XPS.ppt

X-Ray Photoelectron
Spectroscopy (XPS)

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

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:
 Normally, the sample will be radiated with photons of a single
energy. This is known as a monoenergetic X-Ray beam.

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

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.

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.”

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 Department
Front view of the Phi 560 XPS System

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.
Front view of the Phi 560 XPS System and the computer
system that controls the XPS.

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.
Side view of the Phi 560 XPS System

XPS Instrument
X-Ray Source
Ion Source
Analyzer
Sample introduction
Chamber

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 Chamber
Second Chamber UHV

Diagram of the Side View
of XPS System
X-Ray source
Ion source
Axial Electron Gun
Detector
CMA
sample
Analyzer
Sample introduction
Chamber
Sample
Holder
Ion Pump
Roughing Pump Slits

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
 The x-ray photons penetrates
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.

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.

The Atom and the X-Ray
Core electrons
Valence electrons
X-Ray
Free electron
proton
neutron
electron
electron vacancy
The core electrons
respond very well to
the X-Ray energy

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

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.

X-Rays and the Electrons
X-Ray
Electron 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.

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.

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.

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.

Auger Electron
Free e-
e- Vacancy
e- of high energy
that will occupy the
vacancy of the core
level
e- released to
analyze
1
1, 2, 3 and 4 are the order of steps in which the e-s will
move in the atom when hit by the e- gun.
e- gun
2
3
4

Auger Electron Spectroscopy (AES)
Atom layers
e- released from
the top layer
Outer surface
Inner surface
Electron beam
from the e- gun

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.

 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.

Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-Rays
Source
Sample
Holder

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 decreases from left to right
KE increase from left to right
Binding energy
#
of
electrons
(eV)

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
#
of
electrons
N1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise

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.

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

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.

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.

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

XPS.ppt

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XPS.ppt