PASI - Electron Microscopy - Chile 1 Lyman - EDS Qual Energy-Dispersive X-ray Spectrometry in the AEM Charles Lyman Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School

PASI - Electron Microscopy - Chile 2 Lyman - EDS Qual Why Do EDS X-ray Analysis in TEM/STEM? Spatial resolution » 2-20 nm (10 3 times better than SEM/EPMA) Elemental detectability » 0.1 wt% - 1 wt%, depending on the specimen (~same as SEM/EDS) Can use typical TEM specimens (t ~ nm) » EELS requires specimens < nm Straightforward microanalysis » Qualitative analysis => Which element is present? » Quantitative analysis => How much of the element is present? » Easy x-ray mapping

PASI - Electron Microscopy - Chile 3 Lyman - EDS Qual 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 Example of EDS x-ray spectrum from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 4 Lyman - EDS Qual 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 i b increases » I bkg is proportional to Z mean of specimen » I max bkg rises as beam energy rises Peak-to-background ratio » Ratio of I char / I bkg sets limit on elemental detectability Continuum x-rays Absorption of continuum from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 5 Lyman - EDS Qual 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 K  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

PASI - Electron Microscopy - Chile 6 Lyman - EDS Qual Compute Energy of Sodium K  Line Energy levels E K and E L3 are in Bearden's "Tables of X-ray Wavelengths and X-ray Atomic Energy Levels" in older editions of the CRC Handbook of Chemistry and Physics E K = 1072 eV E L3 = 31 eV X-ray energy is the difference between two energy levels: For sodium (Z=11): If beam E > E K, then a K-electron may be ionized: K L M   For Na only see one peak since the K  is only 26 eV from the K  line Beam electron Beam electron loses E K

PASI - Electron Microscopy - Chile 7 Lyman - EDS Qual Families of Lines Note: this is a simplified version of Goldstein Figure 6.9 showing only lines seen in EDS If K-series excited, will also have L-series

PASI - Electron Microscopy - Chile 8 Lyman - EDS Qual 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) 0.3 keV –0.54 for germanium (32) 9.9 keV –0.96 for gold (79) 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  »  L and  M always < 0.5:  L = 0.36 for Au (79)  M = for Au (79) from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 9 Lyman - EDS Qual 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 K  (6.40 keV) can fluoresce the Cr K-series (absorption edge at 5.99 keV) Greater absorption when -- x-ray energy is just above absorber absorption edge -- path length t is large

PASI - Electron Microscopy - Chile 10 Lyman - EDS Qual 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 Schematic courtesy of Oxford Instruments

PASI - Electron Microscopy - Chile 11 Lyman - EDS Qual 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 »  = K (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 Acceptor Valence band Conduction band Donor Energy gap Energy Excited e - Hole For Cu K   8048  eV/3.8 eV = 2118 e-h pairs (after drawing by J. H. Scott)

PASI - Electron Microscopy - Chile 12 Lyman - EDS Qual Details of Si(Li) Crystal –1000 V bias X-ray Silicon inactive layer (p-type) ~100 nm Gold electrode 20 nm Active silicon (intrinsic) 3 mm Ice? Window Be, BN, diamond, polymer 0.1  m — 7  m Anti-reflective Al coating 30 nm (+) (–) Holes Electrons Gold electrode (after drawing by J. H. Scott)

PASI - Electron Microscopy - Chile 13 Lyman - EDS Qual X-ray Pulses to Spectrum from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, slow amplifier fast amplifier charge staircase analog-to- digital converter spectrum energy bins collects e-h as charge

PASI - Electron Microscopy - Chile 14 Lyman - EDS Qual 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 from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, Counting is linear up to 3000 cps (20-30% dead time) fast amplifier slow amplifier slower amplifier

PASI - Electron Microscopy - Chile 15 Lyman - EDS Qual 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) Topics in red explained in next few slides

PASI - Electron Microscopy - Chile 16 Lyman - EDS Qual Energy Resolution Natural line width ~2.3 eV (Mn K   » 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 K   line E = x-ray energy N = electronic noise F = Fano factor (~0.1 for Si) E = 3.8 eV/electron-hole pair Mn K   line from Williams and Carter, Transmission Electron Microscopy, Springer, 1996.

PASI - Electron Microscopy - Chile 17 Lyman - EDS Qual 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 from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 18 Lyman - EDS Qual 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 L  / Ni K   » Cr thin film (Cr L  / Cr K   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) Courtesy of J.R. Michael After operating 1 year Immediately after warmup Windowless detector

PASI - Electron Microscopy - Chile 19 Lyman - EDS Qual Spectrometer Calibration Calibrate spectrum using two known peaks, one high E and one low E » NiO test specimen (commercial) –Ni K  (high energy line) at keV –Ni L  (low energy line) at keV » Cu specimen –Cu K  (high energy line) at keV –Cu L  (low energy line) at keV Calibration is OK if peaks are within 10 eV of the correct value Calibration is important for all EDS software functions keV8.046 keV from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 20 Lyman - EDS Qual 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 from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

PASI - Electron Microscopy - Chile 21 Lyman - EDS Qual Expand Vertically to See EDS Artifacts Escape peaks Si interna l fluor. Sum peaks System peaks from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.

PASI - Electron Microscopy - Chile 22 Lyman - EDS Qual 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 Usually fixed by manufacturers of EDS and TEM

PASI - Electron Microscopy - Chile 23 Lyman - EDS Qual The Physical Setup Want large collection angle,  » Need to collect as many counts as possible Want large take-off angle,  » But  reduced as  is increased Compromise by maxmizing  with  ~ 20˚ at 0˚ tilt angle » can always increase  by tilting specimen toward detector -- but this increases specimen interaction with continuum from specimen from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

PASI - Electron Microscopy - Chile 24 Lyman - EDS Qual 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 EDS detector Rim Thinned Detector Top view of disc Edge of hole furthest from detector from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

PASI - Electron Microscopy - Chile 25 Lyman - EDS Qual 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 The analyst must understand these effects to achieve acceptable qualitative and quantitative results

PASI - Electron Microscopy - Chile 26 Lyman - EDS Qual 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* Mo grid bar NiO film on C Hole Electron beam Hard x-ray from illumination system Spurious (bad) Mo K-series Beam-generated Ni K x-ray (good) * see Egerton and Cheng, Ultramicroscopy 55 (1994) “on film” results ~ “in hole” results - Usually take inverse “hole count” ratio

PASI - Electron Microscopy - Chile 27 Lyman - EDS Qual Spectrum from NiO/Mo Spurious x-rays » Inverse hole count (Ni K  Mo K  » Want high inverse hole count Fiori P/B ratio » Ni K  /B(10 eV) » Increases with kV » Want high to improve element detectability Measure in center of grid square on NiO/Mo specimen Minimize spurious Mo x-rays by using thick C2 aperture Maximize P/B ratio for Ni K  Egerton and Cheng, Ultramicroscopy 55 (1994) 43-54

PASI - Electron Microscopy - Chile 28 Lyman - EDS Qual Figures of Merit for an AEM Fiori PBR = full width of Ni K  divided by 10 eV of background (Ni K  /  Mo K  is inverse hole count Better Less good Obviously, we want to use the highest kV from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

PASI - Electron Microscopy - Chile 29 Lyman - EDS Qual Beam Shape and X-ray Analysis Calculated probes (from Mory, 1985) Effect on x-ray maps (from Michael, 1990) Properly limited Spherically aberrated C2 aperture too large correct C2 aperture size “witch’s hat” beam tail excites x-rays

PASI - Electron Microscopy - Chile 30 Lyman - EDS Qual 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 Which elements are present?

PASI - Electron Microscopy - Chile 31 Lyman - EDS Qual 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.) Specimen

PASI - Electron Microscopy - Chile 32 Lyman - EDS Qual 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

PASI - Electron Microscopy - Chile 33 Lyman - EDS Qual 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 – live sec

PASI - Electron Microscopy - Chile 34 Lyman - EDS Qual 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 3B 1/2

PASI - Electron Microscopy - Chile 35 Lyman - EDS Qual Chart of X-ray Energies (0-20 keV) M-series L-series K-series from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003

PASI - Electron Microscopy - Chile 36 Lyman - EDS Qual Chart of X-ray Energies (0-5 keV) M-series L-series K-series from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003

PASI - Electron Microscopy - Chile 37 Lyman - EDS Qual Know X-ray Family Fingerprints

PASI - Electron Microscopy - Chile 38 Lyman - EDS Qual Some Peaks will Look Similar At low energies each series collapses to a single line From 1 keV to 3 keV, the K, L, or M lines all look similar At 2.0 keV: Z = 15 (P) K keV Z = 40 (Zr) L Z = 77 (Ir) M M-series L-series K-series Z after Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003

PASI - Electron Microscopy - Chile 39 Lyman - EDS Qual Unknown #1 Energy (keV)

PASI - Electron Microscopy - Chile 40 Lyman - EDS Qual Data Analysis for Unknown # Fe(26) K  7.0 Fe(26) K 

PASI - Electron Microscopy - Chile 41 Lyman - EDS Qual Unknown #1 from xray.optics.rochester.edu/.../spr04/pavel/

PASI - Electron Microscopy - Chile 42 Lyman - EDS Qual Unknown #2 after nib htmlwww.pentrace.com/

PASI - Electron Microscopy - Chile 43 Lyman - EDS Qual Analysis of Unknown #2 LineEnergy(keV)Int, ShCandidate 1Candidate wC(6) K (0.25) 2 0.8vw Ru(44) L  escape (0.86) Ni(28) La (0.8) 3 1.4w, sym W(74) M  (1.4) 4 1.8s, sym W(74) M  (1.8) No possible K line 5 2.6m, asym Ru(44) L  (2.6) 6 6.4w, sym Fe(26) K  (6.4) 7 7.1vw Fe(26) K  (7.1) 8 7.4m, symW(74) Ll (7.4)Ni(28) Ka (7.5) 9 8.0vw Cu(29) K  (8.0) Ni(28) Kb (8.3) m, sym W(74) L  (8.4) No possible K line m, overlap W(74) L  1 (9.7) m, overlap W(74) L  2 (10.0) w, sym W(74) L  1 (11.3) vw W(74) L  3 (11.6) Start

PASI - Electron Microscopy - Chile 44 Lyman - EDS Qual Qualitative Analysis W M  W L  W L   W L   W Ll W M  Fe K  Cu Ru L  C Elements present major: W, Ru trace: Fe, Cu, C

PASI - Electron Microscopy - Chile 45 Lyman - EDS Qual Automatic Qualitative Analysis? 1.Are suggested elements reasonable? Tc and Pm are unusual, Cl and S are not 2.Do not use peak energy alone to identify Lines of other elements may have the same energy 3.Consider logic of x-ray excitation All lines of an element are excited in TEM/STEM ( kV) If L-series indicated, K-series must be present If M-series indicated, L-series must be present Check the results of every automatic qualitative analysis

PASI - Electron Microscopy - Chile 46 Lyman - EDS Qual Automatic Qualitative Analysis Blunders Identification by peak energy alone without considering x-ray families or peak shape Identification without considering other lines of same element from Newbury, Microsc. Microanal. 11 (2005)

PASI - Electron Microscopy - Chile 47 Lyman - EDS Qual 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)