumisevaporatedontotheoxidefollowedbyÅofplatinumAfterstrippingtheresist,fourelectrodesremainwhichwillactasthegroundforvoltagesacrossthePZTThePZTpiezoelectricmaterialisthenspunonthesubstrateinasolgelformOnlythinlayersofPZT,nmatatime,canbeformedbecauseofthecondensationandannealingrequirementsofthematerialOncedeposited,thePZTismaskedwithphotoresistandpatternedusingawetetchtechnique(Figc)TheleftoverbacksideoxidehelpsreducepotentialundercuttingduringthisPZTetchBeforepatterningthetopelectrodesforthePZT,anotherinsulatinglayerofoxideisnecessarytoavoidshortingbetweenthetopandbottomelectrodesThisisperformedinthesamemannerasearlierThemaskforthetopelectrodesalsoincludestracestoammsquarecontactpadsforwirebondingtotheexternalelectronics(Figd)Toprovideareflectivesurfaceonthescannermirror,evaporatedaluminumispatternedontotheareausingaliftoffprocesswithachromeadhesionlayerThefinalstepisaDRIEetchtofreeupthemirrorandthecantilevers(Fige)ThePZTactuatorsmustbepoledbeforeuseinordertomaximizethestrainforagivenappliedvoltageConclusionandAcknowledgementMacroscaleandMEMSprototypesoftheproposeddevicearecurrentlybeingconsideredforfabricationandexperimentaltestingFutureanalyticalworkincludesestimationofvirtualpivotlocationerrorsthatmayariseduetoothermodesthatgetexcitedbythePZTactuatorsFeaturesthatallowrealtimefrequencytuningarebeinginvestigatedfordesignaugmentationFurtherinformationonthedesign,modeling,analysis,fabricationandpackagingofthisdevicecanbefoundinadetailedreportavailableatwebmitedushoryawwwThisresearchwasconductedaspartofacourseprojectforMEMSDesignatMITinSpring,andtheauthorswouldliketothankProfSteveSenturiaforhisguidanceReferencesDWWineetal,USPatent,,()HGoto,USPatent()YOhtuka,HNishikawa,TKoumura,THattori,IEEEProcMEMS'()GDJSu,HToshiyoshi,MCWu,IEEEPhotonicsTechnologyLetters,()–MHKiang,OSolgaard,KYLau,RSMuller,SensorsandActuators,A()AGonsalves,WTPlummer,citedinPrecisionMachineDesignbyAHSlocum()DLPolla,LFFrancis,AnnualReviewofMaterialsScience()LPWangetal,MatResSocSympProc()EEEE<<ASCIIEncodePagesfalseAllowTransparencyfalseAutoPositionEPSFilestrueAutoRotatePagesNoneBindingLeftCalGrayProfile(GrayGamma)CalRGBProfile(sRGBIEC)CalCMYKProfile(USWebCoatedSWOPv)sRGBProfile(sRGBIEC)CannotEmbedFontPolicyErrorCompatibilityLevelCompressObjectsTagsCompressPagestrueConvertImagesToIndexedtruePassThroughJPEGImagestrueCreateJDFFilefalseCreateJobTicketfalseDefaultRenderingIntentDefaultDetectBlendstrueColorConversionStrategyLeaveColorUnchangedDoThumbnailsfalseEmbedAllFontstrueEmbedJobOptionstrueDSCReportingLevelEmitDSCWarningsfalseEndPageImageMemoryLockDistillerParamsfalseMaxSubsetPctOptimizefalseOPMParseDSCCommentstrueParseDSCCommentsForDocInfotruePreserveCopyPagetruePreserveEPSInfotruePreserveHalftoneInfofalsePreserveOPICommentsfalsePreserveOverprintSettingstrueStartPageSubsetFontstrueTransferFunctionInfoApplyUCRandBGInfoPreserveUseProloguefalseColorSettingsFile(ColorManagementOff)AlwaysEmbedtrueNeverEmbedtrueAntiAliasColorImagesfalseDownsampleColorImagesfalseColorImageDownsampleTypeBicubicColorImageResolutionColorImageDepthColorImageDownsampleThresholdEncodeColorImagestrueColorImageFilterDCTEncodeAutoFilterColorImagestrueColorImageAutoFilterStrategyJPEGColorACSImageDict<<QFactorHSamplesVSamples>>ColorImageDict<<QFactorHSamplesVSamples>>JPEGColorACSImageDict<<TileWidthTileHeightQuality>>JPEGColorImageDict<<TileWidthTileHeightQuality>>AntiAliasGrayImagesfalseDownsampleGrayImagesfalseGrayImageDownsampleTypeBicubicGrayImageResolutionGrayImageDepthGrayImageDownsampleThresholdEncodeGrayImagestrueGrayImageFilterDCTEncodeAutoFilterGrayImagestrueGrayImageAutoFilterStrategyJPEGGrayACSImageDict<<QFactorHSamplesVSamples>>GrayImageDict<<QFactorHSamplesVSamples>>JPEGGrayACSImageDict<<TileWidthTileHeightQuality>>JPEGGrayImageDict<<TileWidthTileHeightQuality>>AntiAliasMonoImagesfalseDownsampleMonoImagesfalseMonoImageDownsampleTypeBicubicMonoImageResolutionMonoImageDepthMonoImageDownsampleThresholdEncodeMonoImagesfalseMonoImageFilterCCITTFaxEncodeMonoImageDict<<K>>AllowPSXObjectsfalsePDFXaCheckfalsePDFXCheckfalsePDFXCompliantPDFOnlyfalsePDFXNoTrimBoxErrortruePDFXTrimBoxToMediaBoxOffsetPDFXSetBleedBoxToMediaBoxtruePDFXBleedBoxToTrimBoxOffsetPDFXOutputIntentProfile(None)PDFXOutputCondition()PDFXRegistryName(http:wwwcolororg)PDFXTrappedUnknownDescription<<JPN<FEFFeadbafadecfeabcfbdeadecbeffcbdbffeeadbafcffffaeeedadeeadbabfdafcecbfbcfcfc>FRA<FEFFffefefdefeeefcfeecefecedfeeecaffcfeefcecfcfecefffefce>DEU<FEFFeeecceeadccefedfbdeedefeccfecdecffcbfaaceedfbdebfeeedffddeeffe

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