AUGER ELECTRON SPECTROSCOPY MAHBOOB ALAM CA12M004 15/04/2013

Pronunciation and history The Auger effect, pronunciated as AW-zhar. The name Auger effect comes from one of its discoverers Pierre Victor Auger. Although auger emission is intense, it was not used until 1950’s.

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

Auger Electron Spectroscopy Auger Electron Spectroscopy (AES), is a widely used technique to investigate the composition of surfaces. First discovered in 1923 by Lise Meitner and later independently discovered once again in 1925 by Pierre Auger. Pierre Victor Auger Lise Meitner 1. P. Auger, J. Phys. Radium, 6, 205 (1925).

Particle-Surface Interactions electron electron Ions Electrons Photons Ions Electrons Photons Vacuum CATSYMP19 PRESCHOOL

Basic theory Atomic ionization (by removal of a core electron) Auger spectroscopy can be considered as involving three basic steps : Atomic ionization (by removal of a core electron) Electron emission (the Auger process) Analysis of the emitted Auger electrons

Equation KE=hv-BE-Ø KE ===== Kinetic Energy (measure in the XPS spectrometer). hv ===== photon energy from the X-Ray source (controlled). Ø ===== spectrometer work function. It is a few eV, it gets more complicated because the materials in the instrument will affect it. Found by calibration. BE ==== is the unknown variable.

Why UHV for Surface Analysis? Pressure Torr Degree of Vacuum 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. 10 2 Low Vacuum 10 -1 Medium Vacuum 10 -4 High Vacuum 10 -8 Ultra-High Vacuum 10 -11

AUGER PARAMETER Primary Beam=3-20KeV electrons Detection Sensitivity=1% Sampling Distance(depth)=2to4nm Analysis Diameter=80nm to several millimeter Element Detectable=Li and above

Probabilities of X-rays Fluorescence and Auger emission 1 1 Fluorescence yield Auger yield .5 .5 15 30 45 60 75 90 Atomic number

PHOTO ELECTRON VS AUGER ELECTRON EMISSION KE=hv-BE-Work function Photo electron Emitted Auger e Incident Xrays Emitted X-rays WORK FUNCTION Fermi Level BE L2,3 L1

Characteristic of AES Characterstics AES Primary beam Electron Analyzed beam Types of sample conductive Area of analysis 10nm Surface selectivity 1to5nm Elemental identification All except H and He Sensitivity .10% Nature of chemical bombarding Shift and shape Depth profiling Elemental,chemical Destructive nature none

X-rays vs. electron Beam Hit all sample area simultaneously permitting data acquisition that will give an idea of the average composition of the whole surface. Electron Beam It can be focused on a particular area of the sample , determine the composition of selected areas of the sample surface.

AUGER ELECTRONS ARE PRODUCED IN TWO DIFFERENT WAYS 1. The X-Ray source can irradiate and remove the e- from the core level causing the e- to leave the atom. A higher level e- will occupy the vacancy. The energy released is given to a third higher level e-. This is the Auger electron that leaves the atom. 2. The electron gun can irradiate and remove the core e- by collision. Once the core vacancy is created, the Auger electron process occurs the same way.

AES INSTRUMENT CONFIGURATION Elements of Auger System Electron gun Analyser Secondary Electron Detector Ion Gun Sample Stage Intoduction System

ANALYSER CONFIGURATION Generally Two Types Cylindrical Mirror Analyzer(CMA) Hemisphericalsector Analyzer(HAS)

Salient features Resolution scales with Ep. Coaxial design eliminates topography shadowing. Better transmission than an HSA Relatively short working distance

Salient feature Better energy resolution Longer working distance possible Angle dependent measurement possible

Calculation of Auger electron spectroscopy Kinetic energy of each Auger electron (EAUG): Core hole of ionization of K electron BE in O =532 eV(EA) BE of L1 electron in O=24eV(EB) BE of L2,3 electron in O=7eV(EC) Auger electron Kinetic Energy (EKLL) in O=532-24-7=501eV EAUG=ECore-EB-EC ECore State: the core level electron energies EB : first outer shell electron energies EC* : second outer shell electron energies http://mse.hanyang.ac.kr/SNE/

KE versus BE KE can be plotted depending on BE Binding energy No of electrons (eV) KE can be plotted depending on BE Each peak represents the amount of e-s at a certain energy that is characteristic of some element. BE increase from right to left 1000 eV 0 eV E KE increase from left to right

FOUR ways to represent Auger spectra a)N(E) vs E b)dN(E)/d(E)vs E c)d(E.N(E)vs E d) E.N(E)vs E

Examples for AES: Chemical Analysis AES is one of the best complementary technique for XPS in the chemical analysis. Depending on the kinetic energy of the Auger electrons, AES is much more sensitive to the surface. Chemical shifts and Auger line shape can be used to determine the chemical state for a given element in the sample, and is studies as charge transfer in alloys. Differences in the line shape and peak Position for the C Auger (KVV) in different CxHy compounds

Auger chemical state information for a Ge single crystal with layer of oxide 1130 1150 1170 1190 Kinetic Energy(eV)

Auger spectral from element silicon and oxidized silicon showing the chemical shift which occur in the oxide SiO Si-Si 1630 1600 1610 1620

Auger depth profiling Sources of artifacts sample charging topographical features resulting of non-uniform sputtering of the sample preferential sputtering beam effects Ion beam mixing R.Nix, http://www.chem.qmw.ac.uk/surfascc/ ces/

Quantification of the different element on the surface

APPLICATION OF AUGER ELECTRON SPECTROSCOPY Identify surface contaminants and composition. Study composition as a function of depth. Analyze sample features as small as 80nm. Using an electron gun for the primary beam allows small analysis spot sizes. Corrosion Adhesion Catalysis Chemical Characterization Surface reaction

ADVANTAGE AND LIMITATION OF AES It is extremely reliable and reproducible Need UHV Standard surface analysis technique Only surface sensitive More spatial resolution Need careful calibration Can even do depth profiling with light ion sputtering Somewhat difficult to calibrate for adsorbates on surface

Comparison of the main characteristics of AES, XPS and SIMS

References Handbook of auger electron spectroscopy, Physical Electronics Industries Chung and Jenkins, Surface Science,21,253(1970) F. P. Larkins, Application of Surface, Science 13,1982,4-34. ASM. Vol.10 Material Characterization www.wikipedia.org Introduction to Surface science and thin films – J. Venables D. Briggs and M.P. Seah, ‘Practical Surface Analysis. Volume 1 – Auger and X-ray Photoelectron Spectroscopy’, Second Edition, John Wiley and Sons, Chichester, 1990 T.A. Carlson, ‘Photoelectron and Auger Spectroscopy’, Plenum Press, New York, 1975 J.F. Watts, ‘An Introduction to Surface Analysis by Electron Spectroscopy’, Oxford University Press, Oxford, 1990

THANK YOU If we knew what we were doing, It wouldn't be research, now would it? Albert Einstein (1879-1955)