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Introduction and Applications ofAuger Electron Microscopy AEM
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IntroductionThe Auger effect was discovered by Pierre
Auger in 1925 while working with X rays andusing a Wilson cloud chamber. Tracks
corresponding to ejected electrons wereobserved along a beam of X rays.
The aim of Auger electron imaging is to obtainquantitative surface elemental distributionmaps at high spatial resolution
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WorkingPrinciple-1 When an atom is struck by a high energy electron
(typically in the range 1 to 25 keV) there is some
probability of a core level electron being ejected.
The atom is then in an energetic ionic state with anelectron missing from a core level.
One mechanism by which the atom can relax into alower energy state is for another electron
one electron falls from a higher level to fill an initial
core hole in the K-shell and the energy liberated in thisprocess is simultaneously transferred to a secondelectron ; a fraction of this energy is required toovercome the binding energy of this second electron,the remainder is retained by this emittedAugerelectronas kinetic energy.
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WorkingPrinciple -2
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WorkingPrinciple -3Two-step process, generates Auger electrondiffers from XPS
1) generate excited ion as before:
A + hn A+* + e-or using an electron beamA + ei- A+* + ei- + eA-
where:ei-is the beam electron after interactionwith A and loss of kinetic energy.eA-is ejected from A inner orbital
2)excited ion may relax by emitting anAuger electron (eA-) with kinetic energy (Ek)A+* A++ + eA-or by fluorescence (X-ray fluorescence)A+* A+ + hn fKinetic energy of emitted electron (Ek) is independent ofthe energy of photon or electron that
Kinetic energy (EK) of the Auger electron is:EK= (EbEb) Eb= Eb2Eb
where:(Eb-Eb) energy released in relaxation of the excited ionEb energy required to remove the second electron from its
orbit
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WorkingPrinciple -4The Auger electron has a characteristic energy whichdepends upon a number of factors:
The chemical element involved The energy level within which the initial hole was formed The energy level of the electron which eventually fills the
hole
The initial energy level of the electron which eventuallybecomes the the Auger electron
- Auger emissions are described in terms of the typeof orbital transitions involved in the production ofan electron
KLL:1. removes a K electron2. transition of an L electron to the K orbital3. ejection of a second L electron
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AEM: Efficiency of Auger Electron Production
Two competingprocesses:
X-rayfluorescence
Auger electronemission
Auger electronspredominate atlower atomtomicnumber(Z)
createdvacanciesshellKofnumber
producedphotonsKofnumber
K
KAuger 1
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AES: InstrumentationAES instruments are designed like an
SEM often they are integrated with
an SEM/EDXA system
Unlike an SEM, AES instruments aredesigned to reach higher vacuum (10-8torr)
Helps keep surfaces clean andfree from adsorbed gases, etc
Basic components:
Electron source/gun
Electron energy analyzer Electron detector
Control system/computer
Ion gun (for depth profiling)
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Primary Electron SourcesThree kinds of primary electron sources are in commonuse in Auger electron spectrometers
A tungsten cathode source consists of a wire filamentbent in the shape of a hairpin.
Lanthanum hexaboride (LaB6) cathodes provide highercurrent densities
Field Emission electron sources consist of very sharptungsten points at which electrical fields can be >10E7V/cm.
Auger instruments have primary electron beamcolumns similar to electron microscopes. Thecolumns may include bothelectrostaticandmagneticlenses for beam focusing
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Electron Energy Analyzers Electron energy analyzers measure the number of ejected electrons as afunction of the electron energies.
The analyzers must be located in a high vacuum chamber and isolatedfrom stray magnetic fields (including the earths) that deflect electrons
The schematic shows a cross section of a cylindrical mirror analyzer inred.
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The primary electron beam hits the sample surfaceat the source point of the analyzer. Auger electronsmove outward in all directions and some passthrough the grid covered aperture in the inner
cylinder.
A variable negative potential on the outer cylinderbends the Auger electrons back through a secondaperture on the inner cylinder and then through anexit aperture on the analyzer axis.
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Electron DetectorElectron MultipliersDiscrete dynode detector An electron multiplier consists of a series of
electrodes called dynodes
each connected along a resistor string. Thesignal output end of the resistor stringattaches to positive high voltage. The otherend of the string goes to the electronmultiplier case and ground.
When a particle electron strikes the first
dynode it produces secondary electrons.The secondary electrons are acceleratedinto the next dynode where each electronproduces more secondary electrons. Acascade of secondary electrons ensues. Thedynode acceleration potential controls theelectron gain.
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Ultra High Vacuum environment The surface analysis necessitates the use of a UHV
environment
Contamination of the specimen surface is critical for highlyreactive surface materials, where the sticking coefficientfor most residual gases is very high (near unity).
the background pressure is reduced to the low 1010-torr range in order to minimize the influence of
residual gases in surface analysis measurements
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AEM: Image Surface Analysis
ANALYTICAL INFORMATION Spatial resolution is approximately 0.3 microns.
Depth resolution is dependent upon sample and sputteringparameters (less than 100 resolution is not unusual). Typicalsputtering rate is ca. 30 /min.
SAMPLE REQUIREMENTS
Maximum of "x" (18 mm x 12 mm). Height should not exceed" (12 mm). must be conductive or area of interest must be grounded
properly. Insulating samples including thick insulating films(ca. 3000 ) cannot be analyzed
SUPPLEMENTAL INFORMATION: Minimum area of analysis - 0.3 microns 10% relative error (i.e. estimated error in repeated analyses),
20% absolute error (i.e. error between analysis and knownstandard).
hr per sample .Depth profiles may take longer depending on
the total depth being sputtered.
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AEM VS SEM For true surface analysis, AES is
better than SEM/X-ray emission(electron microprobe) because it is
much more surface sensitive
AEM can be easily madequantitative using standards.
The major advantages of AeMover SEM are greater lateral anddepth analytic resolution and easeof light-element detection.
The Spatial resolution of AEM is0.030 pm
The Spatial resolution of SEM is1 - 3 pm
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Advantages of Auger Electron Microscopy
Sensitivity to atoms of low atomic number
Minimal matrix effects High spatial resolution
Detailed examination of solid surfaces
Electron beam more tightly focused than X-raybeam
Almost any solid can be analyzed.
Sample can be analyzed as it is.
Estimated time to obtain the survey spectrumfrom a sample varies from 1 to 5 min.
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Disadvantages of Auger Electron Microscopy
not used to provide structural and oxidative state
information (XPS)
quantitative analysis is difficult
It does not provide for nondestructive depth profiles
It requires that samples be small and compatible with highvacuum
Analyzes conducting and semiconducting samples.
Special procedures are required for nonconductingsamples. Only solid specimens can be analyzed.
Samples that decompose under electron beamirradiation cannot be studied.
G l U
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General Uses
Identification of elements on surfaces ofmaterials
Quantitative determination of elements onsurfaces
Depth profiling by inert gas sputteringPhenomena such as adsorption, desorption,
and surface segregation from the bulk
Determination of chemical states of elements
In situ analysis to determine the chemicalreactivity at a surface
Auger electron elemental map of the system
li i
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Common Applications
Qualitative analysis through
fingerprinting spectral analysis
Identification of different chemical states ofelements
Determination of atomic concentration ofelements
Depth profiling
Adsorption and chemisorption of gases onmetal surfaces
Interface analysis of materials deposited in
situ on surfaces
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