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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