MEMS-in-plane-motion-vibration-measurement-system-

MEMSin-planemotion/vibrationmeasurementsystembasedCCDcameraD.Teyssieux⇑,S.Euphrasie,B.CretinInstituteFEMTO-ST,DepartmentMN2S,CNRSUMR6174,UniversityofFranche-Comté,ENSMM,32av.del’observatoire,F-25044BesançonCedex,FrancearticleinfoArticlehistory:Received11February2009Receivedinrevisedform11May2011Accepted21June2011Availableonline12July2011Keywords:In-planevibrationCCDMotionMeasurementabstractWereportonthedevelopmentofavibrometerforin-planemotionwhichisparticularlysuitableforMicroElectroMechanicalSystem(MEMS)samples.Thesystemcombinesaconventionalmicroscope,coherentelectronicsandaFullFrameCCDcamera.Stroboscopelightingallowsthesystemtofreezethemotion.Theobtainedimagescorrespondtodiffer-entphasesofthesamplemotion.Differentsubpixelmotionmeasurementalgorithmsarecomparedintermsofprecisionandcomputationtime.Analgorithmthatwespeciallydesignforthisapplicationprovedtobethebest.ItisbasedontheshiftFouriertheoremandusesfirstharmonics,wheremostenergyispresent.Thusthesystemallowscomputingthephaseandmagnitudeofthesampledisplacement.Withthesystemwecanobtainameasurementresolutionof100pmdemonstratedontheAFMcantilevervibrationsmea-surement.Theeffectoftheedgeroughnesswasstudied.ItdecreasestheperformanceofthealgorithmbutroughnessisverylowonmostMEMSapplications.ThismethodbasedonaCCDcamera,isverywellsuitedformeasuringin-planeMEMSvibrationsinceitdoesnotrequiresurfaceroughnessforscatteringthelightaswiththespecklemethods(forexample).Itcanquicklyobtainfullfieldmotion,withahighprecision.Furthermore,theusedalgorithmissimple,fastandverynoiseinsensitive.CrownCopyright2011PublishedbyElsevierLtd.Allrightsreserved.1.IntroductionIntheMicroElectroMechanicalSystem(MEMS)andMi-croOptoElectroMechanicalSystem(MOEMS)domains,themeasurementofmicrostructuremotionoverawidemeasurementrangeandwithananometerscaleprecisionisveryimportant.Numerousmeasurementsystemsallowtomeasureoutofplanedisplacements.Theyusuallyuseanopticalprobewhichcanbeverysensitive[1].Howeverthiskindofmethodrequiresascanningofthesampletohaveafullfieldmotionandcannotbeusedforin-planedisplacement.Thereareseveralsystemsforin-planemeasurementsthathavebeendeveloped.LaserDopplervibrometry(LDV)andelectronicspecklepatterninterfer-ometry(ESPI)[2]havebecomereferencetechniquesforthemeasurementofin-planedisplacementswithfullfieldanalyses.Neverthelesstheyarenotsuitedformicrodevices.IndeedtheLDVmethodisbasedonthebackscatteringlightandthespeckleisbasedontheroughnessofthestudysample.UsuallyMEMShaveaverylowroughness(smallerthanthewavelength)andthesurfacesdonothavesuitablepropertiesforthesetwomethods.Thusothervibrationmea-surementsystemsbasedonChargeCoupledDevices(CCD)andbasedondirectobjectobservationhavebeendeveloped[3].Manyofthesesystemsarebasedonstroboscopicillumi-nationofanobjectinharmonicmotion[4].Thesesystemscanbeverysensitiveandallowanin-planeprecisionof2.5nm[5]andupto800kHz[6].ThemainadvantageoftheCCDmethodisthatitcandirectlymeasurethe2Dmo-tionfieldwithaneasyimagecomputation.Wespeciallydesignedanalgorithmforthissystemtomeasurein-planeMEMSvibrations.Itsgoalwastobequickly,preciseandnoiseinsensitive.Thefirstsectionofthispaperdescribesthebasesofthemethod.Wereportonthecurrentdisplacementmeasure-mentalgorithmsusingintheimageprocessingdomain0263-2241/$-seefrontmatterCrownCopyright2011PublishedbyElsevierLtd.Allrightsreserved.doi:10.1016/j.measurement.2011.06.020⇑Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](D.Teyssieux).Measurement44(2011)2205–2216ContentslistsavailableatSciVerseScienceDirectMeasurementjournalhomepage:(AFM)cantilevermotion.Finally,westudytheprecisionofthemethodandtheeffectoftheedgeroughness.2.In-planemotionmeasurements:principleandalgorithms2.1.PrincipleofthemethodTheedgeluminosityofanobjectisvariablewiththemovementofthisobjectwhenthesensorisaCCD.Fig.1showsoneidealcasewhentheobjecthasmovedwithasubpixeldisplacement.ThearraysrepresentaCCDsensorwithafillfactorequalto100%(ThisprincipleisnotvalidforaCCDwithafillfactoroflessthan100%);thegreyzonesrepresenttheobjectandthecurvesAAandBB(Fig.1)representtheintensitiesofapixellineaccordingtothexaxis.TheidealcaseofFig.1isalinearmodelbetweenAAandBBvectors,whenthemovementoftheobjectisrigid(whichisnotthecaseforathermoelasticmovementforexample).Becauseofopticaldiffraction(duetotheRayleighlimitforin-stance)thechangebetweendarkandbrightareastakesplaceonseveralpixels.Onewaytorepresentthisiswiththisonedimensionequation:V1ðxÞ¼ð1DxÞV2ðxÞþDxV2ðx1Þð1ÞwhereDxisthedisplacementaccordingtox,V1isthevec-tor(onedimensionmatrix)correspondingtotheAAcurveandV2isthevectorcorrespondingtotheBBcurve.Thus,despiteadisplacementsmallerthanonepixel,thereexistsarelationbetweenthegreyleveloftheedgeoftheobjectandthedisplacementoftheobject(informationzoneonFig.1)