美国大学透射电镜英文指导书

IntroductoryTransmissionElectronMicroscopyPrimerBobHafnerOctober2011ThisprimerisintendedasbackgroundfortheIntroductoryTransmissionElectronMicroscopytrainingofferedbytheUniversityofMinnesota‘sCharacterizationFacility(CharFac).Theprimeraddressesconceptsfundamentaltoanytransmissionelectronmicroscope(TEM).Learningthismaterialpriortothehands-ontrainingwillimprovetheeffectivenessandefficiencyofyourTEMimaging.TheBigPictureTotherightisanimageofCharfac‘sFEISpirittransmissionelectronmicroscope.Themicroscopecolumn,specimenchamber,andviewingscreenareflankedbythemonitorandpanelcontrolsonthetabletop.Asanoperatoryouwillneedtounderstandwhatishappeninginsidethis―blackbox‖(microscopecolumn)whenapanelorcolumncontrolismanipulatedtoproduceachangeinthemonitorimage.Alookinsidetheblackbox[Figureadaptedfrom1]revealsquiteabitofcomplexity.However,wecanbegintosimplifythisgenericrepresentationbycategorizingthecomponents.Wehavea:source(electrongun)oftheelectronbeamwhichisaccelerateddownthecolumn;seriesofelectromagneticcoils(shifts,tilts,stigmators)whichensurethattheelectronbeamissymmetricalandcenteredasitpassesdowntheopticalcolumnofthemicroscope;seriesofelectromagneticlenses(condenser,objective,intermediate,projector)whichacttoilluminatethespecimenandfocus/magnifythespecimenonthefluorescentscreen/camera;seriesofapertures(micron-scaleholesinmetalfilm)whichthebeampassesthroughandwhicheffectpropertiesofthatbeam;specimenholderwhichpositionsthesampleinthepathoftheelectronbeam.Theareaofbeam/specimeninteractiongeneratesseveraltypesofsignalsthatcanbedetectedandprocessedtoproduceanimage,diffractionpatternorspectra.Therearecontrolsforspecimenpositionandorientation(x,y,zorheight,tilt,rotation);fluorescentscreenorCCDcamerawhichconvertstheelectronsignaltoaformwehumanscansee;alloftheabovemaintainedathighvacuumlevels.Wecanprovidefurtherstructuretothiscomplexitybycomparingthearrangementandfunctionofthesecomponentstosomethingwithwhichwehaveallhadsomeexperience--atransmittedlightmicroscope[Figureadaptedfrom2].Boththeelectronandlightmicroscopehave:an“Illuminationsystem”(areaabovethespecimen)whichconsistsof:asourceofradiation(photons/electrons)andacondenserlenswhichfocusestheilluminatingbeamonthespecimen--allowingvariationsofintensitytobemade;a“Specimenstage”(blackline)situatedbetweentheilluminationandimagingsystems;an“Imagingsystem”(areabelowthespecimen)whichconsistsof:anobjectivelenswhichfocusesthebeamafteritpassesthroughthespecimenandformsanintermediateimageofthespecimen;and,subsequentlenses(eyepiece/intermediateandprojectorlenses)whichmagnifyportionsoftheintermediateimagetoformthefinalimage.an“Imagerecordingsystem”(eye,fluorescentscreenorCCDcamera)whichconvertstheradiation.Inaddition,thetraditionalraydiagramsoflightopticscanbeusedtodisplayhowwemagnify/demagnifyandfocustheelectronbeamandhowelectronsderivingfromapointinanobjectplanecancometoa―well-defined‖pointintheimageplane[Figureadaptedfrom1].Oneadditionalpointneedstobemadeatthistime:Youneedtotaketheconceptofa“column”seriouslyandstartputtingtogetherapictureinyourmindofwherecomponentsareinrelationtooneanother–startingatthetop(gun)andworkingyourwaytothebottom(fluorescentscreen).Thisisessentialtounderstandingalignmentofthemicroscopeaswellaseverydayoperation.HavingmentionedsomeofthesimilaritiesbetweenopticallightmicroscopyandTEM,theTablebelowhighlightstheimportantdifferences.Electronsexhibitbothparticleandwavecharacteristics(λ=h/mv).Justasvisiblelightcanbetreatedasabeamofphotonsoranelectromagneticwave—sotoocanelectronsbecharacterizedasasuccessionofparticlesortreatedbywavetheory.However,electronsarechargedandtheCoulombforcesarestrong.Thenegativechargeoftheelectronsallowsthemtobe:deflectedbymeansofmagneticorelectrostaticfields.Thisisthebasisforhowlensesandcoilsinthemicroscopework;accelerateddownthecolumnbyexperiencingavoltagedifferential.Thesmallmassoftheelectronsnecessitate:highvacuumconditionstonegatetheirdeflectionbyairparticles;ultrathin(<100nm)specimenstoensuretransmissionofthebeam.Theshorterwavelengthoftheelectronresultsinabettertheoreticalresolution(diffractionlimitofresolution).Whenlightraysemanatefromapointandpassthroughalensofsemi-angularapertureα,theyformanimagewhichisnolongerapointbutwiththeintensityspreadoutinwhatisknownasanAirydisk.ThedistancebetweenthetwominimaoneithersideofthemainintensitypeakoftheAirydiskis:D=1.22λ/nsinαwhere:λisthewavelength;nistherefractiveindexofthematerialinwhichtheobjectlies;αisthesemi-angularaperture.[Figuresadaptedfrom3]Whentwoemittingpointsoftheoftheobjectlieveryclosetogether,theintensitypatternsintheimagewilloverlap.Thetheoreticalresolutionofthesystemisdefinedasthedistancebetweenthemaximawhenthemaximumintensityfromonepointiscoincidentwiththefirstminimumfromtheotherpoint.Thisdistanceisthediffractionlimitofresolution:d=0.61λ/nsinαFromthisequationonecanseethatincreasingtheapertureangleanddecreasingthewavelengthwillenablebetterresolution.BottomleftFigure:Smallaperture/largewavelength;BottomrightFigure:Largeaperture/smallwavelength.Inthelightmicroscopethediffractionlimitofresolutionisapproximately200nm.Theeffectiveλ(nm)ofanelectronbeamcanbeapproximatedas1.22/E1/2,whereEistheacceleratingvoltage(eV).Thewavelengthforanacceleratingvoltageof100kVis0.0039nm;0.0022for300kV.Thewavelengthofelectronsisshorterbyordersofmagnitudecomparedtolightmicroscopy,andthusthediffractionlimitofresolutionismuchimproved.OurabilitytoincreasetheapertureangleinTEMendsupbeingconstrainedbyaberrationsintheelectromagneticlenses(moreonthislater).Thusthepractical,oraberrationlimitedresolution,islessthanthetheoreticalresolution.Foragivenmicroscope,theattainableresolutiondependsupontheacceleratingvoltage(higheracceleratingvoltagesproducebetterresolution)andthedesignoftheobjective(focusing)lens.Theobjectivelenshasasphericalaberrationcoefficient.Thesmallerthelensgapthesmallerthesphericalaberrationcoefficientandthebettertheresolution.However,asmallerlensgapwilldecreasecontrast(moreonallofthislater).Finally,thelowmassandnegativechargeoftheelectronsmeansthattheycaneasilybescattered(deflected)bypassingclosetoelectronsorthenucleusofthespecimenatoms.Thisscatteringcanbe[4]:forwardorbackward;coherentorincoherent;andelasticorinelastic(moreonthislater).Thescatteringresultsinanon-uniformdistributionofelectronsemergingfromthespecimen.Thisnon-uniformdistributionisthebasisofthemajorcontrastproducingmechanismsintheTEM:Mass/thicknessDiffractionPhaseAsTEMoperatorswecanselectivelychoosewhichaspectsofthescatterwilltransmitandthuscontributetoanimage[Figureadaptedfrom4].Onecommonwaytodothisistodeterminethe“collectionsemi-angle(θ)”throughtheuseoftheobjectiveaperturebelowthespecimen.Thecollectionsemi-angleisasubsetofthe“generalscatteringsemi-angle(β)”—thelatterbeingdeterminedbythenatureoftheilluminatingelectronbeamandthepropertiesofthespecimen.Theilluminatingbeamhasseveralinterrelatedpropertiesthatwecancontrol:acceleratingvoltage(kV);convergencesemi-angle(α);diameterandcurrentofthebeamonthespecimen.Inwhatfollowswewillworkourwaydownthecolumn--buildingamoredetailedunderstandingaswego.Wewillfirstaddress―brightfield‖and―darkfield‖imagingthatutilizes―amplitude(scattering)contrast‖.Amplitudecontrastincludesboth―mass/thickness‖and―diffraction‖contrast.―Phase(interference)contrast‖willthenbeexplainedandthiswillallowustointroducetheconceptofthe―contrasttransferfunction‖.ThoseindividualswhowishtohaveabetterconceptualunderstandingofthealignmentofthemicroscopeshouldconsultAppendixA.AppendixButilizesthecontrasttransferfunctiontocharacterizehowtheMoosTowerFEITecnaiSpiritandtheNilsHasselmoHallFEITecnaiF30FieldEmissionGunmicroscopesdifferfromoneanotherintermsofimagingcapabilities.IlluminationSystemElectronGunsThepurposeoftheelectrongunistoprovideastablebeamofelectronsofadjustableenergy.Therearethreemaintypesofelectronguns:TungstenhairpinLanthanumhexaboride(LaB6)FieldEmissionTherearethreepropertiesoftheTungstenhairpinandLaB6filamentgunsthatyouhavesomecontrolover.Youcan:heatthefilamenttofacilitatetheemissionofelectronsfromit(emissioncurrent).Thegoalistomaximizeemissioncurrentandminimizefilamenttemperature.Thepointatwhichthisoccursistermedthe“saturationpoint”.applyapotentialdifferencebetweenthefilament(cathode)andtheanodeoftheguntoacceleratetheemittedelectronsdownthecolumn.Theacceleratingvoltagewechoosewillhaveaneffectonthepenetratingpoweroftheelectronbeamaswellasitsresolvingcapabilities.applyasmallnegativebiasbetweenthefilamentandtheWehneltcapwhichhousesthefilament[Figureadaptedfrom4].Withnobias(i)thereishighemissioncurrent,butthatcurrenthasahighdivergenceangleorspread.Averyhighbias(iii)willactuallyretardtheemissionofelectrons.Theoptimumbiaslevel(ii)willtaketheemittedelectronsandproduceacrossoverpoint(gunsource)ofsmalldiameter--asaturatedelectroncloud.Producingasmallgunsourceorcrossoverhasseveralbenefits:Anelectronbeamemanatingfromasmallsourcesizeissaidtohavehighspatialcoherency.Thatis,thewavesemanatingfromthesourcepointareinphasewithoneanother.HighspatialcoherenceoftheilluminatingbeamwillenhancethecontrastproducingmechanismsintheTEM(wewilldiscussotherwaysofenhancingbeamcoherencelateron).Also,byusingthebiastoformasmallsourcesizeweareputtingtheemissioncurrentintoasmallerareaandthusmaximizingthebrightnessoftheelectronbeam.Brightnessisthebeamcurrentperunitareapersolidangle[Figureadaptedfrom4]Β=4i/(πdα)2where:i=beamcurrent;d=diameterofthesource(crossover);and,α=semi-angleofbeamdivergence.Withgreaterbrightnesswecanhaveasmallerdiameterbeamonthespecimenandstillhaveadequatecurrentforsignalproduction.Theresultishighermagnificationimagingcapabilities.Brightness,unlikecurrent,isconserveddownthecolumn.Brightnessincreaseslinearlywithacceleratingvoltage.Electronbeamscanalsobecharacterizedintermsoftemporalcoherency.Abeamwithhightemporalcoherencywillhaveelectronsofthesamewavelength.Inrealitythereisacertain“EnergySpread‖associatedwiththebeam.Theenergyspreadwilldecreasewith:hightensionstability;alowersurfacetemperatureoftheemitter;and,asomewhatloweremissioncurrent(Boerscheffect).Thetablebelowhighlightstheimportantdifferencesbetweenthevariousguntypes.ThereisaprogressionofcapabilitiesfromTungstentoLaB6toFEGintermsofbrightness,sourcesize,andtemporalcoherency.Alongwiththesecapabilitiescomestheneedforbettervacuumsystems.Theadvantagesofamorecoherentbeamsourcewillbenegatedifthebeamisinteractingwithmoleculesonitspathdownthecolumn.Thetablebelowisprovidedsimplytogiveyouafeelingforwhatthesevacuumlevelstranslatetoinsidethemicroscope.Forreference:1Torr=133Pa=1.33mbar.―MeanFreePath‖istheaveragedistancecoveredbyamovingparticlebetweensuccessivecollisions.―TimetoMonolayer‖isthelengthoftimerequired,onaverage,forasurfacetobecoveredbyanadsorbate.Wewon‘tspendtimeheretalkingaboutthevariouskindsofpumpsandgaugesassociatedwiththevacuumsystemsincethosearemaintainedbythestaff.However,asanoperator,youshouldbeveryawareofthevacuumstateofthemicroscopeandensurethat:themicroscopeisatgoodvacuumlevelwhenyoubeginyoursession;ifthereisa―GunValve‖separatingtheuppercolumnfromtherestofthemicroscope–thatitisclosedduringsampleexchange;appropriatevacuumlevelsareachievedpriortoengagingandsteppingupthehightension;yourhandsdonottouchanypartofthespecimenholderthatisinsertedintothecolumn(underhighvacuum);andyoursamplesaredryandfreeofcontaminationandoutgassing.ElectromagneticLenses(Coils[shifts,tilts,stigmators]willbeaddressedinthealignmentsectionoftheprimer)Itisusefultoreasonbyanalogywithglasslensesusedforfocusinglighttohelponeunderstandtheoperationofelectromagneticlenses.Analogiesalsohaveregionsofnon-correspondencewhichareequallyimportanttounderstand.Thediagramtotherightshowsaperfectopticallens.Theraysemanatingfromapointintheobjectplane(adistance―p‖fromthelens)cometoonecommonwelldefinedpointintheimageplane(adistance―q‖fromthelens).Parallelraysenteringthelensarefocusedinthe―backfocalplane‖(adistance―f‖fromthelens).Theopticallenshasafixedfocalpointandtheobjectisinfocusattheimageplane.Themagnificationofthelensisq/p(demagnification=1/magnification)Inlightopticsweexchangelensestochangemagnificationandadjusttheheightofagivenlensalongtheopticaxistofocus.Electromagneticlensesincontrastarestationary--butwecanvarytheirstrengthbyalteringtheamountofcurrentrunningthroughthem.Thuselectromagneticlenseshavevariablefocalpoints.Thediagramtotheleftshowsastationarylenswhichisstronger(larger).Inthisinstancewehaveactuallydemagnifiedtheimage.Anelectromagneticlens[Figurefrom5]consistsofacoilofcopperwiresinsideanironpolepiece.Acurrentthroughthecoilscreatesamagneticfield(symbolizedbyredlines)intheboreofthepolepieceswhichisusedtoconvergetheelectronbeam.Whenanelectronpassesthroughanelectromagneticlensitissubjectedtotwovectorforcesatanyparticularmoment:aforce(HZ)paralleltothecore(Zaxis)ofthelens;andaforce(HR)paralleltotheradiusofthelens.Thesetwoforcesareresponsiblefortwodifferentactionsontheelectrons,spiralingandfocusing,astheypassthroughthelens.AnelectronpassingthroughthelensparalleltotheZaxiswillexperiencetheforce(HZ)causingittospiralthroughthelens.Thisspiralingcausestheelectrontoexperience(HR)whichcausesthebeamtobecompressedtowardtheZaxis.Themagneticfieldisinhomogeneousinsuchawaythatitisweakinthecenterofthegapandbecomesstrongerclosetothebore.Electronsclosetothecenterarelessstronglydeflectedthanthosepassingthelensfarfromtheaxis.Sofarwe‘vementionedthatelectromagneticlensesareunlikeopticallensesinthattheyare:stationary;havevariablefocalpoints;andcausetheimagetoberotated.ThelatteriscorrectedforinmodernTEMs.Electromagneticlensesalsodifferinthat:thedeflectionoftheelectronwithinthelensisacontinuousprocess(noabruptchangesintherefractiveindex);onlybeamconvergence(notdivergence)ispossible;andtheconvergenceanglewithrespecttotheopticaxisisverysmallcomparedwithopticallightmicroscopy(lessthanonedegree!).Finally,itisimportanttokeepinmindthatelectronlenses,comparedtoglasslenses,performmuchmorepoorly.Somehavecomparedthequalityofelectronopticstothatofimagingandfocusingwithacokebottle.Thisismainlyduetothefactthataberrationsarerelativelyeasilycorrectedinglasslenses.LensAberrationsUptothispoint,allofourrepresentationsdepictaperfectlens.Thatis,allraysemanatingfromapointintheobjectplanecometothesamefocalpointintheimageplane.Inreality,alllenseshavedefects.Thedefectsofmostimportancetousherearesphericalaberration;chromaticaberrationandastigmatism.Ratherthanaclearlydefinedfocalpoint,weendupwitha“diskofminimumconfusion”ineachinstance.Sphericalaberration:Thefurtherofftheopticalaxis(theclosertotheelectromagneticpolepiece)theelectronis,thestrongerthemagneticforceandthusthemorestronglyitisbentbacktowardtheaxis.Theresultisaseriesoffocalpointsandthepointsourceisimagedasadiskoffinitesize(dS).Toreducetheeffectsofsphericalaberration,aperturesareintroducedintothebeampath.Aperturesarecircularholesinmetaldisksonthemicronscale.Theneteffectoftheapertureistoreducethediameterofthediskofminimumconfusion.However,thatpositiveeffectcomesatthepriceofreducedbeamcurrent.Also,averysmallaperturewilldisplaydiffractioneffects.Thediameteroftheapertureusedwillalsoaffecttheconvergenceangleofthebeamandthisinturnwillaffectitscoherenceaswellasimagepropertiessuchasdepthoffocus.Chromaticaberration:Theelectronbeamgeneratedbythegunwillhaveacertainenergyspread.Electronsofdifferentenergiesatthesamelocationinthecolumnwillexperiencedifferentforces.Anelectromagneticlenswill―bend‖electronsoflowerenergymorestronglythanthoseofhigherenergy.Aswithsphericalaberration,adiskofminimumconfusion(dC)isproduced.Astigmatism:Finally,theelectromagneticlensesusedintheTEMcannotbemachinedtoperfectsymmetry.Ifthefieldsproducedbythelenseswereperfectlysymmetrical,aconvergedbeamwouldappearcircular(lookingdownthecolumn).Alackofsymmetrywouldresultinanoblongbeam:thenarrowerdiameterduetothestrongerfocusingplane;thewiderdiameterduetotheweakerfocusingplane.Theneteffectisthesameasthatoftheaberrationsabove—adiskofminimumconfusionratherthanawelldefinedpointoffocus.IlluminationsystemlensesandaperturesAlthoughanunderstandingofbeam--specimeninteractionsisnecessarytoinformusercontroloftheilluminationsystem,wecanbegintoputthatpicturetogether.Theilluminationsystemcanbesetupintwobasicmodes:Convergentbeamandparallelbeam.Thelatterwillbeourfocusinthisprimer.ConvergentbeamInconvergentbeammodewefocusareducedimageoftheguncrossoveronthespecimentomakeasmallbrightprobewhichwouldbeusefulinanalyticalSTEMworkorconvergentbeamdiffraction.Thediagramtotherightshowsthedemagnificationoftheguncrossovertoproduceasmallprobeonthespecimen.Theactualdemagnificationisasfollows:thesizeofthegunsourceatd1=dgun(p1/q1);thesizeoftheprobeonthespecimenisd2=d1(p2/q2).Ifwewantedasmalldiameterbeamprobewewouldexcitecondenserlens1(C1)–causingq1todecrease.Thenwewouldfocuscondenserlens2(C2)toformthesmallestspotonthespecimen.