PHOTOELECTRON SPECTROSCOPY (XPS) PRINCIPLES AND …

1 4/9/20171 PHOTOELECTRON SPECTROSCOPY (XPS) PRINCIPLES AND APPLICATIONSProf. NizamM. El-Ashgar PHOTOELECTRON SPECTROSCOPY XPS What is XPS? Aim of XPS Analysis. General Theory How can we identify elements and compounds? Instrumentation for XPS Examples of materials analysis with XPS4/9/20172 What is XPS? X-rayPhotoelectronSpectroscopy(XPS),also knownasElectronSpectroscopyforChemicalAn alysis(ESCA)isawidelyusedtechniquetoinve stigatethechemicalcompositionofsurfaces. Sampleisilluminatedwithsoft( )X-rayradiationinanultrahighvacuum. Thephotoelectriceffectleadstotheproducti onofphotoelectrons, , SPECTROSCOPY ,basedbasedononthethephotoe lectricphotoelectriceffect,effect,11,,22 waswasdevelopeddevelopedininthethemidmid --19601960 s sbybyKaiKaiSiegbahnSiegbahnandandhishisr esearchresearchgroupgroupatatthetheUnive rsityUniversityofofUppsala,Uppsala, H.

2 Hertz, Ann. Physik 31,983 (1887).2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in K. Siegbahn, Et. Al.,Nova Acta Regiae , Ser. IV, Vol. 20 (1967). 1981 Nobel Prize in Atom and the X-RayCore electronsValence electronsX-RayFree electronprotonneutronelectronelectron vacancyThe core electrons respond very well to the X-Ray energyX-Rays on the SurfaceAtoms layerse-top layere-lower layer with collisionse-lower layer but no collisionsX-RaysOuter surfaceInner surface4/9/20174X-Rays on the Surface TheX-Rayswillpenetratetothecoree-oftheat omsinthesample. Somee-saregoingtobereleasedwithoutanypro blemgivingtheKineticEnergies(KE)characte risticoftheirelements. Othere-swillcomefrominnerlayersandcollid ewithothere-sofupperlayers Thesee-willbelowerinlowerenergy. and the Aim of XPS Analysis Identificationofelementsatsurfaces(depth :10nm).

3 Identificationoffunctionalgroupsandredox states. Concentrationanddepthprofiling. Estimationofoverlayerthickness. Comparisonofbulkandsurfacecomposition. Surfacehomogeneity/heterogeneity Possiblesurfacemorphologyofadlayers. Physical Bases Baseduponphotonin/electronoutprocess. PhotonE(Einstein):E=h where:h-Planckconstant( ). -frequency(Hz)oftheradiation. InXPSthephotonisabsorbedbyanatominamolec uleorsolid,leadingtoionizationandtheemis sionofacore(innershell)electron. Ifthephotoninteractswithvalencelevels(io nizationbylostofoneofthem)thetechniqueis differentandcalledUPS, c effect 4/9/20177 Introduction to the Solid StateInsolids,atomicandmolecularenergyle velsbroadenintobandsthatinprincipleconta inasmanystatesasthereareatoms/moleculesi nthesolid. Process of Photoionization A+h A++e-E(A)+h =E(A+)+E(e-)KE=h [E(A+)-E(A)] Thedifferenceinenergybetweentheionizedan dneutralatoms,isgenerallycalledthebindin genergy(BE)oftheelectron:KE = h -BE4/9/20178nnXPSXPS spectralspectrallineslinesareareidentifi edidentifiedbybythetheshellshellfromfrom whichwhichthetheelectronelectronwaswasej ectedejected((11s,s,22s,s,22p,p, )).

4 NnTheTheejectedejectedphotoelectronphoto electronhashaskinetickineticenergyenergy ::KEKE==hvhv BEBE-- nnFollowingFollowingthisthisprocess,proc ess, BandConduction BandValence BandValence BandLL22,L,L33LL11 KKFermiFermiLevelLevelFree Free Electron LevelElectron LevelIncident XIncident X--rayrayEjected PhotoelectronEjected Photoelectron11ss22ss22ppThe Photoelectric Process 4/9/20179 TheBEisadirectmeasureoftheenergyrequired tojustremovetheelectronconcernedfromitsi nitialleveltothevacuumlevelandtheKEofthe photoelectronisagaingivenby:KE = h -BENOTE-thebindingenergies(BE)ofenergyle velsinsolidsareconventionallymeasuredwit hrespecttotheFermi-levelofthesolid, ( )ofthesolid,butforthepurposesofthediscus sionbelowthiscorrectionwillbeneglected. Binding Energy (BE)vTheBindingEnergy(BE) , , + the Core Electrons? AnelectronneartheFermilevelisfarfromthen ucleus,movingindifferentdirectionsallove rtheplace,andwillnotcarryinformationabou tanysingleatom.

5 Fermilevelisthehighestenergyleveloccupie dbyanelectroninaneutralsolidatabsolute0t emperature. Electronbindingenergy(BE)iscalculatedwit hrespecttotheFermilevel. Thecoree-sarelocalclosetothenucleusandha vebindingenergiescharacteristicoftheirpa rticularelement. Thecoree-shaveahigherprobabilityofmatchi ngtheenergiesofAlK andMgK e-Valence e-Atom19 XPS (quantitativeanalysisofthesurfacecomposi tion).4/9/201711KE versus eV0 eVBE increase from right to leftKE increase from left to rightBinding energy# of electrons(eV)KE = hv BE - XPS Spectrum TheXPSpeaksaresharp. InaXPSgraphitispossibletoseeAugerelectro npeaks. TheAugerpeaksareusuallywiderpeaksinaXPSs pectrum. SpectrumO 1sO becauseof Mg sourceCAlAlO 2sO AugerIdentification of XPS Peaks Theplothascharacteristicpeaksforeachelem entfoundinthesurfaceofthesample. TherearetableswiththeKEandBEalreadyassig nedtoeachelement.

6 Vs. e-Beam X-Rays Hitallsampleareasimultaneouslypermitting dataacquisitionthatwillgiveanideaoftheav eragecompositionofthewholesurface. ElectronBeam Technology Considerasnon-destructive. becauseitproducessoftx-raystoinducephoto electronemissionfromthesamplesurface Provideinformationaboutsurfacelayersorth infilmstructures Applicationsintheindustry: Polymersurface Catalyst Corrosion Adhesion Semiconductors Dielectricmaterials Electronicspackaging Magneticmedia Thinfilmcoatings4/9/201714 How Does XPS Technology Work? Amonoenergeticx-raybeamemitsphotoelectro nsfromthefromthesurfaceofthesample. Thepenetrationofthex-rayphotonsaboutamic rometerofthesample TheXPSspectrumcontainsinformationonlyabo utthetop10-100 ofthesample. Ultrahighvacuumenvironmenttoeliminateexc essivesurfacecontamination. Cylindrical Mirror Analyzer (CMA) or hemispherical sector analyzer (HSA)measure the KE of emitted e-s.

7 Thespectrumplottedbythecomputerfromthean alyzersignal. :COMPONENTS OF XPSvA source of X-raysvAn ultra high vacuum (UHV) vAn electron energy analyzervmagnetic field shieldingvAn electron detector systemvA set of stage manipulators4/9/201715 Thebasicrequirements: Asourceoffixed-energyradiation(anx-rayso urce). Anelectronenergyanalyzer(whichcandispers etheemittedelectronsaccordingtotheirkine ticenergy,andtherebymeasurethefluxofemit tedelectronsofaparticularenergy). Ahighvacuumenvironment(toenabletheemitte dphotoelectronstobeanalyzedwithoutinterf erencefromgasphasecollisions).XPS InstrumentX-Ray SourceIon SourceSIMS AnalyzerSample introductionChamberSecondary ion mass spectrometry(SIMS)4/9/201716 X-ray PHOTOELECTRON SpectrometerDiagram of the Side View of XPS SystemX-Ray sourceIon sourceAxial Electron GunDetectorCMAsampleSIMS AnalyzerSample introduction ChamberSample HolderIon PumpRoughing PumpSlits4/9/201717 Schematic Diagram of XPS Energy of LightWavelength( )106 m103 m1 m10-3 m10-6 mEnergy(E)Broad-castShort wave radioX-ray1 MeV1 KeV1 eV10-3eV10-6eV4/9/201718X-Ray Sources Irradiatethesamplesurface,hittingthecore electrons(e-)oftheatoms.

8 TheX-Rayspenetratethesampletoadepthonthe orderofamicrometer. Usefule-signalisobtainedonlyfromadepthof around10to100 onthesurface. Normally,thesamplewillberadiatedwithphot onsofasingleenergy(MgK orAlK ). ThisisknownasamonoenergeticX-Raybeam. TheX-Raysourceproducesphotonswithcertain energies: MgK photonwithanenergy:h = AlK photonwithanenergy:h = Source Requirements 1. High X-ray flux at sample 2. Narrow X-ray line width 3. Multiple-anode capability 4. Ability to move the X-ray source (X-Y) and retract it (Z) 5. Long anode and filament lifetime 4/9/201719 TheX-raysourceforXPS: Incontrasttotheelectronsource,theX-rayso urceenergydependsonthechoiceoftheanodema terial,resultingintheavailabilityofanumb erofdiscreteenergiesratherthanacontinuou svariationoftheenergy,asexistsforelectro nandionguns. Thephotonenergymustbesufficientlyhightoe xciteintensephotoelectronpeaksfromallele mentsoftheperiodictable.

9 ForXPSanalysis,itisveryimportanttoconsid ertheenergyresolutionoftheprimaryX-rays. X-ray tubeEarly x-ray spectrum from x-ray tubeCharacteristic linesfrom the X ray fluorescence process (XRF) and a broad background (Bremsstrahlung), which is strongly depends on the energy of the electronMonochromatic to achieveSampleSampleXX--ray Anoderay AnodeEnergy Energy AnalyzerAnalyzerQuartz Quartz Crystal DisperserCrystal Disperseree--Rowland CircleRowland Circlen =2dsin Forquartz(1010)surface,d= , properties of synchrotron radiation:high intensitytunability in wide range near-coherencepolarized. pulsedwell collimatedNUS has such a source in Singapore!Sample charging effectsThelightforXPSalwayschargessurfac epositively(shiftingofspectrumtohigherbi ndingenergy)andleadstogeneralinstability (spectralnoise).Forthemetalsample, ,forinsulator, 1s shifts due to the chargingFor XPS (even For XPS (even AES) never forget AES) never forget ground the ground the sample !

10 !!sample !!!4/9/201722 Inhomogeneous Surface ChargingChargingcanevenchangethelineshap eduetoInhomogeneousSurfaceCharging, ,thereisonlyspectralshiftduetocharging,w hichcanbedeterminedbycomparisonwithknown elementalXPSlines, System Theinstrumentusesdifferentpumpsystemstor eachthegoalofanUltraHighVacuum(UHV)envir onment. of Texas at El Paso, Physics DepartmentSide view of the Phi 560 XPS/AES/SIMS UHV System4/9/201723 UHV for Surface Analysis? WhyXPS ,ifpossible, 4/9/201724 Instrumentation (analyzers)Analyzer:Mostessentialpartofa nyelectronspectroscopy,itscharacteristic are:energyrange,energyresolution, :retarding oftheincomingelectron,selectionoftheelec tronswithrightkineticenergy(passenergy), detectingoftheelectrons(channeltron)Dete ction of electron energy (Analyzers) MainlytwotypesofdetectorsareusedinAESand XPSsystems:Thecylindricalmirroranalyser( CMA)andthehemisphericalsectoranalyzer(HS A).