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Auger Electron Spectroscopy (AES)
Description
In AES the sample of interest is irradiated with
a high energy (2 - 10 keV) primary electron beam. This bombardment
results in the emission of backscattered, secondary, and Auger
electrons that can be detected and analyzed. The backscattered
and the secondary electrons are used for imaging purposes similar
to that in scanning electron microscopy (SEM).
The Auger electrons are emitted at discrete energies (see Auger
Effect for more details), that are characteristic of
the elements present on the sample surface. When analyzed as a
function of energy, the peak positions are used to identify the
elements and the chemical states present.All elements in the periodic
table, except hydrogen and helium, can be detected, and the depth
of analysis is in the range of 3 - 5 nm or top 2-20 atomic layers.
If a scanning primary beam is used, the secondary electron images
yield information related to surface topography. As the electron
beams can be focused to a very small probe size, excellent spatial
resolution (0.5 um) can be achieved.
Auger point analysis and scanning analysis can be performed with
a spatial resolution down to 250 nm, while SEM resolution is around
100 nm. When ion gun is used for sputtering of top layers, depth
profiles can be run automatically and maps and line scans of Auger
electron distributions can be generated
Comparison to XPS
Auger and X-ray photoelectron spectroscopy give similar information,
and the choice should be based on advantages and disadvantages.
The Auger spot size is much smaller than the XPS and has the capability
of identifying fine features on the surface. The XPS has the capability
of determining surface chemical structure and bonding through
the use of chemical shifts. Although Auger lines also exhibit
chemical shifts, these are not generally as large or as well-documented
as those obtained by XPS. Also, X-radiation used in XPS imparts
less damage to the sample surface than does the electron beam
used in SAM. As mentioned above, the spatial analysis and imaging
capabilities of the scanning Auger microprobe make it a very useful
and complementary technique to XPS.
Modes of Operation
The Auger analysis can include Survey Scans, High-Resolution spectra,
Depth Profiles, Imaging, Mapping, and Point Analysis. Survey scans
of the entire range of Auger electron energies, carried out by
detecting and counting the number of Auger electrons, could reveal
the presence of contaminants on the sample surface. By taking
into account the sensitivity factors of the elements detected,
quantification is possible. This is useful in identifying the
unknown elements and estimating their concentration on the surface.
With argon ion bombardment, the surface layers can be removed
gradually, and analysis carried out on new layers exposed after
each sputtering cycle. This is known as depth profiling, and it
provides the relative concentrations of elements of interest as
a function of depth. Finally, the Auger elemental maps display
the presence and the distribution of elements of interest within
the area analyzed.
A typical analysis of an unknown surface region (survey scan on
the as received surface and after three different sputter intervals)
would take 1 - 2 hours.
Instrumentation
AES is currently available on AXIS 165
spectrometer.
Applications
AES is used mainly to determine the composition of small areas
on the surface, either as point analysis or mapping. Typical applications
include corrosion, plating, integrated circuits etc.different
sputter intervals) would take 1 - 2 hours.
Examples
AES spectrum from a Cu grid: on the top is the measured spectrum,
below is its derivative
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