This document discusses electron spectroscopy techniques for surface chemical analysis. It describes two types of electron spectroscopy: Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). AES uses Auger electrons emitted from the top 10 nm of a material's surface when inner shell electrons are ejected. XPS uses photoelectrons emitted when inner shell electrons are ejected by X-ray photons. Both provide information about a material's surface chemistry through characteristic electron binding energies. The document also outlines the instrumentation required, including ultra-high vacuum conditions and electron energy analyzers.

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Electron Spectroscopy for Surface
Analysis
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XPS and AES
 Electron spectroscopy is a technique for surface chemical analysis which uses
characteristic electrons emitted from a solid for elemental analysis. The
characteristic electrons (either Auger electrons or photoelectrons) exhibit
characteristic energy levels, revealing the nature of chemical elements in
specimens being examined.
 There are two types of electron spectroscopy: 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.
 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).

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XPS
Basic Principle
 The X-ray photoelectron is an electron ejected from an electron shell of an
atom when the atom absorbs an X-ray photon. When an incident X-ray
photon with sufficient energy (hv) interacts with an atom, it may knock out
an inner shell electron (K shell). This K-shell electron would be ejected from
the surface as a photoelectron with kinetic energy EK.
 If the kinetic energy EK of the photoelectron is known then we can calculate
the binding energy of this photoelectron (EB) based on the following
relationship.
 Φ is the parameter representing the energy required for an electron to
escape from a material’s surface, h is Planck’s constant and ν is the
frequency. The value of Φ depends on both the sample material and the
spectrometer.
 The binding energies (EB) of atomic electrons have characteristic values, and
these values are used to identify elements, similar to the way that
characteristic X-ray energy is used in X-ray spectroscopy
 KB EhvE
XPS and AES

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XPS AES
 A typical XPS spectrum is a plot of intensity versus binding energy.
Photoelectrons are ejected from different electronic shells and subshells.
Each binding energy peak is marked as an element symbol plus a shell
symbol from where the photoelectron was emitted.
 For example Al 2p, O 1s. The photoelectrons emitted by subshells p, d and
f are commonly marked with an additional fraction number; for example,
Cu 2p1/2. There are 1/2 and 3/2 for the p-subshell, 3/2 and 5/2 for the
d-subshell, and 5/2 and 7/2 for the f-subshell.
 These fractions represent the quantum number of total angular momentum
(J) for an individual shell electron. An XPS spectrum may also contain peaks
from Auger electrons. For example, the spectrum shown includes Auger
electron peaks marked as OKLL and CKLL.
XPS Spectrum

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AES
 When a high-energy electron (or X-ray photon) strikes an inner shell
electron of an atom, the energy of the incident particle can be high enough
to knock out the inner shell (K shell) electron. Thus, the atom becomes ionized
and in an excited state. The atom will quickly return to its normal state after
refilling the inner electron vacancy with an outer shell electron.
 The energy difference between the outer shell electron and the inner shell
may cause emission of a characteristic X-ray photon which transfers its
energy to electron in another shell, ejecting it from the atom. This ejected
electron is called Auger electron and has a definite kinetic energy.
 The kinetic energy of an Auger electron is approximately equal to the
energy difference between binding energies in the electron shells involved
in the Auger process. It is approximated by the following equation,
3,213,21 BLBLBKLKL EEEE 
AES
 The subscript of EBK is the binding energy of the K shell electron and
similarly EBL1 is the binding energy of electron in subshell L1. Auger electron
spectroscopy identifies chemical elements by measuring the kinetic energies
of Auger electrons.
 In an AES spectrum, an individual kinetic energy peak from an Auger
electron is marked with an elemental symbol and subscripts indicating the
electron shells or subshells involved, for example, AlKLL, OKLL. A typical AES
spectrum is a plot of intensity versus kinetic energy; most commonly, it is a
plot of the first derivative of intensity versus the kinetic energy.
 The Auger peaks appear small against the background of the direct mode
spectrum. This occurs because the signals from Auger electrons are
relatively weak compared with those of secondary electrons escaped from
a solid surface

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AES Spectrum
AES spectra of an oxidized aluminum surface: (a) direct spectrum of
intensity versus kinetic energy of Auger electrons; and (b) differential
spectrum of intensity versus kinetic energy of Auger electrons
AES
Schematic comparison of Auger peak intensity with other electrons
escaped from a solid surface. Eo indicates energy of incident
electrons. The kinetic energy of electrons can be divided into three
regions I, II and III from low to high

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AES
 The primary electrons ejected from a solid surface by inelastic scattering
comprise the background of an AES spectrum in the region of high kinetic
energies while the secondary electrons comprise the background in the
region of low kinetic energies. The number of Auger electrons ejected from
a sample is much less than that of scattered primary electrons and
secondary electrons.
XPS Instrumentation
 A modern instrument for electron spectrometry contains both XPS and AES
in a single chamber as a multifunctional surface analysis system. A scanning
electron microscope (SEM) system may also be included in order to image
the microscopic area to be examined by electron spectroscopy.
 A system combining AES and XPS includes an electron gun, an X-ray gun
and a shared analyzer of electron energy. The electron beam for
generating the Auger electron emission can be easily focused and scanned
over a sample surface to obtain two-dimensional mapping for AES analysis.
Thus, AES analysis is commonly the scanning type, called scanning Auger
microscopy.
 Ultra High Vacuum
Instruments for electron spectrometry require an ultra-high vacuum (UHV)
environment with a vacuum pressure in the range 10−8–10−10 mbar. Such a
high vacuum reduce the chances of low-energy electrons being scattered by
gas molecules on their way to reach the detector, and to keep the sample
surface free from contamination of gas molecules.

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XPS/AES Instrumentation
 Surface contamination by gas molecules is a major concern for surface
chemical analysis. It is possible for a surface to adsorb a monolayer of gas
molecules within one second in a vacuum environment of 10−6 mbar. Low-
energy photoelectrons and Auger electrons are easily scattered by gas
molecules. Scattering will reduce signal intensity and increase background
noise in spectra.
 A typical time to collect XPS and AES spectra is more than several hundred
seconds. Only an UHV environment ensures satisfactory analysis,
particularly for surfaces containing elements that exist in gaseous
environments such as C, O and N.
 The UHV chamber is commonly made from stainless steel, and joints of
chamber parts are made from crushed copper gaskets. UHV pressure can
be achieved using diffusion pumps, sputter ion pumps or turbomolecular
pumps. Currently, sputter ion pumps and turbomolecular pumps are more
commonly used than diffusion pumps that may contaminate the chamber by
leaking oil gas molecules.
XPS/AES Instrumentation
 In order to achieve the necessary UHV and clean the surface of chamber
and sample, the vacuum chamber must be baked at an elevated
temperature (250–350°C) and pumped. This baking process allows the gas
molecules adsorbed onto the chamber walls to be pumped out. An
additional requirement for the chamber is magnetic shielding, because the
trajectory of signal electrons is strongly affected by any magnetic field,
even the Earth’s magnetic field.
 Source Guns (X-ray Gun)
An electron spectrometer system contains an X-ray gun for XPS analysis. The
working principles of the X-ray gun are similar to the X-ray tube used for
X-ray diffractometry. X-ray photons are generated by high-energy
electrons striking a metal anode, commonly Al or Mg for XPS spectrometry.
The X-ray gun produces a characteristic X-ray line to excite atoms of the
surface to be analyzed. XPS uses both non-monochromatic and
monochromatic X-ray sources. The output from a non-monochromatic X-ray

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XPS/AES
Source Guns
 source consists of a continuous energy distribution with high intensity of Kα
characteristic lines. The output of the monochromatic source is produced by
removing continuous X-rays from a radiation spectrum. The monochromatic
source is useful for obtaining XPS spectra with reduced background
intensity.