mm653 esca
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Electron Spectroscopy for Chemical A nalysis
MM653
5
This material is strictly for the MM653 course andshall not be utilised for any other purpose.
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Surface Analysis - Wish ListWhat Can be done?
photons
ions
electrons
EMISSION
TRANSMISSION
Interactionwith material
EXCITATIONProperties and reactivity of the
surface will depend on: bonding geometry of molecules
to the surface physical topography chemical composition chemical structure atomic structure
electronic state
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A sample with a surface of size 1 cm 2 - this will have ~ 1015 atoms in
the surface layer. In order to detect the presence of impurity atoms
present at the 1 % level, a technique must be sensitive to ~ 10 13 atoms. Contrast this with a spectroscopic technique used to analyse
a 1 cm3 bulk liquid sample i.e. a sample of ca. 10 22 molecules. The
detection of 10 13 molecules in this sample would require 1 ppb (one part-per-billion) sensitivity - very few techniques can provide
anything like this level of sensitivity.
Add to this the need of Selectivity to surface atoms only!!!
Surface Analysis constraints
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The Photoelectron ProcessX-rays in photoelectrons out
Sample Surface Layer
binding energy (eV) = photon energy - kinetic energy - work functionBE (eV) = h - KE -
fEv
Ef
KE
BEvalenceband
corelevels
photon
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After Photoemission
Fluorescence or Auger electron emission
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The Photoelectron Spectrum
d
Photoelectrons out(elastically scattered)
X-rays in
d = 3l
Photoelectronsout (inelastically,
sc
attered)
Bonding to anti-bonding orbitaltransitions lead to peaks at higher
binding energy (lower kinetic
energy).
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Penetration depth of the X-ray radiation is 10 2-10 3 nm.Surface sensitivity arises from the short distance thephotoelectrons can travel in the solid before suffering
inelastic scattering.
Surface Sensitivity ofXPS
d
Photoelectrons outX-r ays in
d = 3l
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The average distance from the surface a photoelectroncan travel without energy loss is defined as the inelasticmean free pathlength (IMFP), l .
Sampling depth, d, defined as the average distancefrom the surface for which 95% of photoelectrons aredetected, d = 3 l .
Surface Sensitivity of XPS
universal curve
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X-ray Photoelectron
spectroscopy is...
Surface sensitive - photoelectron signal from first 1-10layers of atoms and molecules.Quantitative (!!?). Provides insight into the chemical state of the element. Sensitive - detection limit ~0.1 atomic %.
Able to detect all elements except H and He. Nondestructive analysis.
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JJ Coupling, Temperature effect and Life time broadening
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The Photoelectron Spectrum
Inelastically scattered photoelectrons
Both photoemission and Auger peaks
observed in a spectrum.Peaks are superimposed on a risingbackground, due to inelasticallyscattered photoelectrons.
Cu 2p
O KLL Auger
O 1s
N 1s
C 1s
Cu LMMAuger
Cu 3p
Cu 3sCl 2p
Inelastically scattered photoelectrons contribute tothe spectral background
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970 960 950 940 930
S SCu 2p 1/2Cu 2p 3/2
S = Shake-up satellites
Photoelectron energy reduced by associated * transition
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Chemical State InformationThe binding energy of an electron is dependent on the atomic orbital the
electron occupies and the chemical environment of the atom.
The variation of binding energy of a specific photoemission peakprovides information on the chemical state of the atom or ion.
Core level electron, high binding energy
Valence electron, low bindingenergy
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Chemical shift due toOxidation stateHybridisationIonic character
ie. nature of electron distribution
Ti 2p
TiO 2
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XPS S Sh i h Ch i l S
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XPS Spectra Showing the Chemical Stateof Si
Si elemental
Si oxide
Si oxide Si elemental
Two samples with different SiO 2 filmthicknesses on Si substrate.
-note large chemical shift betweenelemental Si and silicon dioxidepeaks.
d d
Si elemental
Si oxide
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X-ray sourceElectron energy AnalyserElectron CounterVacuum to
Save hot componentsTube filament
Save surface fromAdsorption forsome time
Avoid scattering ingas phase
Instrumentation for XPS
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X-rays generated by accelerating high energy electrons onto an anode. The coreholes created decay by emission of X-rays.
Commonly used X-ray sourcesanode material energy (eV) Width (eV) Mg 1253.6 0.7Al 1486.6 0.85
Monochromated vs non-monochromated X-ray source
Bremstrahlung radiation
K a 1,2
X - r a y
I n t e n s i
t y ( a r b
i t r a r y u n
i t s )
X-ray Energy / kV
X-ray Intensity as a Function of Energy
K a 3,4
Use of a monochromator preventselectrons, Bremsstrahlung, satelliteX-ray lines and heat radiationstriking the sample. The monochromator alsodecreases the energy spread of theX-rays.
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Thin layer coatings increasingly used inindustry to improve surface properties.Depth profiling combined with XPS allowsvaluable film thickness and chemical stateinformation to be determined.
Depth Profile through a TiN/SiO 2 thin film on Si.
TiN
SiO 2
Si substrate
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High resolution of the Ti regionindicates that Ti is also present asTiOx in the TiN layer.
TiOx persists through the entireTiN layer, as shown in the Ti 3dregion recorded from the subsurface.
Depth Profile through a TiN/SiO 2 thin film on Si.
TiOTiN
TiO 2
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Depth Profile through a TiN/SiO 2 thin film on Si.
Full Chemical stateconcentration depthprofile through TiN filmallowing determinationof film thickness.
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TiNSiO 2
Si
Si 2p region as a functionof depth from the surface
Si 2p region shows
chemicalenvironment of theSi atoms.
Depth Profile through a TiN/SiO 2 thin film on Si.
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X-ray monochromator
Quartz (100) crystal diffraction Line width to 0.25 eV (Al), Cuts Bremsstrahlung, satellitesCan focus to
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Monochromated vsnon-monochromated X-ray source
Monochromated Al K a excited Ag spectrum
Non-monochromated Mg K a excited Ag spectrum
FWHM 0.97 eV
FWHM 0.46 eV
Quantitative Surface Analysis of Poly(ethylene
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Quantitative Surface Analysis of Poly(ethylenetetraphthalate) - PET
C 1s region O 1s regionO(1) 530.8eV 51 at%O(2) 532.1eV 49 at%
C(1) 285.0eV 65 at%C(2) 286.5eV 23 at%C(3) 289.2eV 12 at%
C3C2
C1 O1O2
-(-O-C- -C-O-CH 2-CH 2-)-= = O O
n
2223
1
3 2
1
1
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Angular Dependence of XPS
d
d=3l sinq
d*
photoelectronsX-raysX-rays photoelectrons
d > d*
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Angular Dependence of Phototelectron yield
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Variable take-off angle: sample rotated to increasesurface sensitivity
3l
q
d= 3 l sinq
X-rays
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3l
q
d= 3 l sinq
X-rays
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Angular Dependence of XPS0 deg (bulk sensitive)
60 degrees
45 degrees
75 degrees(surface sensitive)
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Non-destructive depth profile
By rotating the sample about its axis, the samplingdepth can be changed.
collecting data at different angles, will provide a non-destructive depth profile.
this is limited to film thicknesses less than the samplingdepth (~100 ).
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Depth Profile - destructive
Destructive depth profile can be achieved by Ar+ bombardment of the sample to remove
surface atoms, followed by data acquisition.
When the etch / spectrum cycle is repeateda destructive depth profile of several 1000s through the sample may be acquired.
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Thin layer coatings increasingly used inindustry to improve surface properties.Depth profiling combined with XPS allowsvaluable film thickness and chemical stateinformation to be determined.
Depth Profile through a TiN/SiO 2 thin film on Si.
TiN
SiO2
Si substrate
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High resolution of the Ti regionindicates that Ti is also present asTiOx in the TiN layer.
TiOx persists through the entire
TiN layer, as shown in the Ti 3dregion recorded from the subsurface.
Depth Profile through a TiN/SiO 2 thin film on Si.
TiOTiN
TiO 2
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Depth Profile through a TiN/SiO 2 thin film on Si.
Full Chemical stateconcentration depthprofile through TiN filmallowing determinationof film thickness.
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TiNSiO 2
Si
Si 2p region as a functionof depth from the surface
Si 2p region shows
chemicalenvironment of theSi atoms.
Depth Profile through a TiN/SiO 2 thin film on Si.
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Elemental Distribution
Destructive and non-destructive depth profilingprovides information on elemental distribution
from the surface into the bulk.
What about elemental distribution across thesurface?
What modes of acquisition can be used to probelateral (2-dimentional) distribution at the surface?
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Definitions - resolution and sensitivity
Sensitivity - counts per second (cps) at the peakmaximum.
Full width at half maximum (FWHM)- the width of a
peak in eV defined at the point half way from the baselineto the peak maximum
Techniques Available
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Techniques Available
Analytical Technique Signal Measured Elemental Range Depth Resolution Surface info.
SIMS Secondary Ions H-U 5-30 Chemical composition(secondary ion mass spectrometry) Chemical structure
TOF-SIMS Secondary Ions H-U, Large Organic 2000 (Scanning Mode) Adsorbate bonding(time-of-flight SIMS) Molecules / Cluster Ions
TEM Transmitted Electrons X-Rays Na-U EDX N/A(transmission electron microscopy)
FE-SEM, EDX Backscattered or Na-U 1 - 5 micrometres(field emission SEM) Secondary Electrons and X-Rays
ISS Ions H- U monolayer atomic structure(ion scattering spectroscopy) chemical composition
AES/SAM Auger Electrons Li-U 2-30nm chemical composition(Auger electron spectroscopy, scanning Auger microscopy)
ESCA/XPS Photoelectrons Li-U 5 - 30nm chemical composition(electron spectroscopy for chemical analysis, X-ray photoelectron spectroscopy) chemical structure
RAIRS IR photons organic, some inorganics monolayer Adsorbate bonding(reflection-absorption infra-red spectroscopy)STM - solid surfaces upper most atoms physical topography(scanning tunnelling microscopy)
Analytical Technique Signal Measured Elemental Range Depth Resolution surface info,
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X M h
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X-ray Monochromator -The Rowland circle geometry
Energy dispersion E ~ Rowland circle diam.
For 500 mm ~ 0.625 eVmm-1 250 mm ~ 1.25 eVmm-1
Fixed mono spotEnergy dispersivedirection, E
Toroidal quartzbackplane
Electron gun& x-ray anode
Rowlandcircle
diameter
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Cylindrical Mirror Analyser
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