1 / 19

X-Ray Photoelectron Spectroscopy: Capabilities and Exploitation

X-Ray Photoelectron Spectroscopy: Capabilities and Exploitation. Tim Morgan Mike Hawkridge. Outline. Photoemission Instrumentation Spectral Features Quantification Bonding States Depth Profiling Exploiting the XPS. Photoelectron Emission.

nairi
Télécharger la présentation

X-Ray Photoelectron Spectroscopy: Capabilities and Exploitation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. X-Ray Photoelectron Spectroscopy: Capabilities and Exploitation Tim Morgan Mike Hawkridge

  2. Outline • Photoemission • Instrumentation • Spectral Features • Quantification • Bonding States • Depth Profiling • Exploiting the XPS

  3. Photoelectron Emission • Photoelectric effect discovered by Einstein in 1905. Awarded Nobel Prize 1921. • Photoemission as an analytical tool by Kai Siegbahn. Awarded Nobel Prize 1981. KE = hν-BE-φs EVac • Spectrum is plot of electron count/ intensity versus Binding Energy or Kinetic Energy • Quantification of relativesurface composition (± X% (We’ll get to this!)) • Identification of element and chemical state (100ppm ultimate detection limit) • Valence Band Structure (UV best 10-45eV) φs EF photoelectron X-ray (hν) 2p BE 2s 1s

  4. Photoelectron Emission KE = hν-BE-φs EVac EVac φan φs EF EF photoelectron Auger electron X-ray (hν) Analyzer 2p BE 2s KE = hν-BE-φs-(φan-φs) 1s Excited Ion Relaxed Ion KE = hν-BE-φan E(1s)-E(2s) available for either fluorescent x-ray or Auger emission. K-Shell emission

  5. Typical Spectral Features • Sharp Photoelectron peaks • Broader Auger peaks with fine structure • Background from inelastically scattered photoelectrons

  6. Photoemission Spectrometer SCA Lens Ar-ion C-60 Load Lock Main Chamber

  7. Photoemission Spectrometer Quartz crystal monochromator V Energy Analyzer (SCA) Electron Gun Rowland Circle Al Kα x-rays (1486eV) Ion Gun 15-20kV electrons Lens E0 Electron Neutralizer MCD Al Anode 1eV electrons Photoelectrons Pass Energy Sample Resolution All in UHV for electron mfp and to minimize surface contamination.

  8. Charge Neutralization X-ray beam Electron neutralizer +++ Conducting Sample

  9. Charge Neutralization X-ray beam Electron neutralizer Ion Gun - - - - - - - - - - - - - - - - - - - +++ Insulating Sample

  10. Quantification • Assume homogeneous composition throughout analysis volume. • Ii area under peak. • Ni number of ions, i. • σiphotoionization cross-section • λielectron attenuation length • K instrumental factors • First principles approach, or • Re-write equation for number of atoms: • Measure “pure” standards to determine sensitivity factors, Si • Can be done externally or internally Duke C B PNAS 2003;100:3858-3864

  11. Sensitivity Factors in the VersaProbe • RSF Read from tables (inelastic MFP, photo-ionization xs buried in here). • Transmission function, T, in VersaProbe takes general form: • Measure Cu 3p (77eV), 2p3 (934eV) and LMM (568eV) peaks at various pass energies and fit to appropriate abscissa and ordinate. • Geometric correction: • β = asymmetry parameter for a specific atomic orbital of specific atom (tables) • θ = angle between x-ray and lens (= 54.7° = “magic” angle = VersaProbe angle)

  12. Transmission Function Calibration

  13. Peak Area: Background Subtraction • Smooth the data if noisy • Subtract background due to inelastic scattering processes • Peak area (or height) is proportional to the atomic concentration cps si= bi+pi bi • (Iterated) Shirley background model most commonly used; Easy to apply and good enough for most practical purposes, if not physically accurate. • Source of error in concentration determination • Reliability decreases with less intense spectral peaks. KE Shirley Background Modeling

  14. Error on Quantification • Sources: • Accuracy of relative sensitivity factors • Transmission function calibration • Accuracy of background determination • S/N ratio of data • Peak area included (must include all features associated with the subject peak, e.g. Plasmon loss peaks) • Generally accepted relative error of ±5% on atomic concentration • Larger for lower concentrations (lower reliability)

  15. Example Analysis using Multipak

  16. Charge Shift – Bonding States

  17. Depth Profiling - Sputtering C-60 Ar-ion • Ar- and C-60 ion guns can sputter surface material. • Cluster gun uses lower eV/atom and thus less surface damage (not energetic enough to sputter semiconductors) • Alternate sputtering for set time period and data collection (can also collect while sputtering). Tim’s Depth Profiling Data Polymer Profiling Data?

  18. Depth Profiling – Angle Resolved XPS • Equation • Electrons escaping from deeper within the material are attenuated to a greater degree than those escaping from the surface. • Can use this fact and measure at varying escape angles to determine thin layer structure. • Highly accurate, but only for top ~10nm.

  19. Exploiting the Instrument

More Related