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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. Why Do EDS X-ray Analysis in TEM/STEM?. Spatial resolution 2-20 nm (10 3 times better than SEM/EPMA) - PowerPoint PPT Presentation
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PASI - Electron Microscopy - Chile
1Lyman - EDS Qual
Energy-Dispersive
X-ray Spectrometry
in the AEMCharles Lyman
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School
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Why Do EDS X-ray Analysis in TEM/STEM?
Spatial resolution» 2-20 nm (103 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 ~ 50-500 nm)» EELS requires specimens < 20-30 nm
Straightforward microanalysis» Qualitative analysis => Which element is present?» Quantitative analysis => How much of the element is present?» Easy x-ray mapping
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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.
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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
Continuum x-rays
Absorption of continuum
from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.
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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
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Compute Energy of Sodium Ka Line
Energy levels EK and EL3 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
EK = 1072 eVEL3 = 31 eV
X-ray energy is the difference between two energy levels:
For sodium (Z=11):
If beam E > EK, then a K-electron may be ionized:
EK1 EK EL III
E K a1
Na 1072 eV 31 eV 1041 eV
K L M
For Na only see one peak since the Kb is only 26 eV from the Ka line
Beam electron
Beam electron loses EK
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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
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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: KLM » X-ray production is inefficient for L-shell
and M-shell ionizations since» LandM always < 0.5:
L = 0.36 for Au (79)
M = 0.002 for Au (79) from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.
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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)
I Ioe
t
Greater absorption when -- x-ray energy is just above absorber absorption edge-- path length t is large
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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
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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
Acceptor
Valence band
Conduction band
Donor
Energy gapE
nerg
y
Excited e-
Hole
For Cu K :a8048eV/3.8 eV = 2118 e-h pairs
(after drawing by J. H. Scott)
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Details of Si(Li) Crystal
–1000 V bias
X-ray
Silicon inactive layer (p-type) ~100 nmGold
electrode20 nm
Active silicon(intrinsic)
3 mm
Ice?
WindowBe, BN,
diamond,polymer
0.1 mm — 7 mm
Anti-reflectiveAl coating 30 nm
(+)(–)
Holes Electrons
Gold electrode
(after drawing by J. H. Scott)
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X-ray Pulses to Spectrum
from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.
slow amplifier
fast amplifier
charge staircase
analog-to-digital
converter
spectrum
energy binscollects e-h as charge
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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, 2003.
Counting is linear up to 3000 cps (20-30% dead time)
fast amplifier
slow amplifier
slower amplifier
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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
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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 Kaline
FWHM C2E N 2 1/ 2
C 2.35(F)1/ 2
E = x-ray energyN = electronic noiseF = Fano factor (~0.1 for Si)E = 3.8 eV/electron-hole pair
Mn Kaline
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996.
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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.
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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) Courtesy of J.R. Michael
After operating 1 year
Immediately after warmup
Windowless detector
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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
0.930 keV 8.046 keV
from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.
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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
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Expand Vertically to See EDS Artifacts
Escape peaks
Si internal fluor.
Sum peaks
System peaks
from Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003.
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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
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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
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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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
EDSdetector
Rim
Thinned
Detector
Top view of disc
Edge of hole furthest from detector
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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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
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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*
% hole count I on NiO film
MoK
I on NiO filmNiK
x 100%
( Note: " on NiO film count" and
" in hole count" are similar) Mo grid bar
NiO film on C Hole
Electron beam
Hard x-ray from illumination system
Spurious (bad) Mo K-series
Beam-generatedNi K x-ray (good)
* see Egerton and Cheng, Ultramicroscopy 55 (1994) 43-54
- “on film” results ~ “in hole” results- Usually take inverse “hole count” ratio
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Spectrum from NiO/Mo
Spurious x-rays» Inverse hole count
(Ni K /a Mo K )a» Want high inverse
hole count Fiori P/B ratio
» Ni Ka/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 Ka
Egerton and Cheng, Ultramicroscopy 55 (1994) 43-54
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Figures of Merit for an AEM
Fiori PBR = full width of Ni Ka divided by 10 eV of background (Ni K )a /(Mo K )a is inverse hole count
Better
Less good
Obviously, we want to use the highest kV
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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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 largecorrect C2 aperture size
“witch’s hat” beam tail excites x-rays
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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?
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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
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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
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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
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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
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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
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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
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Know X-ray Family Fingerprints
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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) Ka @ 2.013 keV Z = 40 (Zr) La @ 2.042 Z = 77 (Ir) Ma @ 1.977
M-series
L-series
K-seriesZ
after Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003
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Unknown #1
Energy (keV)
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Data Analysis for Unknown #1
26 6.4 Fe(26) Ka
7.0 Fe(26) Kb
4.5
4.9
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Unknown #1
from xray.optics.rochester.edu/.../spr04/pavel/
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Unknown #2
after www.pentrace.com/ nib030601003.html
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Analysis of Unknown #2
Line Energy(keV) Int, Sh Candidate 1 Candidate 21 0.2 w C(6) K (0.25)
2 0.8 vw Ru(44) La escape (0.86) Ni(28) La (0.8)
3 1.4 w, sym W(74) Mz (1.4)
4 1.8 s, sym W(74) Ma (1.8) No possible K line
5 2.6 m, asym Ru(44) La (2.6)
6 6.4 w, sym Fe(26) Ka (6.4)
7 7.1 vw Fe(26) Kb (7.1)
8 7.4 m, sym W(74) Ll (7.4) Ni(28) Ka (7.5)
9 8.0 vw Cu(29) Ka (8.0) Ni(28) Kb (8.3)
10 8.3 m, sym W(74) La (8.4) No possible K line
11 9.6 m, overlap W(74) Lb1 (9.7)
12 10.0 m, overlap W(74) Lb2 (10.0)
13 11.3 w, sym W(74) Lg1 (11.3)
14 11.6 vw W(74) Lg3 (11.6)
Start
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Qualitative Analysis
W Ma
W La
W Lb1,2
W Lg1,3
W LlW Mz Fe K ,a b
Cu
Ru La
C
Elements present major: W, Ru trace: Fe, Cu, C
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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 identifyLines of other elements may have the same energy
3. Consider logic of x-ray excitationAll lines of an element are excited in TEM/STEM (100-300 kV)If L-series indicated, K-series must be presentIf M-series indicated, L-series must be present
Check the results of every automatic qualitative analysis
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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) 545-561
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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)