les Auger.pdf

Auger electrons 

Auger Electron Spectroscopy AES 

Scanning Auger Microscopy SAM 

1 Surface Analysis: Scanning Auger Electron Spectroscopy

2 Surface Analysis: Scanning Auger Electron Spectroscopy 

Sec electron

The Auger Process 

Auger Process 

The Auger effect is named for its discoverer, Pierre Auger, who observed a 

tertiary effect while studying photoemission processes in the 1920s. Auger 

electrons are emitted at discrete energies that allow the atom of origin to be 

identified. The Auger process involves three steps: 

1. Excitation of the atom causing emission of an electron 

2. An electron drops down to fill the vacancy created in step 1 

3. The energy released in step 2 causes the emission of an Auger electron. 


In general, since the initial ionisation is non-selective and the initial hole may therefore be in various shells, there will be 

many possible Auger transitions for a given element - some weak, some strong in intensity. AUGER SPECTROSCOPY 

is based upon the measurement of the kinetic energies of the emitted electrons. Each element in a sample being 

studied will give rise to a characteristic spectrum of peaks at various kinetic energies. 


Auger or X-ray Emission! 

X-ray Fluorescence Auger Electron Emission 

X-ray 

Photon 

Incident 

Beam 


Auger Electron

Auger Transition Probability 

The total Auger probability is given by the Auger emission 

versus the X-ray fluorescence such that ρ A + ρ X = 1 

This does not indicate the probability of a particular 

transition, i.e.,for that there exits the relative probability of 

de-excitation from subshells Xa, Yb, Zc A family of Auger peaks result from transitions between 

subshells. 


Probability to desexcitate by X-rays or Auger 

After ionisation 

either X-ray or Auger 

ω Auger+ ω X-ray =1 

A clear transition from electron to photon emission 

is evident in this chart for increasing atomic number. 

For heavier elements, X-ray yield becomes greater than Auger yield, 

indicating an increased difficulty in measuring the Auger peaks for large Z-values. 

Conversely, AES is sensitive to the lighter elements, 

and unlike X-ray fluorescence, Auger peaks can be detected for elements as light as lithium (Z = 3). 

Lithium represents the lower limit for AES sensitivity since the Auger effect is a "three state" event necessitating 

at least three electrons. Neither H nor He can be detected with this technique. 

For K-level based transitions, Auger effects are dominant 


Auger and Fluorescence Yields 


ρ A + ρ X = 1

ELECTRON BEAM - SAMPLE INTERACTION 

Secondary Electrons 

Backscattered Electrons 

Sample Surface 

Atomic No. 3 

Characteristic X-rays 

> Atomic No. 4 

Volume of 

Primary 

Excitation 

Φ

Electron spectrum in case we use primary electrons 

Secondary electrons 


Auger electrons 

Back scattered electrons

Auger Electron Spectroscopy Cu element 

E KLL = E K - E L - E L’ 

E f 

2p 

2s 

1s 

3/2 

1/2 

E KLL 

Auger 

Electron 

LIII LII 11 Surface Analysis: Scanning Auger Electron Spectroscopy 

L I 

K 

Cu MNN 

Incident Beam 

Cu LMM 

EdN(E)/dE 

E N(E) x 5 

E N(E) 

0 500 1000 1500 2000 2500 3000 

Kinetic Energy (eV)

Examples of Auger spectra 

direct mode and derivative mode 

N(E) or dN(E)/dE versus E spectrum of C 


The Auger electron characteristic energy 

depends 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 eventually becomes the 

Auger electron (a small energy correction takes into account the fact 

that already the atom is ionized 

Φ is the work function of the spectrometer (not the material). for 

which the detector is calibrated. The energy levels in solids are 

conventionally measured with respect to the Fermi-level of the solid, 

rather than the vacuum level. This involves a small correction to the 

equation given above in order to account for the work function (F) of 

the solid 


Possible Auger transitions 

Auger are conventionally XYZ called 

Possible peaks are related with electron- 

electron interactions allowed 

given by quantum physics (electron-electron 

interactions) 

So not all transitions are possible like with X-rays 

This is perfectly known for the Auger transitions 

of all the elements 



Possible Auger 

Transitions

Possible Auger Transitions for 

element Al 

Transitions Energy ev 

KL1L1 1293 

KL1L2 1342 

KL1L3 1343 

KL1M1 1443 

KL2L2 1386 

KL2L3 1387 

KL2M1 1478 

KL3L3 1388 

KL3M1 1551 

L1L2M1 36 

L1L3M1 37 

L1M1M1 109 

L2M1M1 65 

L3M1M1 64 


Analysis Depth 

Surface sensitivity results from the electron inelastic 

mean free path, λ, which is the average distance 

an electron with a given energy travels before 

being inelastically scattered (& therefore losing its 

characteristic energy). 

λ depends on electron energy & material 

λ ~ 1 - 10 monolayers (0.2 - 5 nm) 


Auger Mean Free Path 


Surface sensitivity in AES arises from the 

fact that emitted electrons usually have 

energies ranging from 50 eV to 3 keV and 

at these values, electrons have a short 

mean free path in a solid. The escape 

depth of electrons is therefore localized to 

within a few nanometers of the target 

surface, giving AES an extreme sensitivity 

to surface species. 

Due to the low energy of Auger electrons, 

most AES setups are run under ultra-high 

vacuum (UHV) conditions. Such measures 

prevent electron scattering off of residual 

gas atoms as well as the formation of a thin 

"gas (adsorbate) layer" on the surface of 

the specimen which degrades analytical 

performance

Auger Electron Analysis Depth 

Various thicknesses of Au on Si. The high energy Si KLL peak has a greater 

analysis depth than the low energy Si LMM peak. So you don’ observe the Si 

LMM transition (low energy) for the thick Au film 

Au 

Au 

Si LMM 

Si LMM 

O 

O 


Thickest Au 

Thinner Au 

Clean Si 

Au 

Si KLL 

Au 

Si KLL 

Si KLL 

Au 

Au 

Au 

Au

Intensity of a Auger transition 

Transmission of the analyser 

Mean free path Auger electrons 

Primary current beam 

depends on 

Escape probability of the Auger transition WXY 

Ionisation cross section 

Back scattering effects 

Number of atoms present in the layer 


Analysis Area - Spatial 

Resolution 

Spatial resolution depends on the analytical measurement and the 

definition of resolution 

To a first approximation it depends on the diameter of the primary 

electron beam so it depends on the quality of the used electron gun 

(see previous lectures). This is why the best systems use FE 

technology 

This is why Scanning Auger is one of the best imaging methods in terms 

of lateral resolution and providing surface analysis info 

May also be affected by Auger emission due to backscattered electrons 

(this is way it is not better than 7 nm at the moment) 


Factors Affecting Spatial 

Resolution 

Energy of Auger electrons 

How much energy backscattered electrons can 

lose & still cause ionization & subsequent 

Auger emission 

Inelastic mean free path 

Sample composition 

Sample topography 

System vibration & stray magnetic fields 


Components of an AES 

Instrument 

An AES instrument has three main components: 

An electron source, this should have variable energy and be capable of 

producing a very small spot of electrons. 

An electron energy analyser, for example, a spherical sector analyser 

A secondary electron detector for the production of SEM's 

The measurements must be made in ultra-high vacuum (UHV), for two reasons: 

To allow the Auger electrons to travel from the surface of the sample to the 

detector without striking a gas atom 

If a clean surface is prepared for analysis, it would become contaminated if it was 

not under UHV. 


Auger system 

Primary source : electron gun 

La 6B 

New AES systems are using a Field Emission 

guns 

Energy of the Auger electrons are measured 

Electron detector : cylindrical mirror- CMA, 

hemispherical analyser - HA 


Also in the Equipment 

Ultra High Vacuum (UHV) system 

Sample handling 

Quick sample introduction 

5 axis sample manipulator 

Ion sputter gun 

Hot filament - inert gas 

Hollow cathode plasma source 

duoplasmatron - inert & reactive gases 


Equipment CMA based 



PHI 680

Cylindrical Mirror Analyser 

Relative Energy Resolution 

typically 0,25 % ∆E/E so not so good 

Ideal for geometry as electron gun is placed 

coaxial 


Auger Instrumentation 

Electron Column & Electron Energy Analyzer 

Field Emission 

Electron Source 

Multi-Channel 

Detector 

Cylindrical 

Mirror Analyzer 

Sample 


Eight concentric 

ring anodes 

Ion Gun


Analysis Area - Defects on Al 

Metallization 

Pseudocolor Auger Map of Cu, showing the 

concentration of Cu in the nodules 

SEM image Cu Auger map 


Auger maps resolve 100 nm Cu particles

Scanning Auger microscopy, Scanning Auger Microprobe HA based 

Electrons can be focused to a spot of < 1 nanometer 


Scanning electron microscope 

column 

Penetration depth and scattering within solid typically limit resolution 

to ~20 nanometer. Recent instruments claim resolution of ~ 6 nm. 

Often use hemispherical analyzer because of its better geometric properties 

Electron energy 

Analyzer


Jeol equipment

Hemispherical Analyser 

Variable energy resolution from 0.05% to 

0.6% so very good compared to CMA 

But electron gun is not coaxial 


Information contained in the 

Auger spectrum 

If we measure the energy of the ejected Auger electrons we can learn 

some important facts about the sample under investigation: 

The elements from which it is made 

The relative quantity of each element 

The chemical state of the elements present (provided the energy is 

measured with sufficient resolution) 

The lateral distribution of the elements 

The depth distribution of the elements 


Quantification from first principles 

I i = I P N i σ i γ i (1+r) λ icosθ F T D R 

I i = Auger intensity (current) for ABC transition of element i 

N i = # atoms of element i per unit volume 

I P = Primary electron beam current 

σ i = Ionization cross-section for the A level of element i 

γ i = Auger transition probability for the ABC transition of element i 

r = Secondary ionization for the A level of element i by scattered 

electrons (backscatter factor = 1+r) 

λ i = Inelastic mean free path of emitted Auger electron in the matrix 

θ = Angle between Auger electron and surface normal 

F = Analyzer solid angle of acceptance 

T = Analyzer transmission function 

D = Detector efficiency 

R = Surface roughness factor (0

Quantification 

Sensitivity Factors 

Assuming Auger yield varies linearly with concentration 

Assuming homogeneous distribution 

C X = (I X / S X) / (Σ i (I i / S i) 

S X may depend on sample matrix & chemistry, as well as 

specific analytical instrument 


On this level ??? 


Standards


Sensitivity factors

Concentration calculation 


Sputtering used for cleaning 


+depth profiling

Depth Profiling measures AES intensity as function 

of sputter time while in reality one wants 

concentration as function of depth 


x 104 

3 

2 

1 

0 

c/s 

-1 

-2 

-3 

Depth profile of Chromate-fluoride conversion layer 

on Al 

F 

C Cr 

P Cr 

Al 

O 

200 400 600 800 1000 1200 1400 1600 1800 

Kinetic Energy (eV) 


Al 

P 

100 

Intensity 

(%) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

C 

P 

O 

Cr 

F 

Alox 

Alm 

0 5 10 15 20 

Sputter time (min)

Zalar Depth Profiling 

Many materials exhibit increases in surface 

roughness under extended ion bombardment 

The roughness is often characterized by the 

formation of “cones” that grow in the direction of 

ion bombardment 

Zalar rotation involves the physical rotation of a 

sample during ion bombardment to minimize cone 

formation 

The use of Zalar rotation improves the quality of 

Auger compositional depth profiles 


Zalar Depth Profiling 

Blanket thin film structure 

Auger depth profile results 

without and with Zalar rotation 

Aluminum (500 nm) 

SiO 2 (20 nm) 

Silicon (substrate) 


Aluminum 

Oxygen 

Without Zalar rotation 

Silicon 

Aluminum Silicon 

Aluminum 

Oxide 

Oxygen 

With Zalar rotation 

Silicon 

Oxide

10 µm Via Contact Depth Profile 

5000X 


Without Rotation 

Atomic Concentration Concentration (%) 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

O 

Al in oxide 

Al metal 

Without rotation interface is broad 

0 

0 50 100 150 200 250 300 

Sputter Time (min) 

Si

Depth Profile With Compucentric Zalar 

5000X 


Rotation 

Atomic Atomic Concentration (%) 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

O 

Al in oxide 

Al metal 

0 

0 50 100 150 200 250 300 

Sputter Time (min) 

With Zalar Rotation there is a much sharper interface 

Because of size of bond pad, compucentric Zalar control is required 

Si

EdN(E)/dE 

Chemical Information summary slide 

Energy position 

Line shape 

Loss Structure 

Graphitic C 

W Carbide 

C KLL 

230 240 250 260 270 280 290 300 


C KLL 

Core-Valence-Valence 


EdN(E)/dE 

EdN(E)/dE 

Elemental Al 

Al LMM 

Al Oxide 

40 50 60 70 80 90 


Elemental Si 

Si Oxide 

50 60 70 80 90 100 110 


Si LMM 

EdN(E)/dE 

EdN(E)/dE 

Elemental Al 

Al Oxide 

Al KLL 

1280 1300 1320 1340 1360 1380 1400 1420 


Elemental Si 

Si Oxide 

1500 1520 1540 1560 1580 1600 1620 1640 


Si KLL

Chemical Information 

High Energy Resolution or Numerical Analysis may be used to 

distinguish chemical components 

In many cases numerical analysis is required even with high energy 

resolution 

High energy resolution reduces the count rate and sensitivity of the 

measurement 

High energy resolution often requires a priori knowledge of the chemistry 

High energy resolution does provide sharp spectral line shapes 


Chemical Information 

Variations in an element’s chemical state may affect: 

Binding energies 

Relaxation energies 

Auger transition probabilities 

Valence band density of states 

Conduction band density of states 

Bulk and surface plasmons 

These variations are reflected in the Auger spectrum 

Auger peak energy position 

Auger line shape 

Loss structure 


Chemical shift of the energy levels results in 

netto “small”shift of the Auger energy 


Chemical shift on Al KLL Auger lines for three 

different components Al-pure, Al 2O 3 

amorphous, Al 2O 3 ceramic mineral polymer 


New LVV Auger peaks combining 

energy levels of two neighbours atoms 


eg Si, Al, Mg oxides 

EdN(E)/dE 

Elemental Al 

Al LMM 

Al Oxide 

40 50 60 70 80 90 

Kinetic Energy (eV)

Valence band transitions where energy levels 

of the valence band participates. Density of 

States is reflected in shape of the peaks 


Carbon CVV Auger transition 


Cation effect on the S LVV 


transition

Si KLL Transition taken with high energy resolution 


In summary chemical Information 

Measurements have been made for: 

TiN composition 

C: diamond, graphite, carbide 

Si: elemental, oxide, and silicide 

Metal oxides: 

Al, Si, Mg, Cu, Ce, Ti, Sn. 

Metal silicides 

Ti, W, Mo 


Limitation of Auger 

In most cases Auger analysis is limited to conductive 

samples as a primary electron beam is used and 

conductive coating as used with SEM-EDX is not possible 

here due to the strong absorption of the low energy 

electrons 

Some experiments are performed on non conducting 

samples done by tilting the samples to grazing angles. 

Also some positive charge compensations is done by 

using the ion gun at low energies 

Some experiments are done based on X-rays XAES called 

but this limits the lateral resolution 


Possible Electron Beam Effects 

Sample Charging 

Desorption 

Adsorption 

Oxidation 

Reduction 

Dissociation 

Decomposition 

Erosion 

Diffusion 


Auger Electron Spectroscopy 

Auger Electron Spectroscopy is an analytical technique that provides compositional 

information from the top few monolayers 

Sample restrictions conductive samples as primary electrons are used 

Detect all elements above He 

Detection limits: ~0.1 - 1 atomic % 

Surface sensitive: top 0.4-5 nm 

Spatial resolution:7 nm imaging 

Semi-quantitative: relative sensitivity factors 

Distribution of the elements 

Line Scans 

Depth Profiles 

Maps 

Limited chemical info based on the chemical info resolved either in the chemical 

shits of the peaks or peak shape (for Valence band transitions) 


References 

Slides provided by PHI company 

D. Briggs and M.P. Seah "Practical Surface Analysis", Vol 1, 

"Auger and X-ray Spectroscopy", Wiley, Chichester, 2nd 

Ed, 1980. Vol 2 “Ion and Neutral Spectroscopy” 

T.A. Carlson "Photoelectron an Auger Spectroscopy" Plenen 

Press, New York, 1975 

“X-ray Photoelectron and Auger Electron Spectroscopy” 

TERRYN H. and HUBIN A. Chapter in “Non-destructive 

Microanalysis of Cultural Heritage Materials”, 

Comprehensive Analytical Chemistry, XLII, Edited by K. 

Janssens, R. Van Grieken, Elsevier Science, (2004).

les Auger.pdf

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