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TEM EDS - Analysis Precautions

TEM EDS - Analysis Precautions. Analysis Precautions. You need to think more about experimental conditions in TEM/STEM than in SEM. Spatial resolution is strongly affected by experimental conditions in TEM/STEM. Spurious X-rays are a significant issue in TEM/STEM.

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TEM EDS - Analysis Precautions

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  1. TEM EDS - Analysis Precautions

  2. Analysis Precautions • You need to think more about experimental conditions in TEM/STEM than in SEM. • Spatial resolution is strongly affected by experimental conditions in TEM/STEM. • Spurious X-rays are a significant issue in TEM/STEM

  3. What do you need to think about? • Illumination system issues • Sample and stage issues • Detector issues • Specimen preparation issues

  4. Pt Obj aperture TEM EDS geometry

  5. Spurious X-rays – thinned sample

  6. Spurious X-rays – thin film

  7. EDS improvements

  8. Illumination issues • Spherical aberration • Hard X-rays • Hole count

  9. Spherical Aberration

  10. Spherical Aberration Large fraction of electrons are outside of the FWHM and will generate x-rays FWHM is what your eye will see

  11. Aberrated electrons • More of an issue in nano-probe mode. • Choose the optimum C2 aperture to limit spherical aberration. More beam current is not necessarily better.

  12. Hard X-rays • Bremsstrahlung is generated whenever electrons are slowed down. (e.g. hitting condenser apertures) • TEM/STEM will have bremsstrahlung up to 200 keV compared to ~30 keV in SEMs.

  13. Hard X-rays

  14. Identifying hard X-rays • Use a hole-count test. • X-rays have a smaller ionization cross-section than electrons, therefore more of an effect in “bulk” areas than in “thin” areas. • Ionization occurs deeper in sample, thus low energy lines are preferentially absorbed.

  15. Hole count

  16. Hard X-Ray Mitigation • Thicker apertures will reduce hard X-Rays (“Top Hat” apertures). Do not use Au “thin-foil” apertures • On FEG instruments use the adjustable C1 aperture as the beam limiter and use C2 as a “spray” aperture.

  17. Sample and stage issues • Scattered electrons • Continuum radiation • BSE

  18. Pt Obj aperture Scattered electrons and BSE

  19. Stray scattered electrons • BSE coefficient is ~0.3 for Fe and Cu (~0.7 for Pt) • Lens design can help if the post-field keeps large angle electrons within lower bore… … IF you remove the Objective aperture.

  20. Sample Bremsstrahlung

  21. Sample self-fluorescence • The bremsstrahlung as the electrons go through the sample will be a forward scattered “dipole” (relativistic effects). • Tilted samples (particularly thinned foils) may receive a sizeable dose of sample generated hard X-rays. • Grid bars may also fluoresce.

  22. Trapped BSE

  23. Trapped BSE • This is a function of the BSE coefficient and thickness. Pt caps on FIB samples may backscatter stray electrons. • This may be more of an issue with thinned samples.

  24. Absorption edges

  25. Absorption edges • Visible in thick samples (grid bars, etc.) • Makes it difficult to fit continuum background.

  26. Mo Absorption edge

  27. Source of stray signal • High L/K ratio indicates electron rather than X-ray excitation • Absorption edges in the high energy peaks indicate the source is in a thick area.

  28. Electron channeling • In a strong diffracting condition, the electrons can preferentially excite certain columns of atoms. • This can lead to misleading compositions. • It can also be used to observe atom locations. (ALCHEMI)

  29. Detector issues • BSE detection and damage • Detector Efficiency issues • Detector icing • Escape and sum peaks • Incomplete charge collection

  30. High Energy BSE

  31. BSE in the detector

  32. High energy BSE • Will affect system dead-time calculations esp. if variations in Z across sample. • Will affect peak shape and hence intensity calculations • Will cause damage (point defects) to the detector resulting in incomplete charge collection (low energy tails on peaks).

  33. Detector Efficiency

  34. Detector Icing

  35. Sum Peaks

  36. Detector Si peak

  37. Escape Peak

  38. Sample and preparation issues • Sample issues • Surface films • Ar+ and Ga+ implantation

  39. Typical TEM sample

  40. Precipitates may not be full thickness

  41. Surface films

  42. Surface film geometry

  43. Considerations • FIB samples are thin enough that you should not see significant contributions from hard X-rays (within the sample). • Watch out for sample contamination with small probes. Plasma-clean thinned samples and/or use a cryo-stage. Pre-heating the sample helps too. • Extraction replicas may provide a means to analyze precipitates without matrix interference. • Consider a tilt-rotate holder for looking at interfaces.

  44. SEM Standardless EDS Sample: 123 Superconductor Compositions in Wt %

  45. SEM Standardless EDS Sample: 123 Superconductor Compositions in Wt %

  46. SEM Standardless EDS Sample: 123 Superconductor Compositions in Wt %

  47. SEM Standardless EDS Sample: 123 Superconductor Compositions in Wt % Moral: Be careful about standardless analysis. You can get almost any result you want!

  48. EDS Resources • Monte Carlo simulations • Casino • WinXRay • DTSA and DTSA-II • http://www.cstl.nist.gov/div837/Division/outputs/DTSA/DTSA.htm • DTSA - Macintosh (not OS-X) only • DTSA-II - Java • www.oxinst.com – “TEM Explained.pdf”

  49. Next week • Sample preparation • Specific questions? • Course evaluation • I’ll send link when I get it. • Practical exam scheduling • CM12 alignment • Diffraction (spot and k-line) • Sample tilting, BF/DF

  50. Laboratory 8 • Sample: NiOx thin film on a Mo grid • Hole count in microprobe mode • Electron spectrum of grid • Nano-probe EDS with large and small C2 aperture • EDS at large and small a-tilt angles

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