近代分析實(shí)驗(yàn)原理：第八課-掃描電鏡

八、掃描電鏡

（Scanningelectronmicroscopy）近代分析實(shí)驗(yàn)原理（Introductionofmodernanalyticalmethods）12卡爾·蔡司SUPRA55SAPPHIREFEIInspectF503SolidStateCommunications151,624(2011)4ZnO納米線561.Instrumentation71.1OpticalArrangementeitherthermionicorfieldemissiontypegunshighbeambrightness,andbetterimagingqualityAccelerationvoltage1–40kV,oneordersmallerthanTEMreducethecrossoverdiameteroftheelectronbeamDemagnifies縮小thecross-sectionoftheelectronbeambeamdeflection偏轉(zhuǎn)systemTheSEMlenssystemdemagnifiestheelectronbeambyabout10,000×forathermionicsourceand10–100×forafieldemissionsource.8Magnification:theratioofthelinearsizeofthedisplayscreentothelinearsizeofthespecimenareabeingscanned.fromabout20×togreaterthan100,000×Thethree-dimensionalappearanceofitsimagesbecauseofitslargedepthoffield.lightmicroscopeSEMTheresolutionofanSEMiscontrolledbythesizeoftheelectronprobe探針scanningthespecimen.9Signalsgeneratedwhenahigh-energybeamofelectronsinteractswithathinspecimen.1.2.SignalDetection3–5eVtopographic地形contrastElasticscatteringelementalcompositioncontrastincidentelectronsscatteredbyatomsinthespecimenelectronsejectedfromatomsinthespecimenretain60–80%oftheenergy1011SignalcollectionbytheEverhart–ThornleydetectorB,backscatteredelectrontrajectory;SE,secondaryelectrontrajectory;F,Faradaycage;S,scintillator;LG,lightguide;PM,photomultipliertube.positivelyornegativelycharged(250Vor?50V)Apositivecharge,thedetectorattractsparticularsecondaryelectrons.Anegativecharge,itcanscreenoutsecondaryelectronswithenergylessthan50eV.signalgain(upto～106×)Detectoradiskofabout8–20mmindiametertheEverhart–Thornleydetector121.3.ProbeSizeTheresolutionofSEMimagingprobecurrentconvergence收斂angleoftheprobebeambrightness,bytheelectronsourceafieldemissiongunis1000×brighterthanatungstenthermionicgunand100×brighterthanaLaB6thermionicgun.alargeαflikelyintroducesotheropticalproblems,particularlysphericalaberration.13Brightness:constantcloseto1sphericalaberrationcoefficientthewavelengthoftheelectronsWithanoptimized優(yōu)化αfneglectingchromaticaberration14Probesizeisdeterminedbythetypeoftheelectrongunandprobecurrent.15Signalsandbackgroundnoiseproducedwhenscanningaspecimen.fluctuationoftheelectronbeamcurrentandsignalamplificationinthedetectorRosevisibilitycriterion能見度標(biāo)準(zhǔn)::theminimumrequirementofprobecurrent:thedwelltimeofprobeproportionalityfactorbetweenipandiscontrastcurrentofsignalelectronsthechangeinsignalnoiselevel16Insummary,theresolutionofanSEMisdeterminedbytheprobesizeandtheprobecurrent.Theimportantpointstonotearethattheminimumprobesizeandthemaximumprobecurrentcannotbeobtainedsimultaneously,andacompromise妥協(xié)betweentheprobesizeandtheprobecurrentmustbemadewhenwetrytoobtainahighresolutionimageinSEM.172.ContrastFormation反差形成2.1Electron–SpecimenInteractionsMonteCarloelectrontrajectorysimulationofanelectronbeaminteractionwithironEo=20keV.contrastTopographic地形Compositional(Secondaryelectrons)(backscatteredelectrons)Theinteractionbetweenelectronsandspecimenatomsoccurswithinacertainvolumeunderthespecimensurface.18Theinteractionzoneofelectronsandspecimenatomsbelowaspecimensurface.betterspatialresolutionBothSEsandBSEsgeneratedbyscatteringareusedassignalsourcesforformingSEMimages.However,SEsandBSEs,whicharecollectedbyadetector,escapefromdifferentlocationsinthespecimen.Itssizeincreaseswiththeenergyofincidentelectronsintheprobe.chemicalanalysis19202.2Topographiccontrasttrajectoryeffecthowthespecimensurfaceisorientedwithrespecttothedetectorelectronnumbereffecttrajectory軌跡effectVariationingeometricfeaturesonthespecimensurfaceGenerationoftopographiccontrast:(a)thetrajectoryeffect,whicharisesfromtheorientationofsurfacewithrespecttothedetectorinanSEM,issimilarto:(b)reflectedlighteffectsfromtheorientationofsurfacewithrespecttothelightsourceinalightmicroscope.Secondaryelectronimageofafracturesurfacealonggrainboundaries.Secondaryelectron21electronnumbereffectmoreelectronscanescapefromthespecimensurfaceatanangle.Electronnumbereffectsduetosurfacetopography.Moresecondaryelectronscanescapefromtheedgesoftopographicalfeaturesthanfromaflatsurface.Secondaryelectronimageofacrackedsiliconsurface.Electronnumbereffectsmaketheedgesofsurfacecracksbrighterthantherestofthesurface.222.3.Compositional成分的ContrastthevariationingraylevelsinanSEMimagethatcorrespondtovariationinchemicalcompositioninaspecimen.thecapabilityofBSEstoescapefromthespecimendependsontheatomicnumbersofthespecimenatoms.Thebackscattercoefficient(η)characterizessuchcapability.Backscattercoefficientasafunctionofatomicnumberofspecimenatoms.23Comparisonbetween:(a)asecondaryelectronimage;and(b)abackscatteredelectronimageforthesameareaofnickelalloy.Additionalcompositionalinformationisobtainedfromthebackscatteredimage.anareawithatomsofhigheratomicnumberswillappearbrighter2425Inadditionaltotopographicandcompositionalcontrast,thereareothertypesofcontrastintheSEM,inparticular,crystallographiccontrastandmagneticcontrast.Crystallographiccontrastisalsocalledthecontrastofelectronchanneling.電子通道Itarisesbecausethebackscatteredelectroncoefficientisdependentontheorientationofcrystallographicplaneswithrespecttothedirectionoftheelectronbeam.Theplaneorientationaffectsthepenetration滲透oftheelectronbeamintothecrystalbecausetheatomicpackingdensityvarieswithcrystallographicplane.Forhigherpenetration,thebackscatteringcoefficientislower.Alargesinglecrystalsurfacemayexhibitanelectronchannelingpatternthatrevealsthecrystalorientationofaspecimensurfacelayer(～50nm).TheelectronchannelingimageinanSEMisverysimilartotheKikuchipatterninaTEM.Themagneticdomainsofferromagneticmaterials鐵磁材料mayalsoexhibitspecialcontrastinSEM.263.OperationalVariablesobtainagoodSEMimage3.1WorkingDistanceandApertureSizeresolutionDepthoffieldthepixelsizeofthescreen100mmagnificationapertureradiusworkingdistance2728Increasethedepthoffieldreducestheprobecurrent,andalsomayreducetheratioofsignaltonoise.Increasethedepthoffielddecreaseαfiftheaperturesizeisunchanged.decreasetheresolutionintermediateaperturesizeandintermediateworkingdistanceAsmallaperturesize29Probesize:3.2AccelerationVoltageV0dminBrightnessComparisonoftheeffectofaccelerationvoltageontopographiccontrastofacidetchedtitaniumspecimen.(a)20kV,and(b)5kV.theinteractionzoneinlateraldirectionsLateralspatialresolutionmoresurfacedetails303.3Astigmatism(象散)Effectsofastigmatism:(a)blurredastigmaticimage;and(b)sharpimageafterastigmatismhasbeencorrected.Astigmatismisthelensaberrationresultingfrompowerdifferencesofalensinitslensplaneperpendiculartotheopticalpath314.SpecimenPreparationSEMmicrographofAlonTiN.Thebeamwasfirstfocusedonthecentralregionforabout30seconds,andthenthemagnificationwasreduced.Contaminationbuiltupinthecentralregionisvisibleatreducedmagnification.sizingthespecimenstofitaSEMspecimenholderremovingsurfacecontaminants污染物hydrocarbonsfromoilandgreasedecomposesanyhydrocarbonandleavesadepositofcarbononthesurfaceanartifactofadarkrectangularmark4.1PreparationforTopographicExamination32electricallynonconductivesurfaces電絕緣的表面PreventthesurfacechargingImageartifactsduetosurfacecharging:(a)grosschargingeffectsonTeflonTM;(b)negativechargingofnonconductiveparticlesonanAlsubstrate,whichinducespositivechargeonthesubstrateasshownbydarkcontrast;(c)chargingcausesunstablesignalsasshownindiscontinuitieswhenscanningacalcitecrystal.33Chargi