Energy-Dispersive X-ray Spectrometry in the AEM

1. Energy-Dispersive X-ray Spectrometry in the AEM

3. Example of an X-ray Spectrum 2 Types of X-rays Characteristic x-rays elemental identification quantitative analysis Continuum x-rays background radiation must be subtracted for quantitative analysis

4. Continuum X-rays Interactions of beam electrons with nuclei of specimen atoms Accelerating electric charge emits electromagnetic radiation Here the acceleration is a change in direction The good The shape of the continuum is a valuable check on correct operation The not-so-good I bkg increases as ib increases I bkg is proportional to Zmean of specimen I max bkg rises as beam energy rises Peak-to-background ratio Ratio of Ichar / Ibkg sets limit on elemental detectability

5. Generation of Characteristic X-rays Mechanism Fast beam electron has enough energy to excite all atoms in periodic table Ionization of electron from the K-, L-, or M-shell X-ray is a product of de-excitation Example Vacancy in K-shell Vacancy filled from L-shell Emission of a Ka x-ray (or a KLL Auger electron) Important uses Qualitative use x-ray energy to identify elements Quantitative use integrated peak intensity to determine amounts of elements

6. Compute Energy of Sodium Ka Line

7. Families of Lines

8. Fluorescence Yield ? ? = fraction of ionization events producing characteristic x-rays the rest produce Auger e– ???increases with Z ?K typical values are: 0.03 for carbon (12) K-series @ 0.3 keV 0.54 for germanium (32) K-series @ 9.9 keV 0.96 for gold (79) K-series @ 67 keV X-ray production is inefficient for low Z lines (e.g., O, N, C) since mostly Augers produced ?? for each shell: ?K???L???M X-ray production is inefficient for L-shell and M-shell ionizations since? ?Land??M always < 0.5: ?L = 0.36 for Au (79) ?M = 0.002 for Au (79)

9. X-ray Absorption and Fluorescence X-rays can be absorbed in the specimen and in parts of the detector Certain x-rays fluoresce x-rays of other elements X-rays of element A can excite x-rays from element B Energy of A photon must be close to but above absorption edge energy of element B Example: Fe Ka (6.40 keV) can fluoresce the Cr K-series (absorption edge at 5.99 keV)

10. EDS Dewar, FET, Crystal LN dewar is most recognizable part To cool FET and crystal Actual detector is at end of the tube Separated from microscope by x-ray window Crystal and FET fitted as close to specimen as possible Limited by geometry inside specimen chamber

11. Electron-Hole Pair Creation Absorption of x-ray energy excites electrons From filled valence band or states within energy gap Energy to create an electron-hole pair ? = 3.86 eV @ 77K (value is temperature dependent) Within the intrinsic region Li compensates for impurity holes Ideally # electrons = # holes # electron-hole pairs is proportional to energy of detected x-ray

12. Details of Si(Li) Crystal

13. X-ray Pulses to Spectrum

14. Slow EDS Pulse Processing EDS can process only one photon at a time A second photon entering, while the first photon pulse is being processed, will be combined with the first photon Photons will be recorded as the sum of their energies X-rays entering too close in time are thrown away to prevent recording photons at incorrect energies Time used to measure photons that are thrown away is “dead time” Lower dead time -> fewer artifacts Higher dead time -> more counts/sec into spectrum Processor extends the “live time” to compensate

15. Things for Operator to Check Detector Performance Energy resolution (stamped on detector) Incomplete charge collection (low energy tails) Detector window (thin window allows low-energy x-ray detection) Detector contamination (ice and hydrocarbon) Count rate linearity (counts vs. beam current) Energy calibration (usually auto routine) Maximum throughput (set pulse processor time constant to collect the most x-rays in a given clock time with some decrease in energy resolution)

16. Energy Resolution Natural line width ~2.3 eV (Mn Ka) measured full width at half maximum (FWHM) Peak width increases with statistical distribution of e-h pairs created and electronic noise: Measured with 1000 cps at 5.9 keV Mn Ka line

17. X-ray Windows Transmission curve for a “windowless” detector Note absorption in Si Transmission curves for several commercially available windows Specific windows are better for certain elements

18. Ice Build Up on Detector Surface All detectors acquire an ice layer over time Windowless detector in UHV acquires ~ 3µm / year Test specimens NiO thin film (Ni La / Ni Ka) Cr thin film (Cr La / Cr Ka) Check L-to-K intensity ratio for Ni or Cr L/K will decrease with time as ice builds up Warm detector to restore (see manufacturer)

19. Spectrometer Calibration Calibrate spectrum using two known peaks, one high E and one low E NiO test specimen (commercial) Ni Ka (high energy line) at 7.478 keV Ni La (low energy line) at 0.852 keV Cu specimen Cu Ka (high energy line) at 8.046 keV Cu La (low energy line) at 0.930 keV Calibration is OK if peaks are within 10 eV of the correct value Calibration is important for all EDS software functions

20. Artifacts in EDS Spectra Si "escape peaks” Si Ka escapes the detector Carrying 1.74 keV Small peak ~ 1% of parent Independent of count rate Sum peaks Two photons of same energy enter detector simultaneously Count of twice the energy Only for high count rates Si internal fluorescence peak Photon generated in dead layer Detected in active region

21. Expand Vertically to See EDS Artifacts

22. EDS-TEM Interface We want x-rays to come from just under the electron probe, BUT… TEM stage area is a harsh environment Spurious x-rays, generated from high energy x-rays originating from the microscope illumination system bathe entire specimen High-energy electrons scattered by specimen generate x-rays Characteristic and continuum x-rays generated by the beam electrons can reach all parts of stage area causing fluorescence Detector can't tell if an x-ray came from analysis region or from elsewhere

23. The Physical Setup Want large collection angle, W Need to collect as many counts as possible Want large take-off angle, a But W reduced as a is increased Compromise by maxmizing W with a ~ 20° at 0° tilt angle can always increase a by tilting specimen toward detector -- but this increases specimen interaction with continuum from specimen

24. Orientation of Detector to Specimen Detector should have clear view of incident beam hitting specimen specimen tilting eucentric specimen at 0° tilt Identify direction to detector within the image Analyze side of hole "opposite the detector” Keep detector shutter closed until ready to do analysis

25. Spurious X-rays in the Microscope Pre-Specimen Effects spurious x-rays => hole count due to column x-rays and stray electrons spurious x-rays => poor beam shape from large C2 aperture Post-Specimen Scatter system x-rays => elements in specimen stage, cold finger, apertures, etc. spurious x-rays => excited by electrons and x-rays generated in specimen Coherent Bremsstrahlung extra peaks from specimen effects on beam-generated continuous radiation

26. Test for Spurious X-rays Generated in TEM Detector for x-rays from illumination system thick, high-Z metal acts as “hard x-ray sensor" Uniform NiO thin film used to normalize the spurious "in hole" counts, thus NiO film on Mo grid*

27. Spectrum from NiO/Mo Spurious x-rays Inverse hole count (Ni Ka/ Mo Ka) Want high inverse hole count Fiori P/B ratio Ni Ka/B(10 eV) Increases with kV Want high to improve element detectability

28. Figures of Merit for an AEM Fiori PBR = full width of Ni Ka divided by 10 eV of background (Ni Ka) / ( Mo Ka) is inverse hole count

29. Beam Shape and X-ray Analysis Calculated probes (from Mory, 1985) Effect on x-ray maps (from Michael, 1990)

30. Qualitative Analysis Collect as many x-ray counts as possible Use large beam current regardless of poor spatial resolution with large beam Analyze thicker foil region, except if light elements x-rays might be absorbed Scan over large area of single phase => avoid spot mode Use more than one peak to confirm each element

31. Qualitative Analysis Setup 1 Use thin foils, flakes, or films rather than self-supporting disks to reduce spurious x-rays (not always possible) Orient specimen so that EDS detector is on the side of the specimen hole opposite where you take your analysis Collect x-rays from a large area of a single phase Choose thicker area of specimen to collect more counts Tilt away from strong diffracting conditions (no strong bend contours) Operate as close to 0° tilt as possible (say, 5° tilt toward det.)

32. Qualitative Analysis Setup 2 Microscope Column Use highest kV of microscope Use clean, top-hat Pt aperture in C2 to minimize “hole count” effect Minimize beam tails (C2 aperture or VOA should properly limit beam angle) Use ~ 1 nA probe current to maximize count rate This may enlarge the electron beam (analyze smaller regions later) Remove the objective aperture

33. Qualitative Analysis Setup 3 X-ray Spectrometer Ensure that detector is cranked into position Keep detector shutter closed until you are ready to analyze Use widest energy range available (0-20 keV is normal) 0–40 keV for Si(Li) detector 0–80 keV for intrinsic Ge detector Choose short detector time constant (for maximum countrate) Count for a long time – 100-500 live sec

34. Peak Identification Start with a large, well-separated, high-energy peak Try the K-family Try the L-family Try the M-family Remember -- these families are related Check for EDS artifacts Repeat for the next largest peak Important: Use more than one peak for identification If peak too small to "see", collect more counts or forget about identifying that peak; peak should be greater than 3B1/2

35. Chart of X-ray Energies (0-20 keV)

36. Chart of X-ray Energies (0-5 keV)

37. Know X-ray Family Fingerprints

38. Some Peaks will Look Similar

39. Unknown #1

40. Data Analysis for Unknown #1

41. Unknown #1

42. Unknown #2

43. Analysis of Unknown #2

44. Qualitative Analysis

45. Automatic Qualitative Analysis?

46. Automatic Qualitative Analysis Blunders

47. Summary EDS in the TEM has more pitfalls than in SEM Use the highest kV available Understand the effects of: detector-specimen geometry spurious x-rays from the illumination system post-specimen scatter beam shape and spatial resolution => the “witch’s hat” Identify every peak in the spectrum Even artifact peaks Forget peaks of intensity < 3 x (background)1/2 Collect as many counts as possible Use large enough beam size to obtain about 1 nA current Qualitative analysis use: use long counting times or thicker electron-transparent regions with a short pulse processor time constant, if appropriate Assume data might be used for later quantitative analysis (determine t if possible)