Bruker公司原子力显微镜(AFM)工作原理及发展历史

5/7/2012History,PrincipleandWorkingModesofScanningProbeMicroscopeSunHao,2012.4.23OutlineBasicConceptsandHardwareofSPMTopographyImagingSurfaceModificationPropertiesMappingBasicConceptsandHardwareofSPMTwoImagingMethods“Seeing”MethodWavebased“Touching”MethodProbeBasedComparisonofTwoImagingMethodsWaveBasedProbeBasedResolutionSizeofProbeImagingSpeedFastSlow~MediumImagingDepthPenetrationLimitSurfaceSensitivityNormallyHighMediumVersatilityLimitedMoreModesUserFriendlinessVeryGoodBeingImprovedComparisonofAFMwithOtherImagingModalitiesOpticalMicroscopeSEMAFMOperatingEnvironmentAmbient,Liquid,VacuumVacuumAmbient,Liquid,VacuumResolutioninXY~500nm5nm1~10nmResolutioninZN/AN/A0.1nmSampleRequirementNotcompletelytransparent,refractiveindexdifferentfromenvironmentConductivesurface,vacuumcompatibleVariationindepth10μmG．BinningandH．RohrerinventedthefirstScanningTunnelingMicroscope.1986NobelPrizeSTMisthefirstinstrumentthatcanreflectinformationofmaterialsurfaceinatomicscale.TheFirstSPMintheWorldSTMisbasedonthefactthatthetunnelingcurrentbetweenaconductivetipandsampleisexponentiallydependentontheirseparation.Thiscanberepresentedbytheequation:I~Ve-cdThistechniqueistypicallylimitedtoconductiveandsemiconductingsurfacesScanningTunnelingMicroscopeDemo:STMDetectorSignalAFMSystemDemo:AFMIfthereisasharpenoughandunique(singlevalued)dependenceP=P(z)ofthatparameteronthetip-sampledistance,thenPcanbeusedinthefeedbacksystem(FS)thatcontrolthedistancebetweenthetipandthesample.STMSFMBasicWorkingPrincipleP=P(z)WorkingModeTunnelingCurrentiScanningTunnelingMicroscope(STM)CantileverAmplitudeATappingModeAFMCantileverDeflectionDContactModeAFMCantileverTRAmplitudeAtTorsionalResonanceMode(TRmode)AFMTip-SampleForceFPeakForceTappingAFMScanningProbeMicroscopyEFMKFMSCMSSRMTUNAc-AFMForceSpectroscopyMFMSThMScanningProbeMicroscopyBasicComponentsofSPM-MultiMode8MultiMode8SPMSystemComponentsNanoscopeVControllerandMultiMode8BaseNanoscopeVControllerinsideMultiMode8HeadMultiModeSPMHeadandMajorComponentsQuadPhotodetectorArrangementMultiMode8ScannersVoltagesAppliedtotheX-andY-axesProduceaRasterScanPatternCantileverHoldersVariousCantileverholdersfortheMultiModeSPMBasicComponentsofSPM-DimensionIconSPMInputandDisplayEquipmentComputerControllersNanoscopeVControllerDimensionStageControllerOpticsandMotorStageSystemDimensionSPMHeadTheopticalsystemisalignedsothatthebeamemittedbyadiode-laserisfocusedonthecantilever,andthereflectedbeamhitsthecenterofaphotodetector.Four-sectionsplitphotodiodesareusedasposition-sensitivephotodetectors(PSPD).Position-SensitivePhotodetectorsTheΔIZvalueisusedasaninputparameterinafeedbackloopoftheatomicforcemicroscope.Sum,VerticalandHorizontalPiezoceramicPlateinanExternalElectricFieldTheprobemicroscopescannersaremadeofpiezoelectricmaterials.Piezoelectricmaterialschangetheirsizesinanexternalelectricfield.Thepiezoceramicsispolarizedpolycrystallinematerialobtainedbypowdersinteringfromcrystalferroelectrics.Advantage:Allowingobtaininglargeenoughmovementswithrathersmallcontrolvoltages.Therelativelongitudinaldeformationundertheinfluenceofaradialelectricfieldcanbewrittenas:wherel0isthelengthoftheunstressedtube.Theabsolutelengtheningofthepiezo-tubeis:wherehisthethicknessofthetubewall,Visthepotentialdifferencebetweeninternalandexternalelectrodes.Thus,forthesameappliedvoltage,thetubelengtheningwillbelarger,forlongerandthinnertubes.TubularPiezoelementX,YDirections:Whendifferential-modevoltageisappliedonoppositesectionsoftheexternalelectrode(withrespecttotheinternalelectrode)partofthetubereducesinlengthandincreases(wherefieldandpolarizationdirectionsareopposite).Thisleadstoabendofthetube.ZDirection:ChangeoftheinternalelectrodepotentialwithrespecttoallexternalsectionsresultsinlengtheningorreductionofthetubealongZaxis.TubularPiezo-scannerGenerally(especiallyatlargecontrolfields)thepiezoceramicsarecharacterizedbynonlineardependenceofthedeformationonthefield.Thus,deformationofpiezoceramicsisacomplexfunctionoftheappliedelectricfield:Forsmallcontrolfieldsthegivendependencecanberepresentedinthefollowingway:TypicalvaluesoffieldsE*,atwhichnonlineareffectscannotbeneglected,areabout100V/mm.PiezoceramicsNonlinearityPiezoceramicscreepisadelayintheresponsetosuddenchangeofthecontrolelectricfieldvalue.ThecreepresultsinappearanceofgeometricaldistortionsinSPMimages.Specificallystronginfluenceofthecreepoccurs,oninitialstagesofthescanningprocess,orafteralargedisplacementofthestartingpointofthescannedarea.PiezoceramicsCreepPiezoceramicshysteresisisthatthepiezoceramicdeformationdependsonthesignofpreviouslyappliedelectricfield.ToavoiddistortionsintheSPMimagescausedbypiezoceramicshysteresis,informationisstored,inasamplescanning,onlywhiletracingoneoftheloopbranchesΔZ=f(V).PiezoceramicsHysteresisPiezoceramicsHysteresisThesensitivityofpiezoelectricmaterialsdecreasesexponentiallywithoperationtime.Thiscausesmostofthechangeinthesensitivitytooccuratthebeginningofascanner'slife.Scannersarerunapproximately48hoursbeforetheyareshippedfromthefactorytogetthescannerpastthepointwherethesensitivity