透射电子显微镜结构及工作原理

Chapter2TransmissionElectronMicroscope TEM 1 TEM Electronopticssystem Vacuumsystem PowerSupplyandcontrolsystem 1 illuminationsystem 2 imagingsystem 3 imageviewingandrecording StructureofTEM 2 Illuminationsystem Imagingsystem Imageviewingandrecording 3 2 1Theilluminationsystem Compriseselectrongun provideselectronsource condenserlenses controltheelectronbeam 4 2 1 1Electrongun providessourceofelectronstoilluminatethespecimen Therearetwotypesofelectronsources thermionicsourcetungstenfilamentslanthanumhexaboride LaB6 crystalsfield emissionsource FEG finetungstenneedles 5 ThermionicEmission Ifanymaterialtobeheatedtoahighenoughtemperature theelectronsgainssufficientenergytoovercomethenaturalbarrier workfunction thatpreventsthemfromleakingouttoescapefromthesource TwothermionicsourcesusedinpracticearetungstenandLaB6 Wfilament LaB6crystal 6 Thermionicgun Ahighvoltageisplacedbetweenthefilament actingasacathode andtheanode modifiedbyapotentialontheWehneltwhichactstofocustheelectronsintoacrossover withdiameterd0andconvergence divergenceanglea0 7 FieldEmission TheprinciplebehindfieldemissionisthatthestrengthofanelectricfieldEisconsiderableincreasedatsharppoints becauseifwehaveavoltageVappliedtoa spherical pointofradiusrthenE V r Oneoftheeasiestmaterialstoproducewithafinetipistungstenwire Toallowfieldemission thesurfaceshouldbefreeofcontaminantsandoxide ThiscanbeachievedbyoperatinginUHV betterthan10 11Torr AnFEGtip fineWneedle 8 FieldEmissionGun FEG InordertogetanFEGtowork wemakeitthecathodewithrespecttotwoanodes Afinecrossoverisformedbytwoanodesactingasanelectrostaticlens 9 Idealelectronsource highbrightness highcurrentdensity bettercoherency smallenergyspread smallchromaticaberrationgoodformodernTEMworkgoodstabilitylonglifetime 10 Characteristicsofthethreesourcesoperatingat100kV 11 Comparisonofelectronsources Tungstensourcetheworstinmostrespects butforroutineTEMapplicationstheyareexcellent reliablesourcesandarecheapandeasilyreplaceable LaB6highbrightness improvedcoherencyandtheenergyspread increasedoperatinglifeisarecommendedthermionicsource forallaspectsofTEM butparticularlyAEMexpensive severalhundreddollarseach 12 Comparisonofelectronsources FEGForallapplicationsthatrequireabright coherentsourcetheFEGisthebest ThisisthecaseforAEM HRTEM ForroutineTEM anFEGisfarfromidealbecausethesourcesizeissosmall Itisnotpossibletoilluminatelargeareasofthespecimenwithoutlosingcurrentdensity andthereforeintensityonthescreen needUHV veryexpensive US 10 000 13 2 1 2Condenselens Theilluminationsystemconsistsoftwo three condenselensesThefirstcondenserlens C1 oftencontrolledviaaknobwhichislabeledspotsize setsthedemagnificationoftheguncrossover Thesecondcondenserlens C2 oftencontrolledviaaknobwhichislabeledintensity providesdirectcontrolofthespotsizeatthespecimen anddirectcontroloftheconvergenceangle 14 Theilluminationsystem Twodifferentwaystousetheilluminationsystemformaparallelbeam usedforTEMimaginganddiffraction formaconvergentbeam usedforSTEMimaging microanalysis andmicrodiffraction 15 ParallelBeammode Parallelilluminationisessentialtogetthesharpestdiffractionpatternsaswellasthebestimagecontrast InthetraditionalTEMmodethefirsttwocondenserlenses C1 C2 areadjustedtoilluminatethespecimenwithaparallelbeamofelectronstypicallyseveralmicrometersacross 16 Intheparallel beammodeusuallynoneedtochangeC1 adjusttheC2lenstoproduceanunderfocusedimageoftheC1crossover 17 Convergent beammode FEG TEM ForFEG TEM afocusedC2lensilluminatesasmallareaofthespecimenwithanonparallelbeam CrossoverofFEGis 10nm UnlessyouhaveanFEG itisn tpossibletousejusttheC1andC2lensesasintheFigureshownheretoconvergethebeamtoassmallaprobeasyouwouldlike 10nm ThisisbecausetheC1andC2lensescan tdemagnifythecrossoverofthermionicgun W 50mm LaB6 10mm sufficiently 18 Convergent beammode non FEG Theusualsolutionistoconverttheupperpolepeiceoftheobjectivelensintoathirdcondenserlens whichiscalledacondenser objectivelensC3 WemaketheC3muchstrongerthanusualandweakenC2orturnitoff asshownintheFigure togettheconvergent beam 19 2 2TheImagingsystem ComprisesmainlyobjectivelensintermediatelensprojectorlensFunctionsUsesobjectivelenstoformtheimageortheelectrondiffractionpatternUsesintermediatelensandprojectorlenstomagnifytheimageorthediffractionpatternproducedbytheobjectivelensandtofocusthemontheviewingscreen 20 TheImagingsystem TheImagingsystemiscomposedofObjectivelensIntermediatelensProjectorlensObjectiveapertureSADaperture Thetwoaperturesareneverusedinthesametime 21 TheImagingsystem 22 themostimportantlensinTEMformsthefirstintermediateimageanddiffractionpattern diffractionpatternisinevitablyformedinthebackfocalplaneofthelens magnificationoftheobjectivelensMo 100 200 Objectivelens 23 Intermediatelens magnifythefirstimageanddiffractionpatternformedbytheobjectivelensandprojectittotheobjectiveplaneoftheprojectorlenscontroltogetimageordiffractionpatterncontrolthetotalmagnificationofTEMalenswithavariablemagnification MI 0 20 24 Projectorlens magnifythesecondimageanddiffractionpatternformedbytheintermediatelensandprojectittothefluorescentscreenmagnificationoftheprojectorlensMP 100 25 ThetotalmagnificationoftheTEM Mo magnificationoftheobjectivelens fixed MI magnificationoftheintermediatelens variable Mp magnificationoftheprojectorlens fixed 26 Totalmagnificationiscontrolledbythemagnificationofintermediatelens 27 Aperture diaphragm TheapertureisacircularholeinmetaldiskandthediskismadeofeitherPtorMo Themetalsurroundingtheapertureiscalledthediaphragm Aperture diaphragmisnormallysimplycalledaperture Thediameteroftheapertureisintherangeof10 50 m 28 Aperture diaphragm Weusetheaperturetoallowcertainelectronstopassthroughthelensandexcludeothersbycausingthemtohitthesurroundingdiaphragm i e limitsthecollectionangleofthelens 29 Objectiveaperture tocontroltheimagecontrastsmallaperture highcontrastlargeaperture lowcontrasttheresolutionoftheimageformedbythelensthecollectionangleoftheEELStheangularresolutionofthediffractionpattern 30 Imagemode diffractionmode WeneedtoviewimageordiffractionpatternusingTEM Thisisachievedbyadjustingtheintermediatelens Theintermediatelenscanbeswitchedbetweentwosettings theimagemodethediffractionmode 31 Imagemode Intheimagemode youadjusttheintermediatelenssothatitsobjectplaneistheimageplaneoftheobjectivelens Thenanimageisprojectedontotheviewingscreen Howtoseediffractionpatter 32 Diffractionmode Inthediffractionmode youhavetoadjusttheintermediatelenssothatitsobjectplaneisthebackfocalplaneoftheobjectivelens Thenthediffractionpatternisprojectedontotheviewingscreen 33 SelectedAreaDiffraction SAD Asyouseefromthefigureintheaboveslide thediffractionpatterncontainselectronsfromthewholeareaofthespecimenthatweilluminatewiththebeam Suchapatternisnotveryusefulbecause 1 thespecimenwilloftenbebuckled 2 Thedirectbeamisoftentoointensetodamagetheviewingscreen SoweperformabasicTEMoperationbothtoselectaspecificareaofthespecimentocontributetothediffractionpatternandtoreducetheintensityofthepatternfallingonthescreen 34 SelectedAreaDiffraction SAD Therearetwowayswecouldreducetheilluminatedareaofthespecimencontributingtothediffractionpattern 35 WecouldmakethebeamsmallerbyconvergethebeamatthespecimentoformCBED ConvergentBeamElectronDiffraction pattern Convergingthebeamdestroysanycoherence andspotsinthepatternarenotsharplydefinedbutspreadintodisks Method1 ED CBED 36 SelectedAreaDiffraction SAD Ifwewishtoobtainadiffractionpatternwithaparallelbeamofelectrons thestandardwayistoinsertanapertureabovethespecimenwhichwouldonlypermitelectronsthatpassthroughittohitthespecimen Thisoperationiscalledselected areadiffraction SAD Method2 37 Ifweinsertanapertureinaplaneconjugatewiththespecimen i e inoneoftheimageplanes thenitcreatesavirtualapertureattheplaneofthespecimen Thisisexactlyselectedareadiffractiondoes Butwecan tinsertanapertureatthespecimenplane becausethespecimenisalreadythere 38 SelectedAreaDiffraction SAD Theconjugateplanewechooseistheimageplaneoftheobjectivelens asshowninthefigure WeinserttheSADapertureintotheimageplaneoftheobjectivelensandcentertheapertureontheopticaxisinthemiddleoftheviewingscreen Itmustbefocusedbyadjustingtheintermediatelenssoitisconjugatewith i e exactlyintheplaneof theimageofthespecimen Thenanyelectronthathitsthespecimenoutsidetheareadefinedbythevirtualaperturewillhittherealdiaphragmwhenittravelsontotheimageplane Itwillthusbeexcludedfromcontributingtothediffractionpattern 39 Selectedareaaperture Anotheradvantageofputtingselectedareaapertureintheimageplaneoftheobjectivelensisthatamuchlargeaperturecanbeusedinsteadtoverysmallaperture wecan tmakeaperturessmallerthan 10 m Forexample ifusedintheimageplaneoftheobjectivelenswithmagnification100 x a100 maperturewillselectaregionopticallyequivalentto1 minspecimen 40 2 3Imageviewingandrecording ImageviewingviewingscreenTVimagerecordingfilmCCDcamera showimageoncomputerscreenandtheimagecanbeprocesseddigitally 41 viewingscreenandcamerachamber 42 Viewingscreens TheviewingscreeninaTEMiscoatedwithamaterialsuchasZnS whichemitslightwithawavelengthof450nm TheZnSisusuallymodifiedwithimpuritiestogiveoffgreenlightatcloserto550nm Theresolutionofthescreenisdependentonthegrainsizeofthescreencoating TypicalscreencoatingsaremadewithaZnSgrainsizeofabout50 m 10 mforthehighresolutionscreen 43 Viewingscreen Forhighmagnificationobservation TEMisalsoequippedwithasmallscreenandanauxiliaryfocusingbinocularswithmagnificationof5 10 Assomesignalsarealsogivenoffbytheviewingscreen suchasX rays andwheneveryoulookatthescreenyouareprotectedfromthislethalradiationfluxbyleadglass 44 Imagerecordingmethods FilmTVcamerasCharge CoupledDevices CCD Imagingplate IP 45 Film Filmhasaresolution4 5 m muchhigherthantheviewingscreen soitismostcommonlyusedimagerecordingmethod willbecontinuedtobeusedinTEM HighInformationdensity107pixelsina10cm 10cmimage 46 TV AstandardTVcamerahasaresolutionof500lines frame highresolutionTVcamerahasresolutionof1000lines frame AdvantagetouseTVisthatyoucanrecorddynamicinsituevents Lowinformationdensity2 106pixelsina10cm 10cmimage 47 Charge CoupledDevices CCD CCDsareMOSdevicesthatstorechargegeneratedbylightorelectronbeams CCDarraysconsistofthousandsormillionsofpixelswhichareelectricallyisolatedfromeachotherbycreatingpotentialwellsundereachCCDcellsotheycanaccumulatechargeinproportiontotheincidentbeamintensity 48 Charge coupledDevices CCD TheimagerecordedbyCCDscanbeprocesseddigitally Becausethis CCDsgetmoreandmorepopular Informationdensity1k 1kpixels 106pixels mostcommon 2k 2kpixels 4 106pixels4k 4kpixels 1 6 107pixels latestCCDs lowSpeedframetime 0 01squalityofimagerecordedusingCCD 1k 1kpixels isnotasgoodasthefilm 49 ImagePlate IP TEMimagecanberecordedonimageplate IP IPcouldbeprocessedusingspecialmachine expensive instantly ThequalityofimagestoredonIPisbetterthanthefilmexpensive 50 2 4Vacuumandcontrolparts TEMcanonlyworkinvacuumbecausehighspeedelectronswillinteractwithgasmoleculeresultinginscatteringofrandomelectronswhichreducetheimagecontrastelectronwillionizingandchargingcausingelectronbeamunstableresiduegascancorrodefilamentofelectrongun shorteningthelifeofthefilament andcontaminatethespecimenseriously TheTEMiskeptpermanentlyundervacuum 51 Categoryofvacuum Roughvacuum 1 10 3torrlowvacuum 10 3 10 6torrhighvacuum HV 10 6 10 9torrultrahighvacuum UHV 10 9 10 11torrSIunit pascal Pa non SIunit torr bar1torris 130Pa1Pais7 5 10 3Torr 52 VacuumrequiredforTEM 10 7torrforTEMambientpressure 103torr ItisquiteremarkablethatwecantransferaspecimenintotheTEM reducingtheambientpressureatitssurfaceby10ordersofmagnitudeinamatterofafewseconds 10 9torrforUHVTEM10 11torrforgunregionofFEGTEM 53 Usepumptoachieverequiredvacuum 54 VacuumsystemofTEM 55 Otherparts Hightensionsupplyprovide100 200kVvoltagecomputercontrolledelectronicsystem 56 Otherrequirements hightension theresolutionreductioncausedbythefluctuationofmaximumlenscurrentandchangeofhightensionshouldsmallerthan10 6mechanics vibrationfree makeagoodbase electromagnetism TEMroomshouldbeelectromagnetisminterferencefree 57 2 5Specimenholder Specimenholderhastwodifferentdesignstop entryholdermainlyusedforhighresolutionTEMinthepast israrelyusednow side entryholderisnowthestandard AlmostalltheTEMnowadaysuseit 58 Side entryholder Principalpartsofaside entryholderisheldinthestage Thespecimenisclampedintothecupattheendoftherod Asmalljewelattheendoftherodfitsintoanotherjewelbearinginthestagetoprovideastablebaseformanipulatingthespecimen TheO ringsealstheendoftheholderinsidethevacuum Manipulatingthespecimenisaccomplishedfromoutsidethecolumnviacontrolswithintherod Side entryholder 59 Specimensupportgrid Theactualcupthatholdsyourspecimeniseither2 3mmor3 05mm mostcommon indiameter sothespecimendiskorsupportgridhastobethesamedimensionasshowninthefigure ThegridisusuallyCubutcouldbeNi Au etc Avarietyofspecimensupportgridsofdifferentmeshsizeandshape 60 Differenttypesofholders heatingholder coolingholder double tiltholder single tiltholder 61 Differenttypesofholders Single tiltholderThisisthebasicholder Youcanonlytiltaroundtheaxisoftherod Itisrelativelycheap Double tiltholderThisisthemostpopularholdersinceitgivesyouthemostflexibilityinorientingthespecimen Itisessentialforimaginganddiffractionstudiesofcrystallinespecimens Thetiltaxesarefixedastwoorthogonaldirections 62 Differenttypesofholders Low backgroundholderThecupandclampingringaremadeofBetominimizethegenerationofX rays SotheyarerequiredforEDSstudies Theycanbedoubleorsingletiltandmaybecooledalso HeatingholderSuchholdersinaconventionalTEMcangoto 1300oC Coolingholderavailableforeitherliquid N2orliquid Hetemperatures Thisisidealforpolymersorbiologicaltissue 63 Sideentryholder 64 Specimenholder Specimenholderisverydelicateandisveryexpensive US 10 000 50 000 Youshouldtreatthespecimenholderasifitwerearealjewel 65 2 6TEMalignment Oneofthemostimportantaspectsofgoodelectronmicroscopyisthealignmentoftheelectronbeamalongtheopticalaxisofeachlens ItisonlyifthisalignmentisaccuratethattheaberrationsdiscussedinChapter1canbeminimizedandtheresolutionofthemicroscopecanberealized 66 TEMalignment Themagneticlensesmustthemselvesbemechanicallywellaligned andthiswillgenerallyhavebeendonebythemanufacturer Howeverthereisusuallystilltheneedforminoradjustments whichareperformedbytheoperatorusingsmallelectromagneticdeflectioncoilsplacedatstrategicpointsinthecolumn 67 TEMalignment Alignmentproceduresdifferfrommicroscopetomicroscopebutarealwaysaimedatmakingsurethatthebeamisdirectedalongtheopticalaxis particularlyinthesensitiveregionneartheobjectivelens Furthercoilspermitsmallfieldstobeimposedtocorrecttheeffectofastigmatisminthecondenser objectiveandprojectorsystems 68 TEMalignment Alltheseadjustmentsneedtobemadebytheoperatorbeforeheorsheattemptstotakemicrographs Alignmentcanbedonefollowingsomestandardprocedures ModernTEMhasacomputersystemtohelptodothealignment Youwilllearnhowtodoalignmentinexperiment youcanalsolearntheseknowledgefromtheChinesetextbook 69 2 7TEMspecimenpreparation TheTEMspecimenmustbeelectrontransparentandrepresentativeofthematerialyouwanttostudy Inmostcasesyouwouldlikeyourspecimentobeuniformlythinstableundertheelectronbeamconducingandnonmagneticinthelaboratoryenvironment 70 electrontransparent Theabilityofelectronbeamgothroughthespecimenismainlydependentontheacceleratingvoltage thicknessofthespecimen andatomicnumberofthespecimen Thehighertheacceleratingvoltageandthelowertheatomicnumberofthespecimen theeasiertheelectronbeamgoesthroughthespecimen 71 Requirementofspecimenthickness ForconventionalTEMwork 50 100nmForhighresolutionTEMwork 15nmThethinner thebetter 72 FormofTEMspecimen Self supportingdiskthespecimenitselfismadeasaself supportingdiskof 3mm thenthinnedtillelectrontransparent Specimenplacedonthesupportinggridmakeaspecimensmallerthanthediskof 3mmandthinenough thenplaceitonthesupportinggridtobethinnedtillelectrontransparent 73 FormofTEMspecimen Self supportingdiskthespecimenitselfismadeasaself supportingdiskof 3mm thenthinnedtillelectrontransparent Specimenplacedonthesupportinggridmakeaspecimensmallerthanthediskof 3mmandthinenough thenplaceitonthesupportinggridtobethinnedtillelectrontransparent 74 TherearemanywaystoprepareTEMspecimens Themethoddependonthetypeofmaterialtheinformationyouneedtoobtain bearinmindyourtechniquemustnotaffectwhatyouseeormeasureorifitdoesthenyoumustknowhow 75 VarioustypeofTEMspecimen PowderThinfoilmetalceramicspolymerreplicationcross section forinterfacestudy 76 powderspecimen Grindthepowderspecimenasfineaspossibletillitiselectrontransparentplacethepowderintoliquidthenultrasonicallystirthemtodispersethemplaceadropofthisliquidonaholeycarbonfilmonagridevaporateinadryenvironment leavingadistributionoftheparticlesonthesupportingfilm 77 Wecanplacethesmallparticlesonamorphousorcrystallinefilm Theclassicexampleistheamorphouscarbonfilm theholeycarbonfilm Thethinsupportingfilmshouldhaveauniformthickness theideaisthatyouarenotactuallyinterestedinthismaterialandthereforewanttominimizeitseffectontheimageofthematerialyouareinterestedin Supportingfilm 78 ifthepowdersizeisnotfineenoughforelectrontransparent thenembedthepowdersinepoxyandforcingtheepoxyintoa3mmdiameterbrasstubepriortocuringtheepoxy Thetubeandepoxyarethensectionedintodisks Thenextstepswillbethesameastomakeathinfoilspecimen dimpled andionmilledtotransparency 79 thinfoilspecimen createathinslicefromthebulksamplethickness 0 1 0 2mmcuttingthesliceintoadiskof 3mmprethinningmakethediskasthinasafewtensmicrometerfinalthinningmakethespecimenelectrontransparent 80 creationofathinslicefromthebulksample Thematerialsyoumayneedtothincanvaryenormously sowehavetotreatductilematerials suchasmetals andbrittlematerials suchasceramics differently 81 creationofathinslicefromtheductilematerials useachemicalwire stringsawworksbypassingthestringthroughanacidorsolventandthenacrossthesampleuntilthestring cuts throughthesampleuseawaferingsawnotdiamondsaw thesoftmetalwilldulltheblade usesparkerosionelectro dischargemachiningTogetathinslice 0 2mminthickness 82 creationofathinslicefromthebrittlematerials Materialswithawell definedcleavageplane suchasSi GaAs NaCl MgO canbecleavedwitharazorblade Useadiamondwaferingsawtoprepa