XPS Philips

8/3/2019 XPS Philips

http://slidepdf.com/reader/full/xps-philips 1/4

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



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.



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.



©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.

XPS Philips

Documents