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First of all, do you know any
methods to check chemical
composition?
Or how you know what is what?
Surface sensitive?
Auger Electron Spectroscopy (AES)
Binding energy
Fix kinetic energy
Electron-Surface Interaction
Auger electrons can be generated
by any energetic particles, which
are able to and excite electrons
and leave holes, such as X-Ray
irradiation,
ion-beam
bombardment and electron beam
irradiation. In the sense of AES, it
is excited by electrons.
Electrons interaction with surface
brings:
•X-rays (both continuum and
characteristic)
•Backscattered and transmitted
electrons,
•Secondary electrons
•Auger electrons
•Cathodoluminescence
•Heat.
What does electron spectrum
do?
Certain energetic particles interact with material, there
will be electrons with different energy come out from
the material. Spectrum is to record the intensity (number)
of electrons as a function of energy (kinetic energy).
Two important information from the spectrum:
Where are the peaks? (peak at certain energy)
How intense is the peak? (peak height)
The electron analyzer is device to record spectrum.
x-ray notation
The Auger emission is nominated as x-ray notation:
initial core-hole, initial location of the relaxed electron
and the second core-hole
AES spectra
Auger peaks are very
broad (several eV)…..
I(V+v0sinwt) =
I0+dI/dV*sinwt
+…
Normally login
technique to measure
the dI/dV (dN/dE)
Typical spectra
The electric voltage of the
analyzer lens is modulated
by AC one, then login
technique to detect signal
Experimental aspects 1: e-beam source
Although AES can be generated by both x-ray and high energy e beam, the e beam is
easy to be generated and manipulated (focus, scan) and AES normally use ebeam.
The e-beam generation:
1.
Thermionic emission of heated filament with low work function such as W.
cheap
2.
Filed emission gun (FEG): high electric field gradient remove electrons by
tunneling emission material fashioned to sharp point.
High flux, good
focused.
Thermionic emission
LaB6 Electron Gun
•Single crystal lanthanum hexaboride (LaB6)
cathodes provide higher current densities
•LaB6 has a lower work function and greater
emissivity than tungsten ~100 A/cm2 .
•Narrower electron beams, dg=~10-20u
•Useful for analyzing smaller features
W Electron Gun
•Wire filament in the shape of a hairpin.
•The filament operates at ~2700 K by resistive
heating.
•The tungsten cathodes are reliable and
inexpensive.
•Lateral resolution is limited dg=~50u
•Current densities are only about 1.75 A/cm2.
How electron analyzer works?
Analyzer has certain pass energy (Ep), electrons with this energy in a small
energy range (Ep±ΔE) can pass.
Energy resolution Δ E is proportional to Ep.
As analyzer is works in small range of pass energies. To measure big energy
range, electrons with different energy need to be retarded (or accelerated) by a
potential to change the electron energy to be able to analyze. Retarding
E V=(E-Ep)
There are two methods to retard the energy of electrons:
Constant pass energy mode: retard the electron energy to a fixed pass energy by
varying the retarding voltage, therefore with fixed ΔE for whole spectrum.
Constant retarding ratio mode: retard the electron energy with a fix ratio to a
energy range that the analyzer can use corresponding pass energy to detect.
Therefore the ΔE/E is fixed and not the ΔE is fixed.
Ep
Experimental aspects 2a:The electron energy analyzer:
Principally all the electron energy analyzer can be used, however,
the Cylindrical Mirror Analyzer (CMA) is common. This analyzer has large
angular acceptance and high sensitivity. (AES peak generally broad and
with isotropic angular dependence.)
Experimental aspects 2a:The electron energy analyzer:
CMA
with double pass
(high resolution)
Energy diagram for AES
Ev
Ef
Ekin = Ek - EL1 - EL2
EL2
EL1
Considering the many-electron relaxation
effects (2 holes and 1 electron), there is:
Ezkin = Ezk-EzL1-EzL2 – DE(L1L2)
in a simple model with
EK
DE(L1L2) = ½ * (Ez+1L2- EzL2 +
Ez+1L1 - EzL1)
Z dependence
The strong Z dependence of the kinetic energies of
Auger electrons gives AES elemental sensitivity
AES database
Surface sensitivity
The short
free path
length of the
electron at
energies at
tens to
hundreds
eV gives
AES surface
sensitivity
Check film
thickness!
Theoretical calculation
AES needs the primary e
beam with energies over
several thousand eV to be
enough to generate the core
holes.
Long free path length
Short free path length
The intensity of Auger electrons
n electrons
cm-2 s-1
j
Auger electrons s-1
q
tcosq
Vacuum
t
Solid
escape depth
Incident e beam current
IA=NWI0trF(1-w)/(4pcosj)
The acceptance angle
Correction of xCross section of ionization ray fluorescence
Auger backscattering
factor (Auger from
some secondary)
Angular resolved
The shown formula is more for the incidence angle, it
is integration over large acceptance angle of electron
(for CMA analyzer). When consider the acceptance
angle, only the Auger electrons from the depth of
tcosq contribution can come out.
IA proportional to cosq
The change of detection angle will change the surface
sensitivity. In many case, it is possible to get quantitative
analysis of film thickness from the Auger intensity ratios
of substrate and the coated material.
Quantitative analysis
Quantitative analysis
(1) Required: The same instrumental settings, e.g. resolution,
e-beam energy, for both the determination sensitivity
factors and sample analysis.
(2) Needed: The same peak shapes for all peaks; Reduce effect
of the peak shape using high energy peaks. Alternatively,
different sensitivity factor for different peak shapes.
Sensitivity of AES: ~0.1 atomic% of a monolayer!
Error in AES: analysis: < 15%, Error within a few % can be
achieved with better standards and calibration. Take care
Sensitivities Si for peak to peak height of differentiated
Auger peak different from the one for original Auger
peak(with background subtraction)
Transition possibility of AES
AES by e beam
is simple and
cheap, basically
there are two
steps: creation of
core-hole and the
following Auger
excitation. The
first process is
described by
cross-section s:
s = C(Ei/EA) / EA
Maximum about 3
2
cross section as function of the ratio of the energy of
incident electron Ep and core-hole energy Ew
where C depends on the ratio between the energies of the incident
electron (Ei) and the Auger electron (EA). s typically is between 10-3
to 10-4.
Competition between XRF and AES
There are two processes to fill the core-hole after the 1st step: Auger
emission and X-ray fluorescence (XRF). Auger emission favored for
light Z atoms and X-ray emission for heavier atoms (different
dependences for different core-holes.