透射电子显微镜结构及工作原理

Chapter2

TransmissionElectronMicroscope(TEM)TEMElectronopticssystemVacuumsystemPowerSupplyandcontrolsystem(1)illuminationsystem(2)imagingsystem(3)imageviewingandrecordingStructureofTEMIlluminationsystemImagingsystemImageviewingandrecording2.1TheilluminationsystemCompriseselectrongun(provideselectronsource)condenserlenses(controltheelectronbeam)2.1.1Electrongunprovidessourceofelectronstoilluminatethespecimen.Therearetwotypesofelectronsources:thermionicsourcetungstenfilamentslanthanumhexaboride(LaB6)crystalsfield-emissionsource(FEG)finetungstenneedlesThermionicEmissionIfanymaterialtobeheatedtoahighenoughtemperature,theelectronsgainssufficientenergytoovercomethenaturalbarrier(workfunction)thatpreventsthemfromleakingouttoescapefromthesource.TwothermionicsourcesusedinpracticearetungstenandLaB6.WfilamentLaB6crystalThermionicgunAhighvoltageisplacedbetweenthefilament(actingasacathode)andtheanode,modifiedbyapotentialontheWehneltwhichactstofocustheelectronsintoacrossover,withdiameterd0andconvergence/divergenceanglea0.FieldEmissionTheprinciplebehindfieldemissionisthatthestrengthofanelectricfieldEisconsiderableincreasedatsharppoints,becauseifwehaveavoltageVappliedtoa(spherical)pointofradiusrthenE=V/r.Oneoftheeasiestmaterialstoproducewithafinetipistungstenwire.Toallowfieldemission,thesurfaceshouldbefreeofcontaminantsandoxide.ThiscanbeachievedbyoperatinginUHV(betterthan10-11Torr)AnFEGtip(fineWneedle)FieldEmissionGun(FEG)InordertogetanFEGtowork,wemakeitthecathodewithrespecttotwoanodes.AfinecrossoverisformedbytwoanodesactingasanelectrostaticlensIdealelectronsourcehighbrightness(highcurrentdensity)bettercoherency(smallenergyspread)smallchromaticaberrationgoodformodernTEMworkgoodstabilitylonglifetimeCharacteristicsofthethreesourcesoperatingat100kVComparisonofelectronsourcesTungstensourcetheworstinmostrespects,butforroutineTEMapplicationstheyareexcellent,reliablesourcesandarecheapandeasilyreplaceable.LaB6highbrightness,improvedcoherencyandtheenergyspread,increasedoperatinglifeisarecommendedthermionicsource,forallaspectsofTEM,butparticularlyAEMexpensive(severalhundreddollarseach)ComparisonofelectronsourcesFEGForallapplicationsthatrequireabright,coherentsourcetheFEGisthebest.(ThisisthecaseforAEM,HRTEM)ForroutineTEM,anFEGisfarfromidealbecausethesourcesizeissosmall.Itisnotpossibletoilluminatelargeareasofthespecimenwithoutlosingcurrentdensity,andthereforeintensityonthescreen.needUHV,veryexpensive(>US$10,000)2.1.2CondenselensTheilluminationsystemconsistsoftwo(three)condenselenses

Thefirstcondenserlens(C1,oftencontrolledviaaknobwhichislabeledspotsize),setsthedemagnificationoftheguncrossover.Thesecondcondenserlens(C2,oftencontrolledviaaknobwhichislabeledintensity)providesdirectcontrolofthespotsizeatthespecimen,anddirectcontroloftheconvergenceangle.TheilluminationsystemTwodifferentwaystousetheilluminationsystemformaparallelbeam(usedforTEMimaginganddiffraction)formaconvergentbeam(usedforSTEMimaging,microanalysis,andmicrodiffraction)ParallelBeammodeParallelilluminationisessentialtogetthesharpestdiffractionpatternsaswellasthebestimagecontrast.InthetraditionalTEMmodethefirsttwocondenserlenses(C1,C2)areadjustedtoilluminatethespecimenwithaparallelbeamofelectronstypicallyseveralmicrometersacross.Intheparallel-beammodeusuallynoneedtochangeC1.adjusttheC2lenstoproduceanunderfocusedimageoftheC1crossover.focusunderfocusoverfocusConvergent-beammode(FEG-TEM)

ForFEG-TEM,afocusedC2lensilluminatesasmallareaofthespecimenwithanonparallelbeam.(CrossoverofFEGis<10nm)

UnlessyouhaveanFEG,itisn'tpossibletousejusttheC1andC2lensesasintheFigureshownheretoconvergethebeamtoassmallaprobeasyouwouldlike(<10nm).ThisisbecausetheC1andC2lensescan’tdemagnifythecrossoverofthermionicgun(W:50mm,LaB6:10mm)sufficiently.Convergent-beammode(non-FEG)Theusualsolutionistoconverttheupperpolepeiceoftheobjectivelensintoathirdcondenserlens,whichiscalledacondenser-objectivelensC3.WemaketheC3muchstrongerthanusualandweakenC2orturnitoff,asshownintheFigure,togettheconvergent-beam.2.2TheImagingsystemComprisesmainlyobjectivelensintermediatelensprojectorlensFunctionsUsesobjectivelenstoformtheimageortheelectrondiffractionpatternUsesintermediatelensandprojectorlenstomagnifytheimageorthediffractionpatternproducedbytheobjectivelensandtofocusthemontheviewingscreen.TheImagingsystemSpecimenObjectivelensObjectiveapertureIntermediatelensProjectorlensScreenSADapertureTheImagingsystemiscomposedofObjectivelensIntermediatelensProjectorlensObjectiveapertureSADaperture(Thetwoaperturesareneverusedinthesametime)TheImagingsystemthemostimportantlensinTEMformsthefirstintermediateimageanddiffractionpattern(diffractionpatternisinevitablyformedinthebackfocalplaneofthelens)magnificationoftheobjectivelensMo:100-200ObjectivelensSpecimenBackFocalPlaneImagePlaneObjectivelensIntermediatelensmagnifythefirstimageanddiffractionpatternformedbytheobjectivelensandprojectittotheobjectiveplaneoftheprojectorlenscontroltogetimageordiffractionpatterncontrolthetotalmagnificationofTEMalenswithavariablemagnification(MI=0-20)ScreenProjectorlensIntermediatelensObjectivelensProjectorlensmagnifythesecondimageanddiffractionpatternformedbytheintermediatelensandprojectittothefluorescentscreenmagnificationoftheprojectorlensMP~100ScreenProjectorlensIntermediatelensObjectivelensThetotalmagnificationoftheTEM

Mo:magnificationoftheobjectivelens(fixed)MI:magnificationoftheintermediatelens(variable)Mp:magnificationoftheprojectorlens.(fixed)TotalmagnificationiscontrolledbythemagnificationofintermediatelensAperture/diaphragmTheapertureisacircularholeinmetaldiskandthediskismadeofeitherPtorMo.Themetalsurroundingtheapertureiscalledthediaphragm.Aperture/diaphragmisnormallysimplycalledaperture.Thediameteroftheapertureisintherangeof10-50m.Aperture/diaphragmWeusetheaperturetoallowcertainelectronstopassthroughthelensandexcludeothersbycausingthemtohitthesurroundingdiaphragm,i.e.,limitsthecollectionangleofthelens.Objectiveaperturetocontroltheimagecontrastsmallaperture,highcontrastlargeaperture,lowcontrasttheresolutionoftheimageformedbythelensthecollectionangleoftheEELStheangularresolutionofthediffractionpatternImagemode/diffractionmodeWeneedtoviewimageordiffractionpatternusingTEM.Thisisachievedbyadjustingtheintermediatelens.Theintermediatelenscanbeswitchedbetweentwosettings:theimagemodethediffractionmodeImagemodeIntheimagemode,youadjusttheintermediatelenssothatitsobjectplaneistheimageplaneoftheobjectivelens.Thenanimageisprojectedontotheviewingscreen.Howtoseediffractionpatter?ImageonScreenProjectorlensIntermediatelensObjectivelensDiffractionmodeInthediffractionmode,youhavetoadjusttheintermediatelenssothatitsobjectplaneisthebackfocalplaneoftheobjectivelens.Thenthediffractionpatternisprojectedontotheviewingscreen.DiffractionpatternonscreenProjectorlensIntermediatelensObjectivelensSelectedAreaDiffraction(SAD)Asyouseefromthefigureintheaboveslide,thediffractionpatterncontainselectronsfromthewholeareaofthespecimenthatweilluminatewiththebeam.Suchapatternisnotveryusefulbecause(1)thespecimenwilloftenbebuckled.(2)Thedirectbeamisoftentoointensetodamagetheviewingscreen.SoweperformabasicTEMoperationbothtoselectaspecificareaofthespecimentocontributetothediffractionpatternandtoreducetheintensityofthepatternfallingonthescreen.SelectedAreaDiffraction(SAD)TherearetwowayswecouldreducetheilluminatedareaofthespecimencontributingtothediffractionpatternWecouldmakethebeamsmallerbyconvergethebeamatthespecimentoformCBED(ConvergentBeamElectronDiffraction)pattern.Convergingthebeamdestroysanycoherence,andspotsinthepatternarenotsharplydefinedbutspreadintodisks.Method1EDCBEDSelectedAreaDiffraction(SAD)Ifwewishtoobtainadiffractionpatternwithaparallelbeamofelectrons,thestandardwayistoinsertanapertureabovethespecimenwhichwouldonlypermitelectronsthatpassthroughittohitthespecimen.Thisoperationiscalledselected-areadiffraction(SAD).Method2apertureVirtualaperturespecimenIfweinsertanapertureinaplaneconjugatewiththespecimen,i.e.,inoneoftheimageplanes,thenitcreatesavirtualapertureattheplaneofthespecimen.Thisisexactlyselectedareadiffractiondoes.Butwecan’tinsertanapertureatthespecimenplane,becausethespecimenisalreadythere!SelectedAreaDiffraction(SAD)

Theconjugateplanewechooseistheimageplaneoftheobjectivelens,asshowninthefigure.DiffractionpatternonscreenProjectorlensIntermediatelensSelectedareaaperture

WeinserttheSADapertureintotheimageplaneoftheobjectivelensandcentertheapertureontheopticaxisinthemiddleoftheviewingscreen.Itmustbefocusedbyadjustingtheintermediatelenssoitisconjugatewith(i.e.,exactlyintheplaneof)theimageofthespecimen.Thenanyelectronthathitsthespecimenoutsidetheareadefinedbythevirtualaperturewillhittherealdiaphragmwhenittravelsontotheimageplane.Itwillthusbeexcludedfromcontributingtothediffractionpattern.

SelectedareaapertureAnotheradvantageofputtingselectedareaapertureintheimageplaneoftheobjectivelensisthatamuchlargeaperturecanbeusedinsteadtoverysmallaperture(wecan’tmakeaperturessmallerthan~10m).Forexample,ifusedintheimageplaneoftheobjectivelenswithmagnification100x,a100maperturewillselectaregionopticallyequivalentto1minspecimen.DiffractionpatternonscreenProjectorlensIntermediatelensSelectedareaaperture2.3ImageviewingandrecordingImageviewingviewingscreenTVimagerecordingfilmCCDcamera(showimageoncomputerscreenandtheimagecanbeprocesseddigitally)viewingscreenandcamerachamberViewingscreensTheviewingscreeninaTEMiscoatedwithamaterialsuchasZnS,whichemitslightwithawavelengthof450nm.TheZnSisusuallymodifiedwithimpuritiestogiveoffgreenlightatcloserto550nm.Theresolutionofthescreenisdependentonthegrainsizeofthescreencoating.TypicalscreencoatingsaremadewithaZnSgrainsizeofabout50m(10mforthehighresolutionscreen).ViewingscreenForhighmagnificationobservation,TEMisalsoequippedwithasmallscreenandanauxiliaryfocusingbinocularswithmagnificationof5-10.Assomesignalsarealsogivenoffbytheviewingscreen,suchasX-rays,andwheneveryoulookatthescreenyouareprotectedfromthislethalradiationfluxbyleadglass.ImagerecordingmethodsFilmTVcamerasCharge-CoupledDevices(CCD)Imagingplate(IP)FilmFilmhasaresolution4-5m,muchhigherthantheviewingscreen,soitismostcommonlyusedimagerecordingmethod(willbecontinuedtobeusedinTEM).HighInformationdensity107pixelsina10cm10cmimageTVAstandardTVcamerahasaresolutionof500lines/frame(highresolutionTVcamerahasresolutionof1000lines/frame).AdvantagetouseTVisthatyoucanrecorddynamicinsituevents.Lowinformationdensity2106pixelsina10cm10cmimageCharge-CoupledDevices(CCD)CCDsareMOSdevicesthatstorechargegeneratedbylightorelectronbeams.CCDarraysconsistofthousandsormillionsofpixelswhichareelectricallyisolatedfromeachotherbycreatingpotentialwellsundereachCCDcellsotheycanaccumulatechargeinproportiontotheincidentbeamintensity.Charge-coupledDevices(CCD)TheimagerecordedbyCCDscanbeprocesseddigitally.Becausethis,CCDsgetmoreandmorepopular.Informationdensity1k1kpixels=106pixels(mostcommon)2k2kpixels=4106pixels4k4kpixels=1.6107pixels(latestCCDs)lowSpeedframetime:0.01squalityofimagerecordedusingCCD(1k1kpixels)isnotasgoodasthefilmImagePlate(IP)TEMimagecanberecordedonimageplate(IP).IPcouldbeprocessedusingspecialmachine(expensive)instantly.ThequalityofimagestoredonIPisbetterthanthefilmexpensive2.4VacuumandcontrolpartsTEMcanonlyworkinvacuumbecausehighspeedelectronswillinteractwithgasmoleculeresultinginscatteringofrandomelectronswhichreducetheimagecontrastelectronwillionizingandchargingcausingelectronbeamunstableresiduegascancorrodefilamentofelectrongun,shorteningthelifeofthefilament,andcontaminatethespecimenseriously.TheTEMiskeptpermanentlyundervacuumCategoryofvacuumRoughvacuum~1_10-3torrlowvacuum~10-3

_10-6torrhighvacuum(HV)~10-6

_10-9torrultrahighvacuum(UHV)~10-9

_10-11torrSIunit:pascal(Pa)non-SIunit:torr,bar1torris~130Pa1Pais7.510-3TorrVacuumrequiredforTEM10-7torrforTEMambientpressure:103torr.ItisquiteremarkablethatwecantransferaspecimenintotheTEM,reducingtheambientpressureatitssurfaceby10ordersofmagnitudeinamatterofafewseconds.10-9torrforUHVTEM10-11torrforgunregionofFEGTEMUsepumptoachieverequiredvacuumVacuumsystemofTEMOtherpartsHightensionsupplyprovide100-200kVvoltagecomputercontrolledelectronicsystemOtherrequirementshightension:theresolutionreductioncausedbythefluctuationofmaximumlenscurrentandchangeofhightensionshouldsmallerthan10-6mechanics:vibrationfree(makeagoodbase)electromagnetism:TEMroomshouldbeelectromagnetisminterferencefree2.5SpecimenholderSpecimenholderhastwodifferentdesignstop-entryholdermainlyusedforhighresolutionTEMinthepast(israrelyusednow)side-entryholderisnowthestandard.AlmostalltheTEMnowadaysuseitSide-entryholderPrincipalpartsofaside-entryholderisheldinthestage.Thespecimenisclampedintothecupattheendoftherod.Asmalljewelattheendoftherodfitsintoanotherjewelbearinginthestagetoprovideastablebaseformanipulatingthespecimen.TheO-ringsealstheendoftheholderinsidethevacuum.Manipulatingthespecimenisaccomplishedfromoutsidethecolumnviacontrolswithintherod.Side-entryholderSpecimensupportgridTheactualcupthatholdsyourspecimeniseither2.3mmor3.05mm(mostcommon)indiameter,sothespecimendiskorsupportgridhastobethesamedimensionasshowninthefigure.ThegridisusuallyCubutcouldbeNi,Au,etc.AvarietyofspecimensupportgridsofdifferentmeshsizeandshapeDifferenttypesofholdersheatingholdercoolingholderdouble-tiltholdersingle-tiltholderDifferenttypesofholdersSingle-tiltholderThisisthebasicholder.Youcanonlytiltaroundtheaxisoftherod.Itisrelativelycheap.Double-tiltholderThisisthemostpopularholdersinceitgivesyouthemostflexibilityinorientingthespecimen.Itisessentialforimaginganddiffractionstudiesofcrystallinespecimens.Thetiltaxesarefixedastwoorthogonaldirections.DifferenttypesofholdersLow-backgroundholderThecupandclampingringaremadeofBetominimizethegenerationofX-rays.SotheyarerequiredforEDSstudies.Theycanbedoubleorsingletiltandmaybecooledalso.HeatingholderSuchholdersinaconventionalTEMcangoto~1300oC.Coolingholderavailableforeitherliquid-N2orliquid-Hetemperatures.Thisisidealforpolymersorbiologicaltissue.SideentryholderSpecimenholderSpecimenholderisverydelicateandisveryexpensive(US$10,000-50,000)Youshouldtreatthespecimenholderasifitwerearealjewel.2.6TEMalignmentOneofthemostimportantaspectsofgoodelectronmicroscopyisthealignmentoftheelectronbeamalongtheopticalaxisofeachlens.ItisonlyifthisalignmentisaccuratethattheaberrationsdiscussedinChapter1canbeminimizedandtheresolutionofthemicroscopecanberealized.TEMalignmentThemagneticlensesmustthemselvesbemechanicallywellaligned,andthiswillgenerallyhavebeendonebythemanufacturer.Howeverthereisusuallystilltheneedforminoradjustments,whichareperformedbytheoperatorusingsmallelectromagneticdeflectioncoilsplacedatstrategicpointsinthecolumn.TEMalignmentAlignmentproceduresdifferfrommicroscopetomicroscopebutarealwaysaimedatmakingsurethatthebeamisdirectedalongtheopticalaxis,particularlyinthesensitiveregionneartheobjectivelens.Furthercoilspermitsmallfieldstobeimposedtocorrecttheeffectofastigmatisminthecondenser,objectiveandprojectorsystems.TEMalignmentAlltheseadjustmentsneedtobemadebytheoperatorbeforeheorsheattemptstotakemicrographs.Alignmentcanbedonefollowingsomestandardprocedures.ModernTEMhasacomputersystemtohelptodothealignment.Youwilllearnhowtodoalignmentinexperiment(youcanalsolearntheseknowledgefromtheChinesetextbook).2.7TEMspecimenpreparationTheTEMspecimenmustbeelectrontransparentandrepresentativeofthematerialyouwanttostudy.Inmostcasesyouwouldlikeyourspecimentobeuniformlythinstableundertheelectronbeamconducingandnonmagneticinthelaboratoryenvironment,electrontransparentTheabilityofelectronbeamgothroughthespecimenismainlydependentontheacceleratingvoltage,thicknessofthespecimen,andatomicnumberofthespecimen.Thehighertheacceleratingvoltageandthelowertheatomicnumberofthespecimen,theeasiertheelectronbeamgoesthroughthespecimen.RequirementofspecimenthicknessForconventionalTEMwork:50-100nmForhighresolutionTEMwork:15nmThethinner,thebetter!FormofTEMspecimenSelf-supportingdiskthespecimenitselfismadeasaself-supportingdiskof3mm,thenthinnedtillelectrontransparent.Specimenplacedonthesupportinggridmakeaspecimensmallerthanthediskof3mmandthinenough,thenplaceitonthesupportinggridtobethinnedtillelectrontransparent.FormofTEMspecimenSelf-supportingdiskthespecimenitselfismadeasaself-supportingdiskof3mm,thenthinnedtillelectrontransparent.Specimenplacedonthesupportinggridmakeaspecimensmallerthanthediskof3mmandthinenough,thenplaceitonthesupportinggridtobethinnedtillelectrontransparent.TherearemanywaystoprepareTEMspecimens.Themethoddependonthetypeofmaterialtheinformationyouneedtoobtain.bearinmindyourtechniquemustnotaffectwhatyouseeormeasureorifitdoesthenyoumustknowhow.VarioustypeofTEMspecimenPowderThinfoilmetalceramicspolymerreplicationcross-section(forinterfacestudy)powderspecimenGrindthepowderspecimenasfineaspossibletillitiselectrontransparentplacethepowderintoliquidthenultrasonicallystirthemtodispersethemplaceadropofthisliquidonaholeycarbonfilmonagridevaporateinadryenvironment,leavingadistributionoftheparticlesonthesupportingfilmWecanplacethesmallparticlesonamorphousorcrystallinefilm.Theclassicexampleistheamorphouscarbonfilm:theholeycarbonfilm.Thethinsupportingfilmshouldhaveauniformthickness;theideaisthatyouarenotactuallyinterestedinthismaterialandthereforewanttominimizeitseffectontheimageofthematerialyouareinterestedin.Supportingfilmifthepowdersizeisnotfineenoughforelectrontransparent,thenembedthepowdersinepoxyandforcingtheepoxyintoa3mmdiameterbrasstubepriortocuringtheepoxy.Thetubeandepoxyarethensectionedintodisks.Thenextstepswillbethesameastomakeathinfoilspecimen(dimpled,andionmilledtotransparency)thinfoilspecimencreateathinslicefromthebulksamplethickness:0.1-0.2mmcuttingthesliceintoadiskof3mmprethinning

makethediskasthinasafewtensmicrometerfinalthinningmakethespecimenelectrontransparentcreationofathinslicefromthebulksampleThematerialsyoumayneedtothincanvaryenormously,sowehavetotreatductilematerials(suchasmetals)andbrittlematerials(suchasceramics)differently.creationofathinslicefromtheductilematerialsuseachemicalwire/stringsawworksbypassingthestringthroughanacidorsolventandthenacrossthesampleuntilthestring“cuts”throughthesampleuseawaferingsawnotdiamondsaw(thesoftmetalwilldulltheblade)usesparkerosionelectro-dischargemachiningTogetathinslice<0.2mminthicknesscreationofathinslicefromthebrittlematerialsMaterialswithawell-definedcleavageplane(suchasSi,GaAs,NaCl,MgO)canbecleavedwitharazorblade.Useadiamondwaferingsawtoprepareaspecimenparalleltoaplanethatdoesn’tcleave.UseultramicrotometocutverythinslicesforimmediateexaminationintheTEMDiamondwaferingsawCuttingthediskIfthematerialsisreasonableductileandmechanicaldamageisnotcrucial,thendiskscanbecutusingamechanicalpunch.Awell-designedpunchcancutdiskswithonlyminimaldamagearoundtheperimeter,buttheshockcaninducesheartransformationinsomematerials.CuttingthediskFormorebrittlematerialsthetwoprincipalmethodsaresparkerosionandultrasonicdrilling.Ineachcasethecuttingtoolisahollowtubewithaninnerdiameterof3mm.Sparkerosionisusedforconductingsamplesandintroducestheleastamountofmechanicaldamage.UltrasonicdrillPrethinningTheaimofthisprocessistothinthecenterofthediskwhileminimizingdamagetothesurfaceofthesample,Ingeneralwewillrefertothisstageas“dimpling”.Anydamageyoucreateattheprethinningstagewillhavetoberemovedduringthefinalthinningprocess.dimplerMostcommercialmechanicaldimplersuseasmallradiustooltogrindandpolishthedisktoafixedradiusofcurvatureinthecenter.Youcancontroltheload,preciselydeterminethethicknessofremovedmaterials.Dimplingcanproducedregionsof~10mthick.Tripodpolisherthinsamplemechanicallytolessthan1mTousetripodpolisher,youshoulduseaveryflatpolishingwheelusefinediamondlappingfilmsPrethinningYoucanalsouseabrasivepapertoprethinningthespecimenmanuallyFinalthinningElectropolishingusedforelectricallyconductingsamplessuchasmetalsandalloysIonMillingusedforceramics,composites,semiconductorsandalloysandmanycross-sectionspecimens.fibersandpowderscanalsobethinnedbyionmilling.themostversatilethinningprocess.ElectropolishingThebasicprincipalisthatthereisacertainappliedvoltageatwhichthecurrentduetoanodicdissolutionofthespecimencreatesapolishedsurfaceratherthanetchingorpitting,asshowninthefigure.Twin-jetelectropolishingapparatus

Atwin-jetapparatuscanbeusedtopumpajetofelectrolyteontobothsidesofthedimpleddisk,asshowninthefigure.Alaserbeamorlightsensordetectstransparencyandawarningsoundisgivenifthespecimenisthinenough.

Atthewarning,theelectrolyteflowmustbecutoffimmediatelytopreventlossofthinarea,andthediskmustberapidlyextractedfromtheelectrolyteandwashedinsolventtoremoveanyresidualfilmofelectrolytewhichmayetchthesurface.Electropolishing

usedforelectricallyconductingsamplessuchasmetalsandalloysAdvantagerelativelyquick(afewminutestoanhour)nomechanicaldamageDisadvantagechangethesurfacechemistryofthespecimenIonmillingInvolvesbombardingthethinspecimenwithenergeticionsorneutralatomsandsputteringmaterialsfromyourspecimenuntilitisthinenoughtobestudiedintheTEMIonmillingAcceleratingvoltage:4__6keVArisusedbecauseitisinert,heavy,andnotnaturallypresentinmostsamplesincidenceangle:15-25degreeMostofthethinningparametersaregenerallyfixedexcepttheionenergy,theangleofincidence,andanyrotationofandthetemperatureofthespecimenAtypicalapproachistostartwithrapidthinningconditions(heavyions,highincidenceangle)andslowthethinningrateasperforationapproaches.CoolingthespecimenusingliquidN2isrecommendedforalmostallmaterials;otherwise,itispossiblethattheionbeammightheatthespecimento200oCorhigher.CrosssectionspecimensUsedforinterface/surfacestudymostwidelyusedcrosssectionsamplearethinfilmsandsemiconductordevices(whichoftenhavemultiplelayersandthereforehavemultipleinterfaces).crosssectionspecimenRatherthantryingtothinoneinterfaceonly,thesamplecanbecutandgluedtogethertoproduceseverallayers,ratherlikeaclubsandwich.crosssectionspecimen

Acriticalstepisthegluingofthesectionstoformthesandwichthethicknessoftheepoxylayermustbesuchthatitisthickenoughforgoodadhesion,butnotsothickthatitiscompletelythinnedawayduringfinalionmillingThencutthegluedsectionsinto3-mmrodsusinganultrasonicdrillalternatively,cutthesamplessmallerandencasethemina3-mmthin-walledtube.Sectionthetubeintodiskthinitasyoudoinmakingthinfoil(byuseofionmilling)UltramicrotomyTheultramicrotomeareroutinelyusedforsoftsamples

suchaspolymersorbiologicalsamplesTheultramicrotomeoperatesbymovingthespecimenpastaknifeblade.UltramicrotomyAdvantagechemistryunc