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  • 1

    An Introduction to Auger Electron Spectroscopy

    Spyros Diplas MENA3100

    6th of March 2013

    SINTEF Materials & Chemistry, Department of Materials Physics &

    Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO

  • 2

    Contents

    Background

    An Auger microprobe

    The Auger process

    Auger spectroscopy

    Auger mapping

    Depth profiling

  • 3

    photons

    ions

    electrons

    EMISSION

    TRANSMISSION

    Interaction with material

    EXCITATION

    Properties and reactivity of the surface will depend on: bonding geometry of molecules to the surface physical topography chemical composition chemical structure atomic structure electronic state

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

    Surface Analysis - Techniques Available

  • 4

    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.

  • Auger Electron Spectroscopy (AES) Exciting radiation Electron beam (Scanning)

    Signal Electrons (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

  • 6

    Auger electron vs x-ray emission yield

    5

    B Ne P Ca Mn Zn Br Zr

    10 15 20 25 30 35 40 Atomic Number

    Elemental Symbol

    0

    0.2

    0.4

    0.6

    0.8

    1.0

    Pro

    babi

    lity

    Auger Electron Emission

    X-ray Photon Emission

  • 7

    Auger - lateral resolution

    160 kX SEM

    160 kX Auger Maps

    4m at 20kV

    0,1m 2kV

    n.b. much better than EDS in bulk samples

  • 8

    Auger Electron Spectroscopy

    Basic Specification 3nm SEI resolution 8nm probe diameter for Auger

    analysis Variable energy resolution from

    0.05% to 0.6% Chemical state analysis in several

    10nm areas Ion gun for sputter depth profiling

    Allows some charge neutralisation for analysis of insulating materials

    UHV Chamber FE-SEM quality electron column nm-scale depth resolution Depth profiling

    Additional capabilities: EDS system

    Bulk composition analysis. Backscattered electron detector

    Atomic number contrast. Heating stage

    Diffusion experiments. Liquid nitrogen fracture stage

    Grain boundary and interface studies. Electron Beam Induced Current (EBIC)

    Recombination centre mapping in solar cells, etc.

    The JEOL JAMP-9500F FE Auger Microprobe

  • 9

    Origin of Auger signal

    Auger electron generation

    LMM Auger electron emitted

  • 10

    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.

  • 11

    Auger spectrum - typical

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

  • 12

    Chemical shift in Si compounds

    Comparison of Si, SiNx and SiO2

  • 13

    Auger spectrum - quantification

    Can use peak areas or peak-to-background ratios

  • 14

    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.

  • 15

    Auger mapping

    Pixel-by-pixel, typically calculate;

    (peak-background)/background

  • 16

    Combined SEM/Auger analysis

    200nm

    Carburised alloy

  • Combined Auger EDS mapping

    17

  • 18

    Auger and XPS spectra of Si: p and n Type

    Arb

    itrar

    y U

    nits

    110 108 106 104 102 100 98 96 94 92Binding Energy (eV)

    p-c-Si

    Intrinsic-a-Si

    Intrinsic-c-Si

    n-a-Si

    n-c-Si AES excited Si KLL

    Arbit

    rary U

    nits

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

    X-ray excited Si KLL

    p-c-Si

    Intrinsic-a-Si

    Intrinsic-c-Si

    n-a-Si

    n-c-Si

  • 19

    Auger mapping of Si p and n type

  • AES nm lateral and depth resolution

    20

    Chromated aluminium alloy surface Chromating pretreatment

    corrosion protection before coating, painting,

    bonding, etc Does the chromate

    passivate intermetallic particles?

    430 450 470 490 510 530 550 570 590

    Electron kinetic energy / eV

    1

    2

    Cr metal reference

    1

    2 Auger spectra show that the intermetallic particles are covered by a thin layer of Cr-oxide that is invisible in the SEM images.

    O

  • 21

    Combined SEM/TEM/Auger analysis

    TEM analysis

  • 22

    Depth profiling - schematic

    Process is fully automated, sample can be rotated (Zalar rotation).

    N.B. Can also perform angle-resolved analysis.

    Electron beam

    Argon ion gun

    Sample

  • 23

    Depth profiling raw data

    Surface and after sputtering as grown Pd/Ag membrane

  • 24

    Depth profiling - example

    Multiple quantum well structures

  • 25

    Depth profiling CeO2 buffer layer

  • 26

    Depth profiling CeO2 buffer layer

  • 27

    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

  • 28

    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

  • 29

    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

    An Introduction to Auger Electron SpectroscopyContentsSurface Analysis - Techniques AvailableThe Auger processAuger Electron Spectroscopy (AES)Auger electron vs x-ray emission yieldAuger - lateral resolutionAuger Electron SpectroscopyOrigin of Auger signalElectron spectrometerAuger spectrum - typicalChemical shift in Si compoundsAuger spectrum - quantificationAuger spectrum - quantificationAuger mappingCombined SEM/Auger analysisCombined Auger EDS mappingAuger and XPS spectra of Si: p and n TypeAuger mapping of Si p and n typeAES nm lateral and depth resolutionCombined SEM/TEM/Auger analysisDepth profiling - schematicDepth profiling raw data Slide Number 24Depth profiling CeO2 buffer layerDepth profiling CeO2 buffer layerPotential Auger ApplicationsSummaryReferences