Introduction to Auger Electron Spectroscopy

(AES)

Center for Microanalysis of Materials

Frederick Seitz Materials Research Laboratory

University of Illinois at Urbana-Champaign

What is Auger?

Auger Electron Spectroscopy (AES) is a Auger Electron Spectroscopy (AES) is a widely used technique to investigate the widely used technique to investigate the chemical composition of surfaces.chemical composition of surfaces.

Why the Odd Name?

The Machine

The Man

Auger (as in ‘Pierre’)

1923:Pierre Auger discoversthe Auger process

Electron Spectroscopy

Pierre Victor Auger (May 14, 1899 – December 24, 1993) was a French physicist, born in Paris. He worked in the fields of atomic physics, nuclear physics, and cosmic ray physics. [1] The process where Auger electrons are emitted from atoms is used in Auger electron spectroscopy to study the elements on the surface of materials.[1] This method was named after him, despite the fact that Lise Meitner discovered the process a few years before in 1922. http://en.wikipedia.org/wiki/Pierre_Victor_Auger

Why is Auger so surface sensitive?

Ref: Charles Evans & Assoc. web page tutorial by Ron Flleming http://www.cea.com

IncidentElectron

STEP 3(alternative)an x-ray is emittedto conserve energyreleased in step 2

STEP 3KLL Auger electronemitted to conserveenergy released instep 2

STEP 2L electron fallsto fill vacancy

STEP 1Ejected electron

-or-

K

L1

L2

L3

1s

2s

2p

CONDUCTION BAND

VALENCE BAND

FREEELECTRONLEVEL

FERMILEVEL

E(Auger)=E(K)-E(L2)-E(L3)

E(X-ray)=E(K)-E(L2)

Auger Transition Named:KLL

Auger Energy Scale

The AES instrument measures The AES instrument measures the kinetic energy of all collected the kinetic energy of all collected electrons.electrons.

AES Electron vs. X-ray Photon

Relative Probabilities of RelaxationRelative Probabilities of Relaxationof a K Shell Core Holeof a K Shell Core Hole

5

B Ne P Ca M n Z n Br Z r

10 15 20 25 30 35 40 Atom ic Number

Elem en tal Sym bol

0

0.2

0.4

0.6

0.8

1.0

Pro

ba

bili

ty

Note: The lightNote: The lightelements have aelements have alow cross sectionlow cross sectionfor X-rayfor X-rayemission.emission.

Auger ElectronAuger ElectronEmissionEmission

X-ray PhotonX-ray PhotonEmissionEmission

Auger Stats

Primary Beam = 3 - 20 KeV electrons

Detection Sensitivity = ~1 atomic %

Sampling Distance (depth) = 2 to 4 nm

Analysis Diameter = 80nm to several mm

Elements Detectable = Li and above

Auger Data Formats

Raw Data Differentiated Data

AES Can Identify Elements

Auger Sensitivity Factors

Elemental variation in sensitivity factors3kV Primary Beam Voltage

5kV Primary Beam Voltage

Ref: Charles Evans & Assoc. web page tutorial by Ron Flleming http://www.cea.com

Peak Height / Quantitation

For quantitation the derivativeform of the data is used.

Sensitivity Factors are usedto adjust the peak heights ofall elements present in thesample.

The composition is normalizedto 100% to determine the amountof each element.

Identify surface contaminants and Identify surface contaminants and composition.composition.

Study composition as a function of Study composition as a function of depth.depth.

Analyze sample features as small as Analyze sample features as small as 80nm. Using an electron gun for the 80nm. Using an electron gun for the primary beam allows small analysis primary beam allows small analysis spot sizes.spot sizes.

Additional Capabilities of Auger Spectroscopy

Remove adsorbed gases from Remove adsorbed gases from the sample.the sample.

Eliminate adsorption of Eliminate adsorption of contaminants on the sample. contaminants on the sample.

Prevent arcing and high voltage Prevent arcing and high voltage breakdown.breakdown.

Increase the mean free path for Increase the mean free path for electrons, ions and photons.electrons, ions and photons.

Degree of VacuumDegree of Vacuum1010

1010

1010

1010

1010

22

-1-1

-4-4

-8-8

-11-11

Low VacuumLow Vacuum

Medium VacuumMedium Vacuum

High VacuumHigh Vacuum

Ultra-High VacuumUltra-High Vacuum

PressurePressureTorrTorr

Why UHV for Surface Analysis?

AES Instrument Configuration

Elements of Typical Auger System: Electron Gun Analyzer Secondary Electron Detector Ion Gun Sample Stage Introduction System

Significance of Primary Beam

The Electron Beam is functioning The Electron Beam is functioning both as the imaging beam and the both as the imaging beam and the primary beam for analysis. This meansprimary beam for analysis. This meansthat the area of interest on the samplethat the area of interest on the samplecan be directly aligned for analysis.can be directly aligned for analysis.

MRL Instrumentation

PHI Model 660 Scanning Auger Microprobe

Sputtering Samples

Al/Pd/GaN Thin Film Example

(cross section)

Al/Pd/GaN Profile Data

Al/Pd/GaN Atomic Concentration Data

Area Specific Depth Profile Example

SP UTTER T IM E (M IN .)

PE

AK

-TO

-PE

AK

Fracture surface of Carbon fibers in BN matrix - analysis area outlined in black

Depth profile on fiber to determ ine point of fracture. Variations in fracture surfaceinterface for d ifferent sam ple treatments w ill be reflected in depth profile.

From research by C . Cofer/J. Econom y, M aterials Science D ept.

Summary of Surface Techniques

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