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 ions chemical structure 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 29 .