Transcript X-Ray Photoelectron Spectroscopy (XPS)

Electron Spectroscopy for
Surface Analysis
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Outline
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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
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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 Spectroscopy
Small Area Detection
X-ray Beam
Electrons are extracted
only from a narrow solid
angle.
X-ray penetration
depth ~1mm.
Electrons can be
excited in this
entire volume.
10 nm
1 mm2
X-ray excitation area ~1x1 cm2. Electrons
are emitted from this entire area
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XPS Background
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XPS technique is based on Einstein’s idea about the
photoelectric effect, developed around 1905
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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.
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In 1981, Dr. Siegbahn was awarded the Nobel Prize in
Physics for the development of the XPS technique
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X-Rays
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Irradiate the sample surface, hitting the core electrons (e-) of the
atoms.
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The X-Rays penetrate the sample to a depth on the order of a
micrometer.
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Useful e- signal is obtained only from a depth of around 10 to
100 Å on the surface.
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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
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Normally, the sample will be radiated with photons of a single
energy (MgK or AlK). This is known as a monoenergetic XRay beam.
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Why the Core Electrons?
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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.
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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.
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The core e-s are local close to the nucleus and have
binding energies characteristic of their particular element.
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The core e-s have a higher probability of matching the
energies of AlK and MgK.
Valence eCore e-
Atom
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Binding Energy (BE)
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
B.E.
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.
x
p+
These electrons are
attracted to the
proton with certain
binding energy x
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Energy Levels
Vacumm Level
Ø, which is the work function
Fermi Level
BE
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.”
Lowest state of
energy
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X-ray Photoelectron
Spectrometer
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XPS Instrument
University of Texas at El Paso, Physics Department
Front view of the Phi 560 XPS/AES/SIMS UHV System
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XPS is also known as ESCA
(Electron Spectroscopy for
Chemical Analysis).
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The technique is widely used
because it is very simple to
use and the data is easily
analyzed.
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XPS works by irradiating
atoms of a surface of any
solid material with X-Ray
photons, causing the ejection
of electrons.
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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 Department
Front view of the Phi 560 XPS/AES/SIMS UHV System and
the computer system that controls the XPS.
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XPS Instrument
University of Texas at El Paso, Physics Department
Side view of the Phi 560 XPS/AES/SIMS UHV System
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The instrument uses
different pump systems to
reach the goal of an Ultra
High Vacuum (UHV)
environment.
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The Ultra High Vacuum
environment will prevent
contamination of the
surface and aid an
accurate analysis of the
sample.
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XPS Instrument
X-Ray Source
Ion Source
SIMS Analyzer
Sample introduction
Chamber
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Sample Introduction Chamber
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The sample will be introduced
through a chamber that is in
contact with the outside
environment
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It will be closed and pumped
to low vacuum.
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After the first chamber is at
low vacuum the sample will
be introduced into the second
chamber in which a UHV
environment exists.
First Chamber
Second Chamber UHV
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Diagram of the Side View
of XPS System
X-Ray source
Ion source
Detector
SIMS
Analyzer
Axial Electron Gun
Sample introduction
Chamber
Sample
Holder
sample
Roughing Pump
CMA
Slits
Ion Pump
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How Does XPS Technology Work?
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A monoenergetic x-ray beam
emits photoelectrons from
the from the surface of the
sample.
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Ultrahigh vacuum
environment to eliminate
excessive surface
contamination.
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The X-Rays either of two
energies:
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Cylindrical Mirror Analyzer
(CMA) measures the KE of
emitted e-s.
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The spectrum plotted by the
computer from the analyzer
signal.
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The binding energies can be
determined from the peak
positions and the elements
present in the sample
identified.
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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.
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Why Does XPS Need UHV?
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Contamination of surface
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XPS is a surface sensitive technique.
• Contaminates will produce an XPS signal and lead to incorrect
analysis of the surface of composition.
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The pressure of the vacuum system is < 10-9 Torr
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Removing contamination
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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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Why UHV for Surface
Analysis?
Degree of Vacuum
Pressure
Torr
102
Low Vacuum
Medium Vacuum
High Vacuum
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10-1
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10-4
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10-8
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Ultra-High Vacuum
10-11
Remove adsorbed gases from
the sample.
Eliminate adsorption of
contaminants on the sample.
Prevent arcing and high
voltage breakdown.
Increase the mean free path for
electrons, ions and photons.
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The Photoelectric Process
Incident X-ray
Conduction Band
Ejected Photoelectron
Free
Electron
Level
Fermi
Level
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Valence Band
2p
L2,L3
2s
L1
1s
K
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XPS spectral lines are
identified by the shell from
which the electron was
ejected (1s, 2s, 2p, etc.).
The ejected photoelectron has
kinetic energy:
KE=hv-BE-
Following this process, the
atom will release energy by
the emission of an Auger
Electron.
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X-Rays on the Surface
e- top layer
e- lower layer
with collisions
e- lower layer
but no collisions
Outer surface
X-Rays
Inner surface
Atoms layers
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X-Rays on the Surface
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The X-Rays will penetrate to the core e- of the atoms in
the sample.
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Some e-s are going to be released without any problem
giving the Kinetic Energies (KE) characteristic of their
elements.
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Other e-s will come from inner layers and collide with other
e-s of upper layers
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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 Electrons
Electron without collision
X-Ray
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?
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The CMA not only can detect electrons from the
irradiation of X-Rays, it can also detect electrons
from irradiation by the e- gun.
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The e- gun it is located inside the CMA while the
X-Ray source is located on top of the
instrument.
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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
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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
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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 efrom 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 eby collision. Once the core vacancy is created,
the Auger electron process occurs the same way.
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Auger Relation of Core Hole
Emitted Auger Electron
Conduction Band
Free
Electron
Level
Fermi
Level
Valence Band
2p
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L2,L3
2s
L1
1s
K
L electron falls to fill core level
vacancy (step 1).
KLL Auger electron emitted to
conserve energy released in
step 1.
The kinetic energy of the
emitted Auger electron is:
KE=E(K)-E(L2)-E(L3).
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Auger Electron Spectroscopy (AES)
e- released from
the top layer
Outer surface
Electron beam
from the e- gun
Inner surface
Atom layers
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Cylindrical Mirror Analyzer (CMA)
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The electrons ejected will pass through a device
called a CMA.
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The CMA has two concentric metal cylinders at
different voltages.
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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.
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The voltages on the CMA for XPS and Auger e-s are
different.
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Cylindrical Mirror Analyzer (CMA)
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When the e-s pass through the metal cylinders,
they will collide with one of the cylinders or they
will just pass through.
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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)
X-Rays
Source
Electron Pathway through the CMA
Slit
0V
Sample
Holder
0V
+V
+V
+V
+V
0V
0V
Detector
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Equation
XPS Energy Scale- Kinetic energy
KE = hv - BE - spec
Where: BE= Electron Binding Energy
KE= Electron Kinetic Energy
spec= Spectrometer Work Function
Photoelectron line energies: Dependent on photon energy.
Auger electron line energies: Not Dependent on photon energy.
If XPS spectra were presented on a kinetic energy 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 sample
with data collected using another source.
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XPS Energy Scale- Binding energy
BE = hv - KE - spec
Where: BE= Electron Binding Energy
KE= Electron Kinetic Energy
spec= Spectrometer Work Function
Photoelectron line energies: Not Dependent on photon
energy.
Auger electron line energies: Dependent on photon energy.
The binding energy scale was derived to make uniform
comparisons of chemical states straight forward.
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KE versus BE
KE can be plotted depending
on BE
# of electrons
Each peak represents the
amount of e-s at a certain
energy that is characteristic
of some element.
BE increase from right to left
1000 eV
E
E
E
Binding energy
0 eV
KE increase from left to right
(eV)
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Interpreting XPS Spectrum:
Background
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The X-Ray will hit the e-s
in the bulk (inner elayers) of the sample
e- will collide with other efrom 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.
N = noise
# of electrons
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N4
N3
N2
N1
Binding energy
Ntot= N1 + N2 + N3 + N4
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XPS Spectrum
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The XPS peaks are sharp.
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In a XPS graph it is possible to see Auger
electron peaks.
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The Auger peaks are usually wider peaks
in a XPS spectrum.
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Aluminum foil is used as an example on
the next slide.
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O 1s
XPS
Spectrum
O Auger
O because
of Mg source
C
O 2s
Al
Sample and graphic provided by William Durrer, Ph.D.
Department of Physics at the Univertsity of Texas at El Paso
Al
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Auger Spectrum
Characteristic of Auger graphs
The 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
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The plot has characteristic peaks for each
element found in the surface of the sample.
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There are tables with the KE and BE already
assigned to each element.
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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
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X-Rays
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Hit all sample area simultaneously
permitting data acquisition that will
give an idea of the average
composition of the whole surface.
Electron Beam
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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
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Consider as nondestructive
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because it produces soft
x-rays to induce
photoelectron emission
from the sample surface
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Applications in the
industry:
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Provide information
about surface layers
or thin film structures
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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
Pb3O4
Egyptian Mummy
2nd Century AD
World Heritage Museum
University of Illinois
PbO2
C
O
150
145
140
135
130
Binding Energy (eV)
Pb Pb
N
Ca
Na
Cl
500
400
300
200
Binding Energy (eV)
Pb
100
0
XPS analysis showed
that the pigment used
on the mummy
wrapping was Pb3O4
rather than Fe2O3
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Analysis of Carbon Fiber- Polymer
Composite Material by XPS
XPS analysis identifies the functional
groups present on composite surface.
Chemical nature of fiber-polymer
interface will influence its properties.
Woven carbon
fiber composite
N(E)/E
-C-C-
-C-O
-C=O
-300
-295
-290
Binding energy (eV)
-285
-280
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Analysis of Materials for Solar Energy
Collection by XPS Depth ProfilingThe amorphous-SiC/SnO2 Interface
Photo-voltaic Collector
The profile indicates a reduction of the SnO2
occurred at the interface during deposition.
Such a reduction would effect the collector’s
efficiency.
SnO2
Sn
Solar Energy
Conductive Oxide- SnO2
p-type a-SiC
a-Si
Depth
500
496
492
488
484
480
Binding Energy, eV
Data courtesy A. Nurrudin and J. Abelson, University of Illinois
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Angle-resolved XPS
q =15°
More Surface
Sensitive
q
q = 90°
Less Surface
Sensitive
q
Information depth = dsinq
d = Escape depth ~ 3 l
q = Emission angle relative to surface
l = Inelastic Mean Free Path
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Angle-resolved XPS Analysis of
Self-Assembling Monolayers
SiW12O40
d
Au
Angle Resolved XPS Can
Determine
Over-layer Thickness
Over-layer Coverage
0.6
Expt. Data
0.5
Model
0.4
C(W)
C(Au)
0.3
0.2
0.1
0
20
40
60
80
100
Electron Emission Angle, degrees
Data courtesy L. Ge, R. Haasch and A. Gewirth, University of Illinois
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