Auger Spectrometers: A Tutorial
Review
David H. Narumand and Kenton D. Childs
Physical Electronics Inc Eden Prairie
Applied Spectrocopy Reviews, 34 (3), 139 – 158 (1999)
Presented By: Sutter Kiplangat
Date: 03 October 2008
OUTLINE
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•
•
•
•
•
•
The Basics
The Auger Process
AES Analyzers and Instrumentation
AES Applications
SAM/SEM Microscope
Conclusion
Reference
TERMS
The intensity decay can
be expressed as follows:
I(d) = I0 exp(-d / λ(E))
where I(d) is the
intensity after and The
parameter λ(E), termed
the inelastic mean free
path (IMFP), is defined as
the distance an electron
beam can travel before its
intensity decays to 1/e of
its initial value.
Surface Techniques
Surface Analysis Forum: http://www.uksaf.org/tech/list.html
UHV for Surface Analysis?
Degree of Vacuum
Pressure
Torr
10 2
Low Vacuum
10 -1
Medium Vacuum
High Vacuum


10 -4

10 -8

Ultra-High Vacuum
10 -11
Remove adsorbed gases from the
sample.
Eliminate adsorption of
contaminants on the sample.
Prevent arcing and high voltage
breakdown.
Increase the mean free path for
electrons, ions and photons.
TERMS
Sputtering - Atoms are ejected from a solid target material due to bombardment
of the target by energetic ions.
Etching - Removing atoms by sputtering with an inert gas (Ar) is called `ion
milling' or 'ion etching'.
AES IN BRIEF
•
Study of surfaces especially in Material science.
•
Auger effect - based on the analysis of energetic electrons emitted from an
excited atom after a series of internal relaxation events
•
Pierre Auger in the 1920's.
•
Fast, non-destructive technique.
•
AES characterization technique for probing chemical and compositional
surface environments and has found applications in metallurgy, gas-phase
chemistry, and throughout the microelectronics industry.
The Auger Process
NARUMAND AND CHILDS
Following K shell ionization by interaction with an energetic particle,
this schematic represents relaxation via (a) Auger electron emission, and (b) and
Xray fluorescence.
E (z) = E (z)–E (z)–E* (z)–j
ABC
A
B
C
s
Atomic Excitation
Energy
A
A
[A+]* + e–
E=0
M
Potential Energy
L
K
…..
e–
Fluorescence Transition
[A+]* [A+]* [A+] + h
E=0
M
…..
h = E1
Potential Energy
L
E1
K
Auger Transition
EAuger = E1 – E2
[A+]* [A+]* [A2+] + e–
e–
E=0
M
…..
E2
Potential Energy
L
E1
K
The Auger Process
E (z) = E (z)–E (z)–E* (z)–j
ABC
A
B
C
s
Two views of the Auger process. (a) illustrates sequentially the steps involved in Auger
deexcitation. An incident electron creates a core hole in the 1s level. An electron from the 2s
level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted.
The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b)
illustrates the same process using spectroscopic notation, KL1L2,3.
The Auger Process
NARUMAND AND CHILDS
The interaction between an incident electron beam and a solid sample,
showing the analysis volumes for Auger electrons, back-scattered electrons, and
x-ray fluorescence.
Fluorescence and Auger electron yields:
http://en.wikipedia.org/wiki/Auger_electron_spectroscopy
Auger transitions (red curve) are more probable for lighter elements
while X-ray yield (dotted blue curve) becomes dominant at higher atomic numbers.
Auger Quantification
Measured intensity of an arbitrary Auger peak is a complicated function of a large number
of sample and instrumental factors. These include:
o Number of atoms of that element per unit volume.
o Primary electron current.
o Auger transition probability for that element.
o Ionization cross section of that element by incident and scattered electrons.
o Ionization cross section of that element by scattered electrons.
o Mean free path of the emitted Auger electron.
o Angle between the collected Auger electron and the surface normal.
o Electron detector efficiency
o Surface roughness
Xa = Ia /Sa
∑Ii /Si
A commonly used approach to quantification involves
defining sensitivity factors, Sa, such that, for a measured
Auger intensity, Ia, Ia /Sa is a value proportional to the
concentration of element ‘a’.
A general expression for estimating the atomic
concentration of any constituent in a sample, Xa,
INSTRUMENTATION
Full Featured Scanning Auger Microprobe
Retarding Field Analyser (RFA)
NARUMAND AND CHILDS
N(E) = -S dI(VR ) / dVR D (5)
where N(E) is the desired electron energy distribution,
VR is the retarding grid potential and
Ep is the energy of the electron beam incident on the sample.
Concentric Hemispherical Analyzer (CHA)
(E/E) = RR(E/E0 )
RRT  (E0 /E)  1
NARUMAND AND CHILDS
(where RR is commonly referred to as the retard ratio and E0 is the analyzer
pass energy. As the retard ratio decreases,
AES Instrument Configuration
Elements of Typical Auger System:
 Electron Gun
 Analyzer
 Secondary Electron Detector
 Ion Gun
 Sample Stage
 Introduction System
CYLINDRICAL MIRROR ANALYZER
•
Commercial CMA's are generally based
on a "double pass" design where electrons
travel through the analyser in a figure-ofeight path .
•
This second stage of filtering is intended
to reduce spurious background signal due
to secondary electrons generated within
the analyser.
•
Retarding non retarding modes.
•
In retarding mode the energy resolution is
increased by slowing the electrons before
they enter the analyser using two
hemispherical grids at its snout.
Cylindrical Mirror Analyzer
Outer cylinder
Vouter
Inner Cylinder with
slots cut into it
+
Coaxial electron gun
Sample
Detector
(channeltron)
Rear aperture
1.31Q Vouter
E pass =
 router 

ln 
 rinner 
In order to get best focusing of electrons
(minimization of abberations), CMA’s use a
fixed takeoff angle of 42o from surface normal.
(typically accepts 42o±3o).
Auger Spectra
The Energy distribution of
emitted electrons, N(E),
plotted against KE.
Auger Spectra
a) N(E) spectrum showing the
complete secondary electron
energy
distribution, including the low
energy secondary peak, the
elastically back-scattered
peak, the secondary electron
background, and Auger peaks.
Strong intensity at very low
energies (<50 eV) owing to near
surface secondary electron
emission.
(b) Differentiated N(E)
spectrum, (dN(E)/dE) vs. E.
Secondary
electrons Auger
N(E)
Elastically-scattered
electrons
electrons
Direct spectrum
Energy
Eincident
dN(E)/dE
Derivative
Energy
Eincident
Because Auger transitions are sharp compared with other features, taking the derivative
greatly enhances the signal-to-noise ratio.
Non-differentiated
Differentiated
Scanning Auger: Resolution ~ 100 Angstroms
Steel Fracture Surface
Secondary electron image, 10,000X
Auger Images – Fe (blue), Sb (red), Cr
(green)
AES identified the composition of grain
boundary particles to be Sb and Cr. These
phases resulted in the embrittlement of an
aged steel rotor.
In the Auger map the different region:
titanium (blue), sulphur (green) and silicon (red) are clearly visible with
very good spatial resolution (the horizontal dimension of the picture is 3
µm).
Depth profiling
Example: Al/Pd thin films on GaN
Ion gun can be used
for sputtering –
removing material
from surface. Depth
profiles of the
concentrations of
elements can be
measured:
XPS vs. Auger
XPS/ESCA
Energy resolution < 1 eV
Spot size of analysis
Typically ~1 mm
~1 microns possible
Chemical shifts (oxidation state)
Non-damaging
Auger
Linewidths several eV wide
Typically ~1 mm (CMA,coaxial e gun)
<10 nm possible (Hemi, SEM e-gun)
No chemical shifts
(lineshape analysis possible)
Highly damaging
Al/Pd/GaN Atomic Concentration Data
AES Applications
* Materials evaluation and identification
o Surface contaminants
o Surface homogeneity
o Diffusion profiles
o Particle sizes
o Catalyst degradation
o Interfaces
* Failure analysis
o Corrosion characterization
o Stain identification
o Lifted lead bond evaluation
o Material delamination analysis
o Metal embrittlement evaluation
* Quality control screening
o "Good" to "bad" sample comparison
o Material and plating/coating thickness determination
o Surface process modification
Probe Depth Defines ‘Surface’
•
Infrared Spectroscopy: 1 m
•
Conventional SEM/ EDX: 1 m
•
Auger Electron Spectroscopy 5 nm
•
X-ray Photoelectron Spectroscopy (a.k.a. XPS or ESCA)  5 nm
•
Scanning Tunneling & Atomic Force Microscopy: Top Atomic Monolayer
SAM/Auger Electron Spectroscopy
•
Scanning Auger Microprobe = SEM with e– Energy Analyzer in Vacuum
Chamber
•
P  1.33  10–9 kPa (= 110–8 Torr)
•
Electron Gun + Electron Optics Produce an e– Beam: 2.0 keV  Ekinetic  10.0
keV, Diameter/ Resolution  1 m.
•
Energy Analyzer Measures Energies of the Electrons (not X-rays!) from
Sample.
AES/ SEM: More Comparisons
•
Probe Depth of SEM/ EDX  200  Probe Depth of AES. Elastic Mean Free
Path of e–  2- 3 nm,  Probe Depth  5 nm.
•
As in SEM, Surface Image Can Be Digitized and Stored.
•
As in SEM, Individual Features Can Be Analyzed by e– Beam Positioning.
•
As in SEM/ EDX, Elemental Mapping Is Possible, If Concentrations are High
Enough.
•
When Scanning Auger Microprobe Is Equipped with an Ar+ Ion Gun, Depth
Profile Analyses Are Possible (Ion Milling). 3 keV  Ekinetic(Ar+)  5 keV
Raw Data Survey Spectrum
Min: 65332
Max: 457010
E (Primary Electron Beam) = 5.0 keV
I (Sample) = 50 nA
Backscattered
Electrons
N(E)
Auger Electrons
30 230 430 630 8301030
1230
1430
1630
1830
2030
Kinetic Energy (eV)
Interpretation: Peak Positions
1500
1
· eV )
1000
1
500
dN(E)/dE (Counts · S
0
-500
-1000
268 eV  C1
S(C1) = 0.140
416 eV  Ti4
-1500
382 eV  Ti3
S(Ti3) = 0.314
-2000
-2500
200
300
400
Kinetic Energy (eV)
509 eV  O2
S(O2) = 0.271
500
Interpretation: Peak Intensities
1500
 Int = 1095.343 C·S-1·eV-1
1
· eV )
1000
dN(E)/dE (Counts · S
1
500
0
 Int = 4788.79 C·S-1·eV-1
-500
-1000
 Int =
6363.39 C·S-1·eV-1
[C]  1095.343/ 0.140
-1500
[Ti3]  4788.79/ 0.314
-2000
[O2]  6363.39/ 0.217
-2500
200
300
400
Kinetic Energy (eV)
500
Derivative Survey Spectrum
Min: -3828
Max: 2535
dN(E)
C1
Atomic Concentration
O2
50.4 %
O2
54.2 %
Ti4 Ti3
32.7 %
Ti4
27.7 %
C1
16.8 %
C1
18.1 %
[O]/ [Ti] =
[O]/ [Ti] =
Ti3
1.54
2.00
O2
30 2304306308301030
1230
1430
1630
1830
2030
Kinetic Energy (eV)
Example of Depth Profile
Min: 0Max: 100
Ta2
Ta2
80
O2
60
%
40 Ta2
O2
20
O2
0
160
320
480
640
Depth (angstroms)
800
SED Image
SED Image, Spots Identified
#1
#2
#3
#4
Auger Element Map: C1
#1
#2
#3
#4
Auger Element Map: Sn1
#1
#2
#3
#4
Auger Element Map: Sb1
#1
#2
#4
#3
Auger Element Map: O1
#1
#2
#3
#4
F750, Spot #1 (See Map)
Min: -3086
Max: 1554
Mapped Surface, Spot #1
Cl1
Sb1
dN(E)
O1
Sn1
Atomic Concentration
O1
14.9 %
Sb1 0.7 %
Sn1 7.7 %
C2
76.1 %
Cl1
0.5 %
C2
30 2604907209501180
1410
1640
1870
2100
2330
Kinetic Energy (eV)
F750, Spot #2 (See Map)
Min: -5488
Max: 6006
Mapped Surface, Spot #2
dN(E)
Bi1
Sb1
C1
O1
Sn1
Atomic Concentration
O1
38.4 %
Sb1 2.0 %
Sn1
27.5 %
C1
31.4 %
Bi1
0.7 %
30 2604907209501180
1410
1640
1870
2100
2330
Kinetic Energy (eV)
F750, Spot #3 (See Map)
Min: -3683
Max: 2820
Mapped Surface, Spot #3
Bi1
dN(E)
Sn1
O1
C1
Atomic Concentration
O1
10.3 %
Sb1 27.8 %
Sn1 6.0 %
C1
55.3 %
Bi1
0.6 %
Sb1
30 2604907209501180
1410
1640
1870
2100
2330
Kinetic Energy (eV)
F750, Spot #4 (See Map)
Min: -5487
Max: 6135
Mapped Surface, Spot #4
dN(E)
Bi1
Sb1
C1
O1
Sn1
Atomic Concentration
O1
38.6 %
Sb1 1.6 %
Sn1
25.5 %
C1
34.2 %
Bi1
0.1 %
30 2604907209501180
1410
1640
1870
2100
2330
Kinetic Energy (eV)
Advantages of AES
o
o
o
o
Applicable to all elements except H and He
High spatial resolution
Subsurface analysis can be performed by depth
profiling with inert gases
Rapid analysis
Limitations
o
o
o
o
o
o
Quantitative analysis can be difficult Surface of sample may be
damaged by electron beam
Applicable to many types of samples, but insulators are difficult
due to surface charging
Subsurface analysis by ion sputtering is destructive
Sampling depth: 0.5-10nm
Detection limits: 0.1-1at.%
Accuracy: ± 30% if using published elemental sensitivity
±10% if using standards that closely resemble the sample
Future of AES
•Advances in AES may come in the form of improved software
• Attempt to compile Auger data into a database databases to
allow more reliable peak identification
• These improvements will lead to smaller surfaces being studied
with AES, which would be useful due to the growing trend
toward miniaturization
References
G. Gergely, “Commemoration of the 25th anniversary of Auger electron spectroscopy,”
Vacuum 45, 311 (1994).
D. Briggs and M.P. Seah, Practical Surface Analysis, Wiley, New York (1983), 2nd
Ed. Vol. 1 (1990).
I.F. Ferguson, Auger Microprobe Analysis, Adam Hilger, Bristol (1989).
G.C. Smith, Surface Analysis by Electron Spectroscopy, Plenum Press, New York
(1994).
H. Ibach (ed.), Electron Spectroscopy for Surface Analysis, Springer-Verlag, Berlin
(1977).
D. Roy and D. Tremblay, “Design of electron spectrometers,” Rep. Prog. Phys. 53,
1621 (1990).
http://www.lasurface.com/database/spectres.php