近代分析实验原理：第十二课-二次离子质谱

十二、二次离子质谱

（SecondaryIonMassSpectrometry

forSurfaceAnalysis）近代分析实验原理（Introductionofmodernanalyticalmethods）12surfacechemicalanalysistechniqueAdvantages(ComparedwithAESandXPS):Detectionofallthechemicalelementsintheperiodictable,includinghydrogenwhichcannotbedetectedbytheAESorXPS;Detectionofelementsinconcentrationsaslowas10−6,whileAESorXPSdetectionlimitsareconcentrationlevelsof0.1atom%;Limitationofthedetectiontothetoponeortwoatomiclayersofasolidsurface(<1nm);Distinguishbetweendifferentisotopesofelements.SecondaryIonMassSpectrometry(SIMS)SIMSexaminesthemassofions,insteadofenergyofelectrons,escapedfromasolidsurfacetoobtaininformationonsurfacechemistry.Theterm‘secondaryion’isusedtodistinguish‘primaryion’thatistheenergysourceforknockingoutionsfromasolidsurface.dynamicSIMSstaticSIMSfordepthprofilingofchemistryforsurfacechemicalexaminationQualitativethecomplicatednatureofsecondaryionyieldfromasolidsurface.destructivetypeNondestructivetype31.BasicPrinciplesSecondaryparticlegenerationbyanenergeticprimaryparticle.ElectronsneutralspeciesofatomsormoleculesionsmajorityandnotusefulinSIMSCarrychemicalinformationmorethanasimplyone-to-oneknock-out41.1SecondaryIonGenerationThesputteringprocessinSIMS:(a)directcollisionsputtering;(b)collisioncascade;and(c)thermalsputtering.extremelyfast10−15to10−14seconds10−14to10−12seconds10−13to10−10secondsOnlyasmallportionofsecondaryparticles(∼1%oftotalsecondaryparticles)areionizedandbecomethesecondaryionsthatareanalyzedinSIMS.SlowcollisionsputteringAsputteredparticlefacescompetitionbetweenionizationandneutralizationprocesseswhenitescapesasamplesurface.5secondaryioncurrentprimaryionfluxthesputteryieldtheionizationprobabilitytheprobabilityforpositiveionsthefractionalconcentrationofspeciesminthesurfacelayertransmissionofthedetectionsystemVariationofpositiveionyieldamongchemicalelementsunderbombardmentof13.5keVO−1.(o)indicatesapureelement,(□)indicatesacompound.Underthesameexperimentalconditions6Underthesameexperimentalconditions:Theyieldofelementalsecondaryionscanvarybyseveralordersofmagnitudeacrosstheperiodictable;Also,thesecondaryionyieldforaparticularelementdramaticallyvarieswithchemicalstate.thepositiveAlionyieldinitsoxidestateis100timeshigherthaninitspurestate.7First,thepointatwhichthesecondaryparticlesescapefromthesurfaceisnotthepointoftheinitialimpactbytheprimaryion.Instead,thereisasurfacedamagezonethatincludesthepointsofprimaryparticleimpactandsecondaryparticleescape.Second,thecascadecollisionresultsingenerationofsecondaryionswithmuchlowerenergythanthatofprimaryions.Third,thereissignificantvariationinsecondaryionyieldwithchemicalelementsandchemicalstatesofthesurface,whichmakesquantitativeanalysisdifficult.Theimportantfeaturesofthesputter溅射ionizationprocess81.2DynamicandStaticSIMSHighfluxofprimaryionbombardmentremovemanylayersofatomsinthesurfaceregionandprovideelementaldistributionsinadepthprofilelowfluxofprimaryionsStaticSIMS.Primaryionbombardmentresultsinsurfacedamageandemissionofionsandneutralatoms.Thedamagedareaisquantifiedbythedamagecross-section(σ).InstaticSIMSthetotalprimaryiondoseshouldbelimitedsothatonlysmallportionofthesurfaceisdamaged.a,surfacelayer;b,bulk;c,damagedzone;d,depthofdamagezone;r,radiusofdamagezone.thedensityofsurfaceatomsasabout1015atomscm−2lessthan10%ofthesurfaceatomsormoleculesarebombarded.9theprimaryionflux(Ip)damagecross-section(σ)1A=6.2×1018chargedparticlespersecond10−13cm2Ip=1Acm−2(=6.2×1012ionscm−2)t~1s<1013ionscm−2,forstaticSIMSfora10-mindurationIp<2.7nAcm−2102.InstrumentationSIMSinstrumentation.massanalyzersystemultra-highvacuumtrajectoriesofionsremainundisturbedduringSIMSsurfaceanalysis.112.1PrimaryIonSystemionsourceprimaryionswithacertainkineticenergyionfilterpurifiestheprimaryionsandrejectsunwantedionsintheprimaryionbeamdeflectormakesthefocusedionbeamrasteronthesamplesurfaceintwoorthogonaldirectionsAr+,Xe+,O2+,Ga+,Cs+,Bi+ionsElectronbombardmentsource.(∼105Am−2persolidangle)2.1.1.1electronbombardmentsources2.1.1Ionsources12Duoplasmatronionsource.ThemagneticfieldmarkedBintensifiestheplasmabyconfiningelectronsclosetotheaxis.upto107Am−2persolidangleLiquidmetalionsource.TheprimaryionbeamisextractedfromtheTaylorconeofliquidmetal.(∼1010Am−2persteradiansolidangle)highspatialresolution2.1.1.4Surfaceionizationsources(Cs+)2.1.1.2Plasmaionsources2.1.1.3LiquidmetalionsourcesTheO2+ionbeamiscommonlyusedbecauseitcanincreasepositivesecondaryionyield.Ga+(29.8◦C)assmallas10nmuniformkineticenergyandreasonablespatialresolutionBi(271.3◦C)132.1.2WienFilterTheWienfilterforionmassselection.Underthesameelectricfieldforacceleratingtheprimaryions,thetimefortwoionstoreachthesamplewillbedifferentiftheirmassisdifferent.TimeaccuracyisextremelyimportantinthemostwidelyusedformofstaticSIMS,time-of-flightSIMS.Theprimaryionbeamlikelycontainsionswithdifferentmasssuchasisotopicionsandimpurityions.samekineticenergyMass-dependentvelocity142.2MassAnalysisSystemDetect:ratiosofmasstoelectricchargemagneticsectoranalyzer,quadrupoleanalyzer,time-of-flightanalyzer2.2.1MagneticSectorAnalyzerextractionpotentialmagneticfieldstrengthmass-to-chargeratiosuitablefordynamicSIMSmassresolutionbysequentialselectioninwhichafewspecificelementalionsaremeasured.扇形磁场分析器四极杆分析器飞行时间分析器152.2.2QuadrupoleMassAnalyzerAquadrupoleanalyzer.TheoscillationsofionsaregeneratedbycombinedDCandACelectricfieldsusingfourcylindricalrods:(a)travelpathofsecondaryionsintheanalyzer;and(b)electrodearrangementoftheanalyzer.Toobtainawholemz−1spectrum,theanalyzerincreasesthevoltagesbutkeepstheratioofDC:ACvoltagesconstant.162.2.3Time-of-FlightAnalyzerTime-of-flightmassanalyzer.Thesecondaryionbeamisreflectedbyamirrortocorrecttheflighttimeofionswithidenticalmz−1.themostwidelyusedanalyzerinstaticSIMSsamekineticenergyMass-dependentvelocitypulsedprimaryions~10ns17popularforstaticSIMShighresolution,hightransmissionandhighsensitivityThemajorshortcomingoftheToFanalyzerisitsuseofpulseprimaryions.Theratioofprimarybeamon-tooff-timeisonlyabout10−4.Thus,itisnotefficientforanalysissuchasdepthprofilingofchemicalelements.183.SurfaceStructureAnalysisthestaticSIMStechnique(ToFSIMS)Time-of-flightSIMSinstrumentation.3.1ExperimentalAspectsPrimaryIonsFloodGunsolvestheproblemofsurfacechargingofinsulatingsamplesSampleHandlingavoidchemicalcontaminationandsurfacerougheningThepositioningofthesampleHigherenergymaynotbefavorablebecauseitcancauseexcessivefragmentationandmakeSIMSspectradifficulttoanalyze.stand-aloneinstrumenthigh-massionsincreasetheyieldofsecondaryions193.2SpectrumInterpretationPositiveionToFSIMSspectrumofpolystyrene.聚苯乙烯massisinatomicmassunits20NegativeionToFSIMSspectrumofpolystyrene.thespectracanbeeitherthatofpositiveionsornegativeions.21NegativeToFspectrumofNaNO3.SIMSspectraappearmorecomplicatedthanthoseofAESandXPSbecausetherearemorepeaksthanthebasicioncomponents.223.3ElementIdentificationThegreatmajorityofmetalsgiveexclusivelypositiveionsinstaticSIMSconditions.Themostelectropositiveelements,suchasalkaliandalkaliearthmetals,giveintensepositiveionpeaks.Themostelectronegativeelements,suchasO,F,Cl,BrandIgiveintensenegativeionpeaks.HydrogengivesahighyieldofbothH+andH−ions.TheCH−peakiscommonlymoreintensethantheC−peak.Nisnotusuallydetectablebyanelementalpeak.PisusuallydetectedfrompeaksofPO2−andPO3−.sub-monomer,n-merandoligomerAspectrumforpolymer:massvalues<themonomermassasmallnumberofmonomerunitsormonomerunitsplusfragmentsathighmz−1,inwhichpeaksaregeneratedbyaprocesscalledcationization.单体identifyingpolymermoleculesbasicchemicalinformationmoreinformationonthemacromolecularstructure.低聚物complicated23Cationizationisthelayingdownofathinlayeroforganicsubstanceorpolymeronasilversubstrate.ToFSIMSisabletodetectthepeaksoforganicmolecules(M)attachedtoAgas[Mx+Ag]+.Thepeaksofcationizationcanreachmz−1levelsatascaleof103.PositiveToFSIMSspectrumofapolymericsubstancethathasarepeatunitofC10H20.estimatingtheaveragemolecularweightdistributionofpolymericmaterials24254.SIMSImaginginascanningmode,similartoEDSandAESSIMSimagesofceramiccoatingontitaniumobtainedwithscanningToFSIMS.Ga+primaryionbeamgeneratesthepositiveionimagesofCa,Ti,OandOH.ChemicalgradientsofOandOHinthecoatinglayer(left-handsidesofimages)arerevealed.acrosssectionthecalciumphosphateCoatingTisubstrateThecommonlyuseddiametersofionbeamsforscanningimagesareabout200nmto1μm.磷酸钙26ImageQualityhighspatialresolutionwithhighsignal-to-noiseratiosSmallerpixelsizeNotsufficientsecondaryionsincreasetheionizationprobabilityprimaryionswithheavymasssurfacetopographicfeaturespossibleartifactsignalsfromsecondaryionswillvaryeventhereisnotchemicalvariation1.2.Particularly,forinsulatingsamplessampleshouldasflataspossibleWemayalsocheckwhetherthereisatopographiceffectbycomparingSIMSimagesforseveraltypesofsecondaryions.275.SIMSDepthProfiling5.1GenerationofDepthProfilesmagneticsectoranalyzer(highmassresolutionGoodtransmission)SIMSdepthprofileoftheGaAs–Si–Al2O3system.10m×10mto500m×500mbeamenergy1–20keVbeamdensity1Acm−2AnO2+ionbeamiscommonlyusedforpositiveionsprofiles.Cesiumions,Cs+,arewidelyusedforgeneratinghighnegativeionyieldsoftheelementsthatexhibitlowpositiveionyields.thedetectionlimit28Definitionofdepthresolutionanditseffectondepthprofiling.5.2OptimizationofDepthProfilingdetectionsensitivitydetectionlimitdepthresolutiontheabilitytodetectagivenelementinasamplethelowestatomconcentrationofagivenelementthatcanbedetectedinasampletheaccuracyofdepthmeasurementforele