X-RAY PHOTO ELECTRON SPECTROSCOPY

1. Presented by:
Zaahir salam
M.Tech NS &T
University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System

2. Background
1905 Photoelectric effect
discovered by Albert
Einstein
Nobel Prize
1961 Photoemission as an
analytical tool
demonstrated by Kai
Siegbahn (Electron
Spectroscopy for
Chemical Analysis –
ESCA)
Nobel Prize

3. X-Rays
 Irradiate the sample surface, hitting The core e-s are local close to the nucleus and
have binding energies characteristic of their
the core electrons (e-) of the atoms. particular element.
 The X-Rays penetrate the sample to a The core e-s have a higher probability of
matching the energies of AlK and MgK.
depth of the order of a micrometer.
Valence e-
Core e- Atom
 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

4. X-Rays
 Irradiate the sample surface, hitting The core e-s are local close to the nucleus and
have binding energies characteristic of their
the core electrons (e-) of the atoms. particular element.
 The X-Rays penetrate the sample to a The core e-s have a higher probability of
matching the energies of AlK and MgK.
depth of the order of a micrometer.
Valence e-
Core e- Atom
 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

5. Spectroscopy
 Spectroscopy- the study of the light from an object.
 Spectrometer- an instrument which spreads out light
making a spectra.
 Spectra- range of electromagnetic energy separated by
wavelength.

6. Working Equation
 Because the energy of an X-ray
with particular wavelength is
known, the electron binding
energy of each of the emitted
electrons can be determined by
using an equation that is based on
the work of Ernest Rutherford
(1914):
KE=hv-BE-Ø
 where BE is the binding energy of
the electron, hv is the energy of
the X-ray photons being used, KE
is the kinetic energy of the
electron as measured by the
instrument and φ is the work
function of the spectrometer (not
the material).

7. XPS Instrument
 XPS is also known as ESCA
(Electron Spectroscopy for
Chemical Analysis).
 It is a quantitative
spectroscopic technique that
measures the
 Elemental composition
 Empirical formula
 Chemical state
 Electronic state
 The technique is widely used
because it is very simple to use
and the data is easily analyzed.
University of Texas at El Paso, Physics Department
Front view of the Phi 560 XPS/AES/SIMS UHV System

8.  XPS works by irradiating atoms of a surface of
any solid material with X-Ray while
simultaneously measuring the kinetic energy
and number of electrons that escape from the
top 1 to 10 nm of the material being analyzed
 The XPS is controlled by using a computer
system.
 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
Front view of the Phi 560 XPS/AES/SIMS UHV System and the computer system that controls the XPS.

9. XPS Instrument
X-Ray Source
Ion Source
SIMS Analyzer
Sample introduction
Chamber
University of Texas at El Paso, Physics DepartmentSide view of the Phi 560 XPS/AES/SIMS UHV System

10. 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

11. 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
CMA
Roughing Pump Slits
Ion Pump

12. How Does XPS Technology Work?
 A monoenergetic x-ray beam  Ultrahigh vacuum environment
emits photoelectrons from the to eliminate excessive surface
surface of the sample. contamination.
 The X-Rays either of two  Cylindrical Mirror Analyzer
energies: (CMA) measures the KE of
 Al Kα (1486.6eV) emitted e-s.
 Mg Kα(1253.6 eV)
 The spectrum plotted by the
 The x-ray photons The
computer from the analyzer
penetration about a micrometer signal.
of the sample
 The XPS spectrum contains  The binding energies can be
information only about the top determined from the peak
10 - 100 Ǻ of the sample. positions and the elements
present in the sample identified.

13. 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

14. X-Rays on the Surface
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.

15. 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.

16. 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.
2
e- of high energy that
Free e- 3 will occupy the vacancy
of the core level
e-released to
analyze
4 1
e- gun
e- Vacancy
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.

17. Cylindrical Mirror Analyzer (CMA)
Electron Pathway through the CMA
Slit
 The electrons ejected will pass X-Rays
through a device called a CMA. Source
0V 0V
 The CMA has two concentric metal
cylinders at different voltages.
+V +V
+V +V
 One of the metal cylinders will
have a positive voltage and the Sample
Holder
other will have a 0 voltage. This will 0V 0V
create an electric field between the
two cylinders.
 The voltages on the CMA for XPS
and Auger e-s are different.

18. KE versus BE Noise
N = noise
No. of electrons
No. of electrons
N4
N3
N2
N1
Binding energy
Binding energy (eV) Ntot= N1 + N2 + N3 + N4
KE can be plotted depending on BE
e- will collide with other e- from top
Each peak represents the amount of layers, decreasing its energy to
e-s at a certain energy that is contribute to the noise, at lower
characteristic of some element. kinetic energy than the peak .
BE increase from right to left
The background noise increases
with BE because the SUM of all
1000 eV 0 eV noise is taken from the beginning
of the analysis.
KE increase from left to right

19. 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
XPS Spectrum Auger Spectrum
O 1s
Characteristic of Auger graphs
O Auger The graph goes up as KE increases.
O because
of Mg source
C
Al
Al
O 2s

20. 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.

21. Use of XPS Technology
 Elements and the quantity of those elements that are present within the top 1-12 nm of the sample surface.
 Detects all elements with an atomic number (Z) of 3 (lithium) and above. It cannot detect hydrogen (Z =
1) or helium (Z = 2) because the diameter of these orbitals is so small, reducing the catch probability to
almost zero.
 Chemical state analysis of the surface of polymers readily reveals the presence or absence of the chemical
states of carbon known as: carbide (C 2-), hydrocarbon (C-C), alcohol (C-OH), ketone (C=O), organic
ester (COOR), carbonate (CO3), fluoro-hydrocarbon (CF2-CH2), trifluorocarbon (CF3).
 Is routinely used to analyze
 Inorganic compounds.
 Metal alloys.
 Semiconductors.
 Polymers.
 Catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones,
medical implants, bio-materials, viscous oils, glues, ion modified materials and many others.
 Organic chemicals are not routinely analyzed by XPS because they are readily degraded by either the
energy of the X-rays or the heat from non-monochromatic X-ray sources.

22. References
 Dr.William Durrer for explanations on XPS
technique, Department of Physics at UTEP.
 www.uksaf.com
 www.casaxps.com
 www.nwsl.net
 XPS instrument from the Physics Department.

23. Thank You