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X-Ray Photoelectron Spectroscopy (XPS) Surface Analysis
Electron Spectroscopy for Chemical Analysis (ESCA) Surface Analysis
XPS surface analysis (also known as ESCA) is an essentially
non-destructive surface analysis technique that can be used to
determine the composition of the outermost atomic layers of a solid
material. With the exception of hydrogen, all elements can be
detected. The average detectability limit is approximately 0.1 atom
percent. The average depth of surface analysis is approximately 50Å.
Both elemental identification and chemical speciation are possible
with careful surface analysis of the obtained binding energy
information.
XPS surface analysis is accomplished by flooding the sample
with x-rays of a known energy (typically Al Kα 1486.6ev). Absorption
of these x-rays by the sample causes photoelectrons to be emitted. The
kinetic energy of the emitted photoelectrons is measured with an
electron spectrometer. Binding energy is determined from the following
equation: BE = hυ - KE where BE is the binding energy associated with
the emitted photoelectron, hυ is the energy of the x-rays being used
and KE is the measured kinetic energy of the emitted photoelectron.
The binding energy associated with a peak is then used to establish
it's elemental identity and chemical state.
The incoming x-rays penetrate microns into the surface of the
sample. However, the emitted photoelectrons, because of their low
kinetic energy (less than 2000ev), can only travel a short distance
without being scattered by colliding or interacting with other atoms
and losing energy. This short distance is referred to as the escape
depth of the electron. Escape depths range from 5 to 100Å, depending
upon the kinetic energy of the photoelectron. Photoelectrons that are
close enough to the surface to escape without loss of energy will be
detected as photoelectron peaks. Those photoelectrons that lose energy
before leaving the sample surface will add to the background of the
spectrum. This escape depth limitation makes XPS a surface analysis
technique with an average depth of surface analysis of approximately
50Å.
Quantification is possible with the use of elemental
sensitivity factors to compensate for the transmission function of the
spectrometer and the change in photo-ionization cross-sections from
element to element. These sensitivity factors have been determined
empirically and found to be in agreement with the current theoretical
models for quantification of XPS data.
Elemental and in some cases chemical depth profiling is
possible with the use of an inert gas sputter ion gun. The ion gun
removes material exposing a new surface to be analyzed. By
sequentially sputtering and taking XPS surface analysis data, a
compositional depth profile may be generated. If the chemical matrix
of the sample is not damaged by the sputter ion beam, chemical
information may also be obtained as a function of depth.
If the sample is insulating, an electrical charge may develop
on the surface of the sample causing all of the peaks to shift to a
higher binding energy. To correct for this charging phenomena we
reference all the peaks to a peak of known energy. Typically carbon is
used for a reference because hydrocarbons are present on most
surfaces. This charge correction method allows us to obtain useful
chemical state information from insulating samples.
In summary, XPS is a surface analysis technique that can
provide: elemental identification, chemical speciation, accurate
quantification, and depth distribution information.
Auger Electron Spectroscopy (AES) Surface Analysis
Auger Electron Spectroscopy (AES) surface analysis is a fast,
non-destructive analytical technique used to determine the elemental
composition of the top few atomic layers of a surface or exposed
interface in a solid material. AES surface analysis can detect all
elements except hydrogen and helium and can provide semiquantitative
information with an average detection limit of 0.1 to 1 atomic
percent. Modern Auger instruments with field emission electron sources
can provide characterization of sample features as small as 100Å.
Auger Electron Spectroscopy surface analysis is performed under
ultra-high vacuum conditions, using an electron beam typically in the
3 to 25keV range. The Auger process begins with electron bombardment
of the sample material causing the ejection of an inner shell electron
to form a vacancy. A second electron from a higher shell fills this
inner shell vacancy. This is an energy gain process. This energy can
then cause the ejection of a third electron, referred to as an Auger
electron. A Frenchman, Pierre Auger, first described this process in
1925. The energy of the escaping Auger electron is analyzed by an
electron spectrometer. Since each element has a unique set of Auger
peaks, the resulting spectrum provides a fingerprint of the probed
surface. The kinetic energy of the Auger electrons is typically
between 40 and 2500 eV. In this energy regime, electrons have escape
depths of approximately 5-100Å. This shallow escape depth gives Auger
its surface sensitivity.
Scanning Auger Microscopy (SAM) is accomplished by scanning an
electron beam across the surface of a sample while measuring resultant
electron signals. This process generates Secondary Electron Microscope
(SEM) images, Backscattered Electron (BSE) images, and Auger maps. SEM
images, which provide a topographic view of the sample by detecting
low energy electrons emitted from the surface, are used to locate
specific areas for more detailed study. BSE images, involving higher
energy electrons, which have undergone scattering processes before
escaping from the sample, reveal atomic number contrast and
crystallographic information. Auger maps, obtained by measuring the
emitted Auger electron intensity while scanning the electron beam,
reveal the lateral distribution of elements across the sample surface.
Additional information can be obtained by using inert gas ions
to sputter etch the sample surface. Such sputter etching erodes the
sample in a controlled manner and is typically performed with argon
ions. A sputter depth profile, which plots the Auger signal as a
function of sputter time, shows elemental concentration as a function
of depth into the material. This surface analysis technique is
particularly effective for thin film analysis and for solving
materials problems which require identifying the thickness of a thin
surface layer, the composition of a thin film deposit, the presence of
interdiffusion between thin films, or the presence of contamination at
an interface between two layers.
While AES surface analysis is normally used for analyzing
conductive solids, it is also possible to analyze inorganic oxides. An
advantage of Auger analysis over Energy Dispersive X-ray Spectroscopy
(EDS) analysis is that light elements such as B, C, N, O and F can
easily be detected. Auger surface analysis is also more appropriate
for submicron particles since the analyzed volume of material with
Auger is much less than that for EDS.
The nomenclature used to identify the various Auger peaks
involves listing the 3 electron shells involved in the formation of
transition. For example, for low atomic number elements, the most
probable transitions occur when a K-level electron is ejected by the
primary electron beam, an L-level electron drops into the vacancy, and
another L-level electron is ejected. This is referred to as a KLL
Auger electron.
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)Surface Analysis
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is an
analytical surface analysis technique that uses a high-energy, primary
ion beam to probe the surface of a solid material. The transfer of
momentum from the primary ions to the solid results in the ejection of
ions. Greater than 90% of these secondary ions originate from the
outermost 1-2 layers of the solid thus defining TOF-SIMS surface
analysis as an extremely surface sensitive technique. Once desorbed
from the surface, the ions are extracted into a TOF mass spectrometer
where their masses are determined with high accuracy. The resulting
mass spectrum allows for unambiguous identification of chemical
moieties, typically in the 1-1500 amu range, that can be correlated to
the original surface structure. Moreover, by using a finely focussed
ion beam it is possible to record the lateral distribution of species
across a surface with micron to sub-micron spatial resolution. Lastly,
a third dimension can be added by using high primary ion fluxes. This
allows atomic and diatomic species to be monitored as a function of
depth.
In TOF-SIMS surface analysis, the desorption/ejection of
secondary ions from a surface is initiated by a primary ion that
impinges on a surface at high angles of incidence. The momentum
transfer from the primary ion to the solid initiates a ‘collision
cascade’ within the solid, much like a microscopic billiards game. A
portion of this momentum is redirected back toward the surface
resulting in the ejection of atomic and molecular ions. The emitted
secondary ions are extracted into a TOF analyzer by applying a
potential between the sample surface and the mass analyzer.
Secondary ion detection sensitivities are maximized by using a
pulsed, primary ion source (≤ 1 ns) in conjunction with a TOF
analyzer. Once extracted into the mass spectrometer, the secondary
ions travel through a field free region where they separate in time,
based on their mass to charge ratio (KE=1/2mv2). For each primary ion
pulse, a full mass spectrum is obtained by measuring the arrival times
of the secondary ions at the detector and performing a simple time to
mass conversion. Chemical images are generated by collecting a mass
spectrum at every pixel (256 x 256) as the finely focussed, primary
ion beam is rastered across the sample surface.
Sample integrity is preserved during analyses by operating
under “static” conditions. This mode dictates that ≤ 1x1012 ions/cm2
impinge on a surface ensuring that ≤ 1 percent of the characterized
region is consumed.
Because of its surface sensitivity and non-destructive nature,
TOF-SIMS surface analysis has been utilized to characterize a variety
of conducting and non-conducting surfaces. Common applications include
metals, lubricants, semiconductors, pharmaceuticals, biomaterials,
plastics/polymers, glasses, ceramics, catalysts, coatings/thin films
and paper.
Lastly, shallow depth profiling can be performed by TOF-SIMS
surface analysis. A low energy ion gun is operated in DC mode for
sputtering with high depth resolution. Data acquisition is
accomplished using the same or a second ion gun operated in pulsed
mode. Depth profiling by TOF-SIMS surface analysis allows monitoring
of all species of interest simultaneously, and with high-mass
resolution to remove any interferences.
One of the Techniques in Surface Analysis is Dynamic Secondary Ion Mass Spectrometry (SIMS)
In D-SIMS surface analysis, as a solid material is bombarded
(sputtered) by a primary beam of energetic ions, a variety of species
are emitted from the sample surface. These include neutrals, excited
state species, electrons, photons, and both positive and negative
secondary ions. It is these secondary ions which comprise the
analytical signal for the Secondary Ion Mass Spectrometry (SIMS)
experiment.
In the Dynamic SIMS surface analysis mode of operation the
sample surface is rapidly eroded by a primary beam of ions in order to
generate as many secondary ions per unit time as possible. The
secondary ions emitted from the surface are energy filtered and mass
separated so that the number of ions at any given mass can be counted.
The surface analysis technique is typically used for depth profiling
where the intensities of specific elements with depth are monitored.
Survey spectra can also be acquired. These spectra show the
intensities of all the elements in the periodic table and are used to
monitor dopants and impurities on the surface and in the bulk of
materials and also at interfaces between layers. At any given time,
the depth of analysis is only about 20Å, making it an ideal tool for
interface surface analysis.
In order to maximize the secondary ion yield for different
elements, reactive primary ion beams are typically used to etch the
sample. An oxygen beam is used to enhance positive secondary ions and
cesium is most often used to enhance negative secondaries.
The surface analysis technique is capable of detecting
extremely small concentrations of elements in a wide range of
materials and compounds. Detection limits in the ppm range are
routinely achieved, and for certain elements, the limits may reach the
ppb range. Unfortunately, unless standards are available,
quantification for these dopants and impurities can be very difficult.
This is due to the fact that secondary ion yields vary by orders of
magnitude depending on the matrix being sputtered. For example, the
positive secondary ion yield for Si in a pure Si matrix is
approximately 3 orders of magnitude less than for Si in a SiO2 matrix.
Si and GaAs are two semiconductor substrate materials in which many
impurity standards are commonly available.
The lateral resolution of the surface analysis technique is, at
best, a few μm, however, areas a few hundred μm square are typically
bombarded with the primary beam to ensure uniformity of the center of
the sputter crater, from where the secondary ion signal is acquired.
