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X-Ray Photoelectron
Spectroscopy (XPS/ESCA)
Binding Energy
Kinetic Energy
5d
4f
4d
4p
4p 4s
XPS (X-ray Photoelectron Spectroscopy) or ESCA
(Electron Spectroscopy or Chemical Analysis) is based
on the principle that X-rays hitting atoms generate
photoelectrons. It is a typical example o a surace-sensitive
technique. Only electrons that are generated in the top
ew atomic layers are detected. In this way quantitative
inormation can be obtained about the elemental
composition o the surace o all kinds o solidmaterial (insulators, conductors, polymers). An important
strength o XPS is that it provides both elemental and
chemical inormation.
•surfaceanalysis
•chemicalcomposition
•airsensitivesamples
•informationdepth
1nm–2µm
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Basic principle
BombardingasampleinvacuumwithX-rays
givesrisetotheemissionofelectrons.If
monochromaticX-raysareusedwitha
photon energy hν,thekineticenergyofthe
emitted electrons Keisgivenby:
whereBeisthebindingenergyofthe
atomicorbitalfromwhichtheelectron
originates and φistheworkfunction.The
workfunctionistheminimumamount
ofenergyanindividualelectronneedsto
escapefromthesurface.
Eachelementproducesauniqueset
ofelectronswithspecicenergies.By
measuringthenumberoftheseelectrons
asafunctionofkinetic(orbinding)energy,
anXPSspectrumisobtained.Allelements
canbedetected,exceptHandHe.Figure1
showsanexampleofasurveyspectrumof
aSAMlayer(self-assembledmonolayer)on
gold.XPSpeaksofC,O,Au,NandScan
beobserved.
Chemical state
Bindingenergiesofphotoelectronsdepend
onthechemicalenvironmentoftheatoms.
Accuratemeasurementoftheexactpeak
positionoftheelementspresentgives
informationonthechemicalstateofthese
elements.Figure3showsamoredetailed
XPSspectrumofthesulphurpeakof
theSAMlayer.Agoodtofthesignalis
obtainedusingtwodoublepeaksindicating
twodifferentchemicalenvironmentsofthe
sulphuratoms.
Depth profling
Toobtaininformationatlargerdepths
(1–2µmatmost),concentrationproles
canberecordedbyalternatingsputtering
withAr+-ionsandspectrumcollection.
Typicalsputterratesare5-10nm/min.
Adisadvantageofsputteringisthatthe
chemicalstateoftheelementspresent
maychangeduetotheionbombardment.
Inaddition,theelementalcompositionmay
beinuencedbypreferentialsputtering.
Inert atmosphere
Itisalsopossibletoanalyzesamplesthat
aresensitivetooxygen,nitrogenand/or
water.Thesesamplescanbepreparedin
an argon atmosphere and transported to
theXPSinstrumentwithoutexposureto
ambientair.
Micro XPS
WiththeaidofafocusedX-raybeamitis
possibletoobtaindatafromareasassmall
as10µmindiameter.Scanningthebeam
overthesampleallowstheacquisitionofa
two-dimensionalelementmapasshownin
gure4.Thismapmakesaccurate
positioningoftheX-raybeampossible
enablingspatiallyresolvedstudies.
01002003004005006007000
1
2
3
4
5
6x 104
Binding Energy (eV)
C P S
A u 4 p 1
A u 4 p 3
A u 4 d 3
A u 4 d 5
A u 4 f
O 1
s
O 2
s
C
1 s
S 2 s
S 2 p
N 1
s
208
170 168 166 164
Binding Energy (eV)
162 160 158
210
212
214
216
218
220
222
224
226
C P S
S 2p
S 2p1-low
S 2p3-lowS 2p1-high
S 2p3-high
SH NH O
OO NH
S
N
H
NH
O
OO
φ ν −−= Bh K
Fig. 1: XPS spectrum o a sel-assembled monolayer (SAM-layer) o
biotinylated alkyl thiol (see fgure 2) on gold. XPS peaks o C, N, O, S
and Au, can be obser ved. The appendix to the element symbol, e.g. 1s
or 2p. denotes the atomic orbital rom which the electrons originate.
Fig. 2: Molecular structure o biotinylated alkyl thiol (BAT).
Fig. 3: XPS spectrum o S in BAT on gold (see fgure 2). Two doublets
(high and low) can be ftted to the signal indicating two dierent
chemical states o sulphur. The S 2p-low doublet at 161.9 eV
corresponds to the sulphur as Au-thiolate, the S 2p-high doublet at
163.9 eV corresponds to sulphur in the unbound thiophene groups.
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Quantitative analyses
Thenumberofdetectedelectronsisa
measurefortheelementalconcentration.
Inordertoobtainquantitativeresults,peak
areasaredividedbystandardsensitivity
factorsandnormalizedto100%toobtain
atomicconcentrations.Inthisway,for
bulkmaterialsthesurfacecompositioncan
bedeterminedwitha20%inaccuracyin
concentration.However,mostmaterials
donothaveahomogeneouschemical
compositionintheupperfewnanometers,
butratheracompositionthatvariesas
afunctionofdepth.Inthecaseofareal
multi-layersystemthesignalofanelement
inalowerlayerwillbeattenuatedmore
stronglythanthesignalfromanelement
inthetoplayer.Toobtainquantitative
informationfornon-homogeneoussamples,
eitherangle-resolvedmeasurementsor
modelcalculationscanbeperformed.
Angle-resolvedmeasurements
Awaytogetmoreinsightintothe
compositionofanon-homogeneous
sampleistomeasureanumberofspectra
atdifferentmeasuringangles.Variationof
theemissionanglecauseschangesinthe
effectiveinformationdepthofanalysis.At
glancingincidence(smallangles)onlythe
upperlayersofthesampleareexamined;
athighmeasuringanglesdeeperlayers
aredetected.WithrespecttoSAM
layersangle-resolvedmeasurementsgive
qualitativeinformationabouttheposition
ofthesulphurinthelayer.Ingure6ratios
areshownoftherelativeconcentrations
ofdifferentelementsinaSAMlayer,
measuredatdifferentangles.Thelowerthe
concentration ratio the deeper the element
ispositionedinthesample.Obviously,the
sulphurisclosesttotheAusubstrate
(seegure5).
Modelcalculation
Fortheanalysisofmultilayersystems,
amodelcalculationmethodhasbeen
developed. With the model only one
measurementatonemeasuringangle
is needed to determine thickness and
compositionofeachlayeronthesubstrate.
SAMlayersongoldcanbeseen
asmultilayersystemsduetotheirordered
structures.Forsuchlayersthemodelgives
insightintothelayercompositionandallows
thecoverageofthegoldwithsulphurtobe
calculated(gure7).
Au
Gold Substrate
S
R
Au
S
R
Au
S
R
Au
S
R
Au
S
R
Au
S
R
0
0.5
1
1.5
2
2.5
3
3.5
4
20 40 60 80
measuring angle (degrees)
c o n
c e n t r a t i o n
r a t i o
Au
C
O
S
Fig. 4: At the let a photograph o a pattern o gold
lines in a biosensor is shown. These gold lines are
present at dierent depths within the device. At
the right the Au X-ray image o the same sensor
is shown. Only the gold line on the upper surace
shows up. This area is covered by a SAM layer.
Fig. 5: Schematic o a sel-assembled alkane-thiol layer on gold. SAM
layers play an important role in the development o biosensors. The
strong chemical interaction between the thiol (SH) and the gold
surace plus the chain-to-chain interaction o the molecules (e.g. van
der Waals orces) orces the molecules to align parallel to each other
on the gold surace.
Fig. 6: Concentrations measured at 25 and 45 degrees divided by the
concentrations measured at 90 degrees or a SAM layer based on CH3 – O -
(CH2 – CH2 – O)3 – (CH2)6 – SH on gold. The ratios (especially at 25 degrees)
give a good idea about the position o the dierent elements in the sample. C
and O are present in the top layer with O at the outer surace. S is closest to
the Au substrate.
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©2008 Koninklijke Philips Electronics N.V.
All rights reserved.
Characteristics
Sample type
•solidmaterials(bulk,powders,
multi-layers)
Sample sizes
•fromafewmm2upto70mmin
diameter;themaximumallowable
thicknessofsamplesis25mm
Informationdepth
•1-10nm,bysputteringwithAr+-ions
uptoapproximately2µm
Lateralresolution
•≥5µmforbothimagingand
spectroscopy
Detection limit
•0.01atom%forheavyelementsand
0.1atom%forlightelements
Elemental range
•AllelementsexceptHandHe
MiPlaza Materials Analysis
offersafullrangeofanalyticalmethods
andexpertisetosupportbothresearch
andmanufacturing,servingcustomers
bytakinganintegral,solution-oriented
approach.
World-class expertise –
working or youFormoreinformation:
Tel./fax:+31-40-2748044/42944
E-mail:[email protected]
www.miplaza.com/materials
TechnicalNote3
August2008
Remote analysis
TheavailableXPS-instrumentsarepart
oftheVirtualLaboratoryofMiPlaza
MaterialsAnalysis.TheVirtualLaballows
customerstocollaboratereal-timewiththe
XPS-operatorduringtheanalysisoftheir
samples.
TheremotecustomeronlyneedsaPCwith
Internet-browserandobtainsaccessviaa
fullyprotected,encryptedconnection.To
setuptheconnectiontheremotecustomer
only needs a session ID; the session ID is
suppliedbytheoperatorandisvalidforone
session only.
Moreinformationtobefoundat
http://s2s.hightechcampus.nl
Applications
•Characterizationofthinlayers(<10nm)
Determinationofcompositionand
effectivelayerthicknessofmultilayer
systems(high-koxidelayersonSi,self-
assembledmonolayersonagoldsubstrate,
monolayersofbiologicalmaterialssuchas
proteins,antibodiesandDNA)
•Characterizationofthicklayersby
sputtering
Determinationofthecompositionof
layered-structureslikeITO-Cr-SiN
systemsandofadditional(oxide)interface
layers herein
•Contamination
ofsurfacesofwires,glass,leadframes,
ribbons,innersurfaceoflampbulbs
•Effectofspecictreatments
likecleaning,heating,oxidationorgas
treatmentsonthesurfacecomposition
•Causeofbadadherence
ofe.g.crystalsonleadframes,glass-metal
interfaces
•Chemicalinformation
Identicationofthechemicalstate
inwhichelementsarepresent,i.e.
determinationwhetherametalisoxidized,
ifasaltispresent,valenceofspecic
elements in glasses
“CO”, O, N, S S-high (thiophene)
“CH2” S-low (Au-thiolate)
Au substrate
SAM layer4.8 nm thick S coverage: 3.9 1014 at/cm2
80 atom % C9.2 atom % O7.3 atom % N1.6 atom % S-low
1.4 atom % S-high
Fig. 7: Multilayer model o BAT on gold (see fgure 5). This SAM layer can be seen as two organic layers on top
o the gold substrate: a lower layer containing the –(CH2)x – part o the molecules and a top layer containing
predominantly CO groups and N, O and S o the thiophene group. The S o the thiol group is present at the
gold surace only. One single measurement results in the ‘raw’ concentration. A subsequent model calculation
provides the real layer composition, as shown next to the fgure.
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