Electron Spectroscopy for Surface Analysis

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Outline Introduction XPS Background XPS Instrument How Does XPS Technology Work? Auger Electron Cylindrical Mirror Analyzer (CMA) Equation KE versus BE Spectrum Background Identification of XPS Peaks X-rays vs. e- Beam XPS Technology

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Introduction

Electron spectroscopy (ES) is a technique that uses characteristic electrons emitted from solid for elemental analysis, not for imaging as in electron microscopy;

The characteristic electrons (either Auger electrons or photoelectrons) exhibit characteristics energy levels, revealing the nature of chemical elements in specimen being examined;

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Auger or photoelectrons can only escape from the uppermost atomic layers of solid (a depth of 10 nm or less) because their energies are relative low (generally 20-2000 eV);

While the characteristic X-rays can escape from a much greater depth (several micrometers from the surface);

Thus, ES is a technique for surface analysis; There two types of ES: Auger electron

spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS);

Auger electrons and photoelectrons are different in their physical origins, but both types of electrons carry similar information about chemical elements in material surfaces.

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X-ray Photoelectron SpectroscopySmall Area Detection

X-ray BeamX-ray Beam

X-ray penetration X-ray penetration depth ~1depth ~1m.m.Electrons can be Electrons can be excited in this excited in this entire volume.entire volume.

X-ray excitation area ~1x1 cmX-ray excitation area ~1x1 cm22. Electrons . Electrons are emitted from this entire areaare emitted from this entire area

Electrons are extracted Electrons are extracted only from a narrow solid only from a narrow solid angle.angle.

1 mm1 mm22

10 nm10 nm

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XPS Background XPS technique is based on Einstein’s idea about the

photoelectric effect, developed around 1905

The concept of photons was used to describe the ejection of electrons from a surface when photons were impinged upon it

During the mid 1960’s Dr. Siegbahn and his research

group developed the XPS technique.

In 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique

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X-Rays Irradiate the sample surface, hitting the core electrons (e-) of the

atoms.

The X-Rays penetrate the sample to a depth on the order of a micrometer.

Useful e- signal is obtained only from a depth of around 10 to 100 Å on the surface.

The X-Ray source produces photons with certain energies: MgK photon with an energy of 1253.6 eV AlK photon with an energy of 1486.6 eV

Normally, the sample will be radiated with photons of a single energy (MgK or AlK). This is known as a monoenergetic X-Ray beam.

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Why the Core Electrons? An electron near the Fermi level is far from the nucleus,

moving in different directions all over the place, and will not carry information about any single atom. Fermi level is the highest energy level occupied by an

electron in a neutral solid at absolute 0 temperature. Electron binding energy (BE) is calculated with respect to the

Fermi level.

The core e-s are local close to the nucleus and have binding energies characteristic of their particular element.

The core e-s have a higher probability of matching the energies of AlK and MgK.

Core e-

Valence e-

Atom

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Binding Energy (BE)

These electrons are attracted to the

proton with certain binding energy x

This is the point with 0 energy of attraction

between the electron and the nucleus. At this point the electron is free from

the atom.

The Binding Energy (BE) is characteristic of the core electrons for each element. The BE is determined by the attraction of the electrons to the nucleus. If an electron with energy x is pulled away from the nucleus, the attraction between the electron and the nucleus decreases and the BE decreases. Eventually, there will be a point when the electron will be free of the nucleus.

0

x

p+

B.E.

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Energy Levels

Vacumm Level

Fermi Level

Lowest state of energy

BE

Ø, which is the work function

At absolute 0 Kelvin the electrons fill from the lowest energy states up. When the electrons occupy up to this level the neutral solid is in its “ground state.”

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X-ray Photoelectron Spectrometer

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XPS Instrument

XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis).

The technique is widely used because it is very simple to use and the data is easily analyzed.

XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System

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XPS Instrument

The XPS is controlled by using a computer system.

The computer system will control the X-Ray type and prepare the instrument for analysis.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System and

the computer system that controls the XPS. 13

XPS Instrument

The instrument uses different pump systems to reach the goal of an Ultra High Vacuum (UHV) environment.

The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of the sample.University of Texas at El Paso, Physics Department

Side view of the Phi 560 XPS/AES/SIMS UHV System

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XPS Instrument

X-Ray Source

Ion Source

SIMS Analyzer

Sample introductionChamber

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Sample Introduction Chamber

The sample will be introduced through a chamber that is in contact with the outside environment

It will be closed and pumped to low vacuum.

After the first chamber is at low vacuum the sample will be introduced into the second chamber in which a UHV environment exists.

First ChamberSecond Chamber UHV

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Diagram of the Side View of XPS System

X-Ray source

Ion source

Axial Electron Gun

Detector

CMA

sample

SIMS Analyzer

Sample introduction Chamber

Sample Holder

Ion PumpRoughing Pump Slits

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How Does XPS Technology Work?

A monoenergetic x-ray beam emits photoelectrons from the from the surface of the sample.

The X-Rays either of two energies:

Al Ka (1486.6eV) Mg Ka (1253.6 eV)

The x-ray photons The penetration about a micrometer of the sample

The XPS spectrum contains information only about the top 10 - 100 Ǻ of the sample.

Ultrahigh vacuum environment to eliminate excessive surface contamination.

Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s.

The spectrum plotted by the computer from the analyzer signal.

The binding energies can be determined from the peak positions and the elements present in the sample identified.

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Why Does XPS Need UHV? Contamination of surface

XPS is a surface sensitive technique.• Contaminates will produce an XPS signal and lead to incorrect

analysis of the surface of composition.

The pressure of the vacuum system is < 10-9 Torr

Removing contamination To remove the contamination the sample surface is bombarded

with argon ions (Ar+ = 3KeV). heat and oxygen can be used to remove hydrocarbons

The XPS technique could cause damage to the surface, but it is negligible.

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Remove adsorbed gases from Remove adsorbed gases from the sample.the sample.

Eliminate adsorption of Eliminate adsorption of contaminants on the sample. contaminants on the sample.

Prevent arcing and high voltage Prevent arcing and high voltage breakdown.breakdown.

Increase the mean free path for Increase the mean free path for electrons, ions and photons.electrons, ions and photons.

Degree of VacuumDegree of Vacuum1010

1010

1010

1010

1010

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-1-1

-4-4

-8-8

-11-11

Low VacuumLow Vacuum

Medium VacuumMedium Vacuum

High VacuumHigh Vacuum

Ultra-High VacuumUltra-High Vacuum

PressurePressureTorrTorr

Why UHV for Surface Analysis?

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XPS spectral lines are XPS spectral lines are identified by the shell from identified by the shell from which the electron was which the electron was ejected (1s, 2s, 2p, etc.).ejected (1s, 2s, 2p, etc.).

The ejected photoelectron has The ejected photoelectron has kinetic energy:kinetic energy:

KE=hv-BE-KE=hv-BE- Following this process, the Following this process, the

atom will release energy by atom will release energy by the emission of an Auger the emission of an Auger Electron.Electron.

Conduction BandConduction Band

Valence BandValence Band

L2,L3L2,L3

L1L1

KK

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Incident X-rayIncident X-rayEjected PhotoelectronEjected Photoelectron

1s1s

2s2s

2p2p

The Photoelectric Process

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X-Rays on the Surface

Atoms layers

e- top layer

e- lower layer with collisions

e- lower layer but no collisions

X-Rays

Outer surface

Inner surface

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X-Rays on the Surface The X-Rays will penetrate to the core e- of the atoms in the

sample.

Some e-s are going to be released without any problem giving the Kinetic Energies (KE) characteristic of their elements.

Other e-s will come from inner layers and collide with other e-s of upper layers These e- will be lower in lower energy. They will contribute to the noise signal of the spectrum.

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X-Rays and the ElectronsX-RayElectron without collision

Electron with collision

The noise signal comes from the electrons that collide with other electrons of different layers. The collisions cause a decrease in energy of the electron and it no longer will contribute to the characteristic energy of the element.

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What e-s can the Cylindrical Mirror Analyzer Detect?

The CMA not only can detect electrons from the irradiation of X-Rays, it can also detect electrons from irradiation by the e- gun.

The e- gun it is located inside the CMA while the X-Ray source is located on top of the instrument.

The only electrons normally used in a spectrum from irradiation by the e- gun are known as Auger e-s. Auger electrons are also produced by X-ray irradiation.

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X-Rays and Auger Electrons

When the core electron leaves a vacancy an electron of higher energy will move down to occupy the vacancy while releasing energy by: photons Auger electrons

Each Auger electron carries a characteristic energy that can be measured.

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Two Ways to Produce Auger Electrons

1. The X-Ray source can irradiate and remove the e- from the core level causing the e- to leave the atom

2. A higher level e- will occupy the vacancy. 3. The energy released is given to a third higher

level e-. 4. This is the Auger electron that leaves the atom.

The axial e- gun can irradiate and remove the core e- by collision. Once the core vacancy is created, the Auger electron process occurs the same way.

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L electron falls to fill core level L electron falls to fill core level vacancy (step 1).vacancy (step 1).

KLL Auger electron emitted to KLL Auger electron emitted to conserve energy released in conserve energy released in step 1.step 1.

The kinetic energy of the The kinetic energy of the emitted Auger electron is: emitted Auger electron is:

KE=E(K)-E(L2)-E(L3).KE=E(K)-E(L2)-E(L3).

Conduction BandConduction Band

Valence BandValence Band

L2,L3L2,L3

L1L1

KK

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Emitted Auger ElectronEmitted Auger Electron

1s1s

2s2s

2p2p

Auger Relation of Core Hole

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Auger Electron Spectroscopy (AES)

Atom layers

e- released fromthe top layer

Outer surface

Inner surfaceElectron beam from the e- gun

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Cylindrical Mirror Analyzer (CMA)

The electrons ejected will pass through a device called a CMA.

The CMA has two concentric metal cylinders at different voltages.

One of the metal cylinders will have a positive voltage and the other will have a 0 voltage. This will create an electric field between the two cylinders.

The voltages on the CMA for XPS and Auger e-s are different.

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Cylindrical Mirror Analyzer (CMA)

When the e-s pass through the metal cylinders, they will collide with one of the cylinders or they will just pass through. If the e-’s velocity is too high it will collide with the

outer cylinder If is going too slow then will collide with the inner

cylinder. Only the e- with the right velocity will go through the

cylinders to reach the detector.

With a change in cylinder voltage the acceptable kinetic energy will change and then you can count how many e-s have that KE to reach the detector.

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Cylindrical Mirror Analyzer (CMA)

Slit

Detector

Electron Pathway through the CMA

0 V

+V

0 V 0 V

0 V

+V

+V

+V

X-RaysSource

SampleHolder

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KEKE = hv - = hv - BEBE - - specspec

Where: Where: BEBE= Electron Binding Energy= Electron Binding Energy

KEKE= Electron Kinetic Energy= Electron Kinetic Energy

specspec= Spectrometer Work Function= Spectrometer Work Function

Photoelectron line energies: Photoelectron line energies: DependentDependent on photon energy.on photon energy.

Auger electron line energies: Auger electron line energies: Not DependentNot Dependent on photon energy.on photon energy.

If XPS spectra were presented on a kinetic energy If XPS spectra were presented on a kinetic energy scale, one would need to know the X-ray source energy used to scale, one would need to know the X-ray source energy used to collect the data in order to compare the chemical states in the collect the data in order to compare the chemical states in the sample with data collected using another source.sample with data collected using another source.

XPS Energy Scale- Kinetic energy

Equation

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XPS Energy Scale- Binding energy

BEBE = hv - = hv - KEKE - - specspec

Where: Where: BEBE= Electron Binding Energy= Electron Binding Energy

KEKE= Electron Kinetic Energy= Electron Kinetic Energy

specspec= Spectrometer Work Function= Spectrometer Work Function

Photoelectron line energies: Photoelectron line energies: Not DependentNot Dependent on photon on photon energy.energy.

Auger electron line energies: Auger electron line energies: DependentDependent on photon on photon energy.energy.

The binding energy scale was derived to make uniform The binding energy scale was derived to make uniform comparisons of chemical states straight forward.comparisons of chemical states straight forward.

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KE versus BE

E E E

KE can be plotted depending on BE

Each peak represents the amount of e-s at a certain energy that is characteristic of some element.

1000 eV 0 eV

BE increase from right to left

KE increase from left to right Binding energy

# o

f ele

ctr

on

s

(eV)

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Interpreting XPS Spectrum: Background The X-Ray will hit the e-s

in the bulk (inner e- layers) of the sample

e- will collide with other e- from top layers, decreasing its energy to contribute to the noise, at lower kinetic energy than the peak .

The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis. Binding energy

# o

f ele

ctr

on

sN1

N2

N3

N4

Ntot= N1 + N2 + N3 + N4

N = noise

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XPS Spectrum The XPS peaks are sharp.

In a XPS graph it is possible to see Auger electron peaks.

The Auger peaks are usually wider peaks in a XPS spectrum.

Aluminum foil is used as an example on the next slide.

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XPS Spectrum

O 1s

O becauseof Mg source

CAl

Al

O 2s

O Auger

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

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Auger Spectrum

Characteristic of Auger graphsThe graph goes up as KE increases.

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

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Identification of XPS Peaks

The plot has characteristic peaks for each element found in the surface of the sample.

There are tables with the KE and BE already assigned to each element.

After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.

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X-rays vs. e- Beam

X-Rays Hit all sample area simultaneously

permitting data acquisition that will give an idea of the average composition of the whole surface.

Electron Beam It can be focused on a particular area

of the sample to determine the composition of selected areas of the sample surface.

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XPS Technology Consider as non-

destructive because it produces soft

x-rays to induce photoelectron emission from the sample surface

Provide information about surface layers or thin film structures

Applications in the industry: Polymer surface Catalyst Corrosion Adhesion Semiconductors Dielectric materials Electronics packaging Magnetic media Thin film coatings

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Applications of X-ray Photoelectron Spectroscopy (XPS)

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XPS Analysis of Pigment from Mummy Artwork

150 145 140 135 130

Binding Energy (eV)Binding Energy (eV)

PbO2

Pb3O4

500 400 300 200 100 0Binding Energy (eV)Binding Energy (eV)

O

Pb Pb

Pb

N

Ca

C

Na

Cl

XPS analysis showed XPS analysis showed that the pigment used that the pigment used on the mummy on the mummy wrapping was Pbwrapping was Pb33OO44

rather than Ferather than Fe22OO33

Egyptian Mummy Egyptian Mummy 2nd Century AD2nd Century ADWorld Heritage MuseumWorld Heritage MuseumUniversity of IllinoisUniversity of Illinois

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Analysis of Carbon Fiber- Polymer Composite Material by XPS

Woven carbon Woven carbon fiber compositefiber composite

XPS analysis identifies the functional XPS analysis identifies the functional groups present on composite surface. groups present on composite surface. Chemical nature of fiber-polymer Chemical nature of fiber-polymer interface will influence its properties.interface will influence its properties.

-C-C--C-C-

-C-O-C-O

-C=O-C=O

-300 -295 -290 -285 -280

B inding energy (eV )

N(E

)/E

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Analysis of Materials for Solar Energy Collection by XPS Depth Profiling- The amorphous-SiC/SnO2 Interface

The profile indicates a reduction of the SnOThe profile indicates a reduction of the SnO22

occurred at the interface during deposition. occurred at the interface during deposition. Such a reduction would effect the collector’s Such a reduction would effect the collector’s efficiency.efficiency.

Photo-voltaic CollectorPhoto-voltaic Collector

Conductive Oxide- SnOConductive Oxide- SnO22

p-type a-SiCp-type a-SiC

a-Sia-Si

Solar EnergySolar Energy

SnOSnO22

SnSn

Depth500 496 492 488 484 480

Binding Energy, eV

Data courtesy A. Nurrudin and J. Abelson, University of Illinois46

Angle-resolved XPS

=15° = 90°

More Surface More Surface SensitiveSensitive

Less Surface Less Surface SensitiveSensitive

Information depth = dsinInformation depth = dsind = Escape depth ~ 3 d = Escape depth ~ 3 = Emission angle relative to surface= Emission angle relative to surface==Inelastic Mean Free PathInelastic Mean Free Path

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Angle-resolved XPS Analysis of Self-Assembling Monolayers

Angle Resolved XPS Can Angle Resolved XPS Can DetermineDetermineOver-layer ThicknessOver-layer ThicknessOver-layer CoverageOver-layer Coverage

Data courtesy L. Ge, R. Haasch and A. Gewirth, University of IllinoisData courtesy L. Ge, R. Haasch and A. Gewirth, University of Illinois

0 2 0 4 0 6 0 8 0 1 0 00 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

C (W )

C (A u)

A u

S iW O12 40 d

E lectron E m iss ion A ng le , degrees

Expt. Data

M odel

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