掃描電子顯微術(shù)：例子

掃描電子顯微術(shù)：例子TheeffectofAcceleratingVoltageonSEMImages30kV10kV5kV3kVSpecimen:Toner墨粉Whenhighacceleratingvoltageisusedasat(a),itishardtoobtainthecontrastofthespecimensurfacestructure.Besides,thespecimensurfaceiseasilychargedup.Thesurfacemicrostructuresareeasilyseenat(b).(a)30kVx2,500(b)5kVx2,500Specimen:EvaporatedAuparticles.Theimagesharpnessandresolutionarebetteratthehigheracceleratingvoltage,25kV.(a)5kVx36,000(b)25kVx36,000Specimen:Filterpaper.At5kV,themicrostructuresofthespecimensurfaceareclearlyseenasthepenetrationanddiffusionareaofincidentelectronsisshallow.(a)5kVx1,400(b)25kVx1,400Fig.6Specimen:Sinteredpowder.Atlowacceleratingvoltage,whilesurfacemicrostructurescanbeobserved,itisdifficulttoobtainsharpmicrographsathighmagnifications.Insuchacase,clearimagescanbeobtainedbyshorteningtheWDorreducingtheelectronprobediameter.(a)5kVx7,200(b)25kVx7,200Specimen:Paintcoat.Whenahighacceleratingvoltageisused,morescatteredelectronsareproducedfromtheconstituentsubstanceswithinthespecimen.Thisnotonlyeliminatesthecontrastofsurfacemicrostructures,butproducesadifferentcontrastduetobackscatteredelectronsfromthesubstanceswithinthespecimen.(a)5kVx2,200(b)25kVx2,200SE(secondaryelectron)imagingHighresolution(betterthan5nm)isobtainablewithmostSEM’sBetterthan2nmresolutionispossibleinsomecases10nmresolutionisveryroutine(unlessthesamplelimitstheresolution,asisoftenthecase)Edgeeffect(secondaryelectronemissiondifferingwithsurfacecondition).InfluenceofedgeeffectonimagequalityAmongthecontrastfactorsforsecondaryelectrons,thetilteffectandedgeeffectarebothduetothespecimensurfacemorphology.Secondaryelectronemissionfromthespecimensurfacedependslargelyontheprobe’sincidentangleonthespecimensurface,andthehighertheangle,thelargeremissioniscaused.TheobjectsoftheSEMgenerallyhaveunevensurfaces.Therearemanyslantsalloverthem,whichcontributemosttothecontrastofsecondaryelectronimages.Ontheotherhand,largequantitiesofsecondaryelectronsaregeneratedfromtheprotrusionsandthecircumferencesofobjectsonthespecimensurface,causingthemtoappearbrighterthanevenportions.SpecimenICchip.Thehighertheacceleratingvoltage,thegreateristheedgeeffect,makingtheedgesbrighter.InfluenceofedgeeffectonimagequalityThedegreeoftheedgeeffectdependsontheacceleratingvoltage.Namely,thelowertheacceleratingvoltage,thesmallerthepenetrationdepthofincidentelectronsintothespecimen.Thisreducesbrightedgeportions,thusresultinginthemicrostructurespresentinthembeingseenmoreclearly.Normally,secondaryelectronimagescontainsomebackscatteredelectronsignals.Therefore,ifthetiltdirectionofthespecimensurfaceandthepositionofthesecondaryelectrondetectoraregeometricallyinagreementwitheachother,morebackscatteredelectronsfromthetiltedportionsaremixed,causingthemtobeseenmorebrightlyduetosynergism.(a)5kVx720TiltAngle:50°(b)25kVx720TiltAngle:50°Specimen:ICchip.5kVx1,100Thesidesofpatternsareviewedbytiltingthespecimen.Theamountofsignalsisincreased.Useofspecimentilt：a)Dependenceofimagequalityontiltangle1)Improvingthequalityofsecondaryelectronimages;2)Obtaininginformationdifferentformthatobtainedwhenthespecimenisnottilted,thatis,observingtopographicfeaturesandobservingspecimensides.3)Obtainingstereomicrographs.Fig.13showsaphototakenatatiltangleof0°(a)andaphototakenat45°(b).Theircomparisonshowsthatthelatterisofsmoothqualityandstereoscopicascomparedwiththeformer.Whenthespecimenistilted,howeverlengthsobservedaredifferentfromtheiractualvalues.Whenmeasuringpatternwidths,etc.,therefore,itisnecessarytomeasurewithoutspecimentiltingortocorrectvaluesobtainedformatiltedstate.(a)Tiltangle:0°(b)Tiltangle:45°Specimen:Backsidesofoleasterleaves.Moreinformationisobtainedfromstereo-pairphotos.Useofspecimentilt：b)StereomicrographsWithSEMimagesitissometimesdifficulttocorrectlyjudgetheirtopographicalfeatures.InsuchacaseobservationofstereoSEMimagesmakesiteasytounderstandthestructureofthespecimen.Besides,stereoobservationallowsunexpectedinformationtobeobtainedevenfromspecimensofsimplestructure.Instereoobservation,afterafieldofinterestisphotographed,thesamefieldisphotographedagainwiththespecimentiltedfrom5oto15o.Viewingthesetwophotosusingstereoglasseswiththetiltingaxisheldverticallyprovidesastereoimage.Useofspecimentilt：c)DetectorpositionandspecimendirectionTheamountofsecondaryelectronsproducedwhenthespecimenisilluminatedwithanelectronbeam,dependsontheangleofincidencetheoretically.However,therearisesadifferenceintheimagebrightnessdependingonwhetherthetiltedsideofthespecimenisdirectedtothesecondaryelectrondetectorortheoppositeside.Withalongspecimen,forexample,thebrightnessdiffersbetweenthesidefacingthedetectorandtheoppositeside.Insuchacase,directingthelongitudinalaxisofthespecimentothedetectormakesthebrightnessuniform.Fig.16Specimen:FiberDetectorpositionandspecimendirection.Specimen:Fiber7kVx2,200Directingthelongitudinalaxisofthespecimentothesecondaryelectrondetectormakestherightandleftsidesequallybright.(AnSRTunitisusedtodirecttheimagelongitudinally.)Useofspecimentilt：c)Detectorpositionandspecimendirection(a)Specimendirectedasat1(b)Specimendirectedasat2(c)Specimendirectedasat3Backscatteredelectronsvaryintheiramountanddirectionwiththecomposition,surfacetopography,crystallinityandmagnetismofthespecimen.Thecontrastofabackscatteredelectronimagedependson(1)thebackscatteredelectrongenerationratethatdependsonthemeanatomicnumberofthespecimen,(2)angledependenceofbackscatteredelectronsatthespecimensurface,and(3)thechangeinthebackscatteredelectronintensitywhentheelectronprobesincidentangleuponacrystallinespecimenischanged.UseofbackscatteredelectronsignalsRetractableBSEdetectorinchamberObjectivelensSecondarydetector(ETD)BSEpathsanddetectorlocationsObjectivelensBSEfromadjacentregionsonflatsampleSEMCompositionalimageBackscatteredSEMimageofanPbSnalloyshowingcontrastbasedontheatomicnumber.ThebrighterareasarePb-rich.UseofbackscatteredelectronsignalsPrinciplesofcompositionimageandtopographyimageThebackscatteredelectronimagecontainstwotypesofinformation:oneonspecimencompositionandtheotheronspecimentopography.Toseparatethesetwotypesofinformation,apairedsemiconductordetectorisprovidedsymmetricallywithrespecttotheopticalaxis.Additionofthemgivesacompositionimagewhilesubtractiongivesatopographyimage.Andwithcompositionimagesofcrystallinespecimens,thedifferenceincrystalorientationcanbeobtainedastheso-called“channelingcontrast,”byutilizingtheadvantagethatthebackscatteredelectronintensitychangeslargelybeforeandafterBragg’scondition.SEversuBSEimagesofalloyObjectivelensCu/ZnAlloy,SE(left),BSE(right).

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