An Introduction to Auger Electron Spectroscopy

Spyros Diplas MENA3100

SINTEF Materials & Chemistry, Department of Materials Physics & Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO

1 Contents

 Background

 An Auger microprobe

 The Auger process

 Auger spectroscopy

 Auger mapping

 Depth profiling

2 Surface Analysis - Techniques Available

 Properties and reactivity of the surface will depend on:  bonding geometry of molecules to the surface  physical topography photons EXCITATION EMISSION  chemical composition  chemical structure ions  atomic structure electrons  electronic state

Interaction No one technique can provide all these with material pieces of information. However, to solve a specific problem it is seldom necessary to use every technique available.

TRANSMISSION

3 The Auger process

 Named after Pierre Auger  1925

 Actually discovered by Lise Meitner  1922

 Emission of an intial electron leads to the emission of a characteristic (Auger) electron.

4 Auger Electron Spectroscopy (AES) Signal Exciting radiation Electrons Electron beam (Scanning) (Spectrometer)

UHV vacuum

Analysis depth (typically a few nm) SAMPLE

Auger emission

EKL2,3 L2,3 ≈ EK – EL2,3 – EL2,3

E = energy of emitted electron EK = K-shell ionisation energy EL2,3 = L-shell electron energies Auger electron vs x-ray emission yield

1.0 Auger Electron Emission

0.8

0.6

0.4 Probability 0.2 X-ray Photon Emission 0 5 10 15 20 25 30 35 40 Atomic Number B Ne P Ca Mn Zn Br Zr Elemental Symbol

6 Auger - lateral resolution

160 kX SEM

4µm at 20kV

0,1µm 2kV 160 kX Auger Maps

n.b. much better than EDS in bulk samples

7 Auger Electron Spectroscopy The JEOL JAMP-9500F FE Auger Microprobe  UHV Chamber  FE-SEM quality electron column  nm-scale depth resolution  Depth profiling

Basic Specification Additional capabilities:  3nm SEI resolution EDS system  8nm probe diameter for Auger •”Bulk” composition analysis. analysis Backscattered electron detector  Variable energy resolution from •Atomic number contrast. 0.05% to 0.6% Heating stage  Chemical state analysis in several •Diffusion experiments. 10nm areas Liquid nitrogen fracture stage  Ion gun for sputter depth profiling •Grain boundary and interface studies. Allows some charge neutralisation Electron Beam Induced Current (EBIC) for analysis of insulating materials •Recombination centre mapping in solar cells, etc.

8 Origin of Auger signal

LMM Auger electron emitted

Auger electron generation

9 Electron spectrometer

Hemispherical spectrometers often used. Tend to be used in constant retard ration mode (CRR), as this supresses the strong low energy signal. Constant analyser energy (CAE) also has uses. Cylindrical mirror analyser used to be most common, still in use.

10 Auger spectrum - typical

It is quite common to deal with the differential form of the spectrum.

11 Chemical shift in Si compounds

Comparison of Si, SiNx and SiO2

12 Auger spectrum - quantification

Can use peak areas or peak-to-background ratios

13 Auger spectrum - quantification

Concentration of element NA is:

NA = IA/(IA + FABIB+FACIC+....)

I is the element intensity F is a sensitivity factor determined from binary standards such that:

FAB = (IA/NA/IB/NB)

This is highly simplified but can work reasonable well.

14 Auger mapping

Pixel-by-pixel, typically calculate; (peak-background)/background

15 Combined SEM/Auger analysis Carburised alloy

200nm

16 Combined Auger EDS mapping

17 Auger and XPS spectra of Si: p and n Type

AES excited Si KLL n-c-Si

n-a-Si

Arbitrary Units Intrinsic-c-Si

Intrinsic-a-Si

p-c-Si

110 108 106 104 102 100 98 96 94 92 X-ray excited Si KLL Binding Energy (eV)

n-c-Si

n-a-Si Intrinsic-c-Si Arbitrary Units

Intrinsic-a-Si

p-c-Si

1608 1610 1612 1614 1616 1618 1620 1622 Kinetic Energy (eV)

18 Auger mapping of Si p and n type

19 AES – nm lateral and depth resolution Chromated aluminium alloy surface • Chromating pretreatment – corrosion protection – before coating, painting, bonding, etc • Does the chromate ”passivate” intermetallic particles? O 2 2 • Auger spectra show that the intermetallic 1 1 particles are covered by a thin layer of Cr-oxide Cr metal reference that is invisible in the 430 450 470 490 510 530 550 570 590 SEM images. Electron kinetic energy / eV

20 Combined SEM/TEM/Auger analysis

TEM analysis

21 Depth profiling - schematic Electron beam

Argon ion gun

Sample

Process is fully automated, sample can be rotated (Zalar rotation). N.B. Can also perform angle-resolved analysis.

22 Depth profiling – raw data

Surface and after sputtering – as grown Pd/Ag membrane

23 Depth profiling - example

Multiple quantum well structures

24 Depth profiling CeO2 buffer layer

25 Depth profiling CeO2 buffer layer

26 Potential Auger Applications  Conducting / semiconducting materials  Corrosion studies  Coatings  Depth profiling  Catalysis  Carbon/other material fibres  Metallurgy  Grain boundaries in steels  Can be extended to low conductivity materials  Use low energy ion gun to flood surface  Probably not polymers

27 Summary

 Combine SEM and electron spectroscopy  High lateral and depth resolution

 Chemical state information  Need to explore the possibilities

 Depth profiling  ”Bread and butter” applicaion.

 Segregation and interface studies  Surfaces and internal interfaces

 Complementart with other techniques  XPS/TEM……

28 References  Scanning Auger Electron Microscopy; M. Prutton and M. M. El. Gomati, Eds., John Wiley and sons, 2006.

 Practical Surface Analysis by Auger and Photoelectron Spectroscopy; D. Briggs and M. Seah, John Wiley, 1983

 An Introduction to Surface Analysis by XPS and AES; J. F. Watts and J. Wolstenholme, Wiley, Chichester, 2003.

 Surface analysis by Auger and x-ray photoelectron spectroscopy; D. Briggs, J. T. Grant, Eds.; IM: Chichester, 2003

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