Quantification of XPSMost analyses use empirical calibration constants (called atomic sensitivity factors) derived from standards:Note: ASF for H, He very small - undetectable in conventional XPS!Note: XPS spectrum will show all peaks for each element in same ratioNote: Not all XPS peaks for an element same intensity (in area ratio proportional to ASF's) -choose peak with largest ASF to maximize sensitivityNote: Sensitivity for each element in a complex mixture will varyHow to measure peak area IMust include or correct for (i) x-ray satellites (ii) chemically shifted species (iii) shake-up peaks (iv) plasmon or other lossesNon-destructive Angle resolved analysis Sampling depths 10 to 100 Can use different x-raysourcesDestructive Energetic ion beam sputtering Sputter rates 10 to 100 /minXPS depth profiling analysisX-rayAngle resolved XPSHowcan we change the degree of the surface sensitivity?(1) To reduce kinetic energy of photoelectrons to the energy corresponding to the minimum in the universal curve for the IMFP, through decreasing of X-ray photo energy.(2)To collect photoelectrons at more grazing emission angleX = dcosThe emission angle () is increased, the surface sensitivity is increased.Angle-dependent of Si 2p spectra obtained from a Si wafer with a native oxide Non-destructive depth profileARXPS measurementsCalculate peak intensityGenerate profile with modelsMaximum-entropy modelProg. Surf. Sci. 69(2002)125Factor limiting depth resolution and accuracy of profiles Physical factors:Ion energyAngle of ion incidence Ion species Instrumental factors:Adsorption from residual gas atmosphereRedeposition of sputtered speciesimpurities in beam(including neutrals, multiple charge etc)Non-uniform ion beam intensity and time-dependent ion beam intensityCrater qualityinformation depth Sample characteristics:Original surface roughnessCrystalline strusture and defectsAlloys, compounds, second phases, insulators Radiation-induced effects:Primary ion implantationAtom mixingSputtered-induced roughnessPreferential sputtering and decomposition of compoundsEnhanced diffusion and segregationCharging of insulatorsOptimized depth profiling conditionsIon Beam: Lowbeam energyLarge incidence angle for smooth sampleSmall incidence angle for rough sampleHigh mass ion speciesSample rotationLarge sputtered areaPure gas feedInstrumental: Low partial pressure of reactive gasesSmall analysis area and centred in craterLow kinetic energy signal (low information depth)Issue Energetic ion beam can change sample chemistry All organic information lost Preferential sputtering may changeobserved stoichiometrySample characteristics:Smooth, polished surfaceNon-crystalline, no second phasesComponents with similar sputtering yieldsGood electrical and thermal conductivitiesLow interdiffusivitiesLow Gibbsean segregation tendenciesSurfaceafter sputtering Before sputtering2 keV 75o0oWith sample rotation10 keV Imaging XPSGold x-ray photoelectron image (Au4f)Chemical state x-ray photoelectron imageThe spatial resolution is poorer than that of other imaging techniques such as SEM, STM et al.An XPS instrument has two main components: An X-ray source An electron energy analyzerOther optional items: A low energy electron flood gun An ion sourceXPS instrumentationAll measurements must be made in ultra-high vacuum (UHV)X-ray tubeEarly x-ray sourceStandard lab X-ray source is byvery high energy ebeam hitting theanode.X-ray GenerationConduction BandValence Band1s2s2pConduction BandValence BandL2,L3L1KFermiLevelFree Electron Level1s2s2pSecondary electronIncidentelectronX-ray PhotonWhy were Mg and Al k X-ray used in XPS?Twin anode (Mg/Al) sourceSimple, relatively inexpensiveHigh flux (1010-1012photonss-1)PolychromaticBeam size ~ 1cm Better energy resolution that allows to separate photoelectron lines with close binding energies Improved signal to background ratio by removing background caused by satellites and bremstrahlung. This allows to obtain useful signals from elements present at small concentrations, or from small analysis areas Minimizes sample damage from heat and unwanted radiation by separating X-ray source from sampleWhy Use an X-Ray Monochromator?Monochromators are expensive and decrease the X-ray flux, which means longer measurement times and less throughputPrinciple of monochromatization of x-rayBragg relation:n = 2dsinn:diffraction order:x-ray wavelengthd: crystal spacing: Bragg angleSchematic illustrating Bragg angleSynchrotron RadiationThe synchrotron storage ring is a tubular vacuum chamber made to: Hold an electron beam travelling through it at nearly the speed of light. Maintain the high energy of the electron beam. As the accelerating electrons circle the ring at relativistic velocities, they give off intense beams of light including x-rays. By using a monochromator the light will be Monochromatic.Key properties of synchrotron radiation:high intensitytunability in wide rangenear-coherencepolarized. pulsedwell collimatedNUS has such a source in Singapore!resolutionAcceptance angleAnalyzer: mostessential part of anyelectronspectroscopy, itscharacteristic are:energy range, energyresolution, sensitivityand acceptanceangle. Normally itsfunctions involve:retarding of theincoming electron,selection of theelectrons with rightkinetic energy (passenergy), detecting ofthe electrons(channeltron)Electron Energy AnalyzersMost common type of electrostatic deflection-type analyzer called the concentric hemispherical analyzer (CHA) Negative potential on two hemispheres V2 > V1Potential of mean path through analyzer isAn electron of kinetic energy eV = V0will travel a circular orbit through hemispheres at radius R0Since R0, R1and R2are fixed, in principle changing V1and V2will allow scanning of electron KE following mean path through hemispheres1/2Energy of resolution of Electron Energy AnalyzersA schematic cross section of the ESCALAB spectrometerSimplified optical schematics for the ESCALAB input lens system -Large area XL spectroscopy modeXPS at the Magic AngleOrbital Angular Symmetry FactorLA() = 1 + A(3sin2/2 - 1)/2where: = source-detector angle = constant for a given sub-shell and X-ray photonAt 54.7 the magic angleLA= 1XPS systems in IMREESCALAB 220i-XL system Small area XPS (20m) Imaging XPS (~2 m) Superior depth profiling He+ ion scattering UPSTheta Probe system Analysis area (15m to 400 m) Parallel ARXPS without sample tilting Superior depth profiling SEM AESSynchrotron realisationLaboratory realisationStrengths and WeaknessesStrengths Elemental identification of all elements except H and He Chemical state identification (oxidation state) Quantitative analysis Can analyze insulating samples Non-destructive depth profile by changing take-off angleWeaknesses Complex, expensive instrumentation, UHV technique Detection limits typically ~500 ppm Not usually spatially sensitive (smallest analytical area ~20 m) Limited organic information Sampling depth varies with electron KE (and material, samplingdepth typically 20-100 )