pasi - electron microscopy - chile 1 lyman - eds qual energy-dispersive x-ray spectrometry in the...

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


2Lyman - EDS Qual

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


3Lyman - 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.


4Lyman - 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 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



5Lyman - 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


6Lyman - EDS Qual

Compute Energy of Sodium K 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:

€

EKα1 = EK − EL III

€

EK a1

Na =1072 eV − 31 eV =1041 eV

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 EK


7Lyman - 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


8Lyman - 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) 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)



9Lyman - 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)

€

I = Ioe−

μ

ρ

⎛

⎝ ⎜

⎞

⎠ ⎟ρt

Greater absorption when -- x-ray energy is just above absorber absorption edge-- path length t is large


10Lyman - 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


11Lyman - 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

» = 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 8048 eV/3.8 eV = 2118 e-h pairs

(after drawing by J. H. Scott)


12Lyman - EDS Qual

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 m — 7 m

Anti-reflectiveAl coating 30 nm

(+)(–)

Holes Electrons

Gold electrode

(after drawing by J. H. Scott)


13Lyman - EDS Qual

X-ray Pulses to Spectrum


slow amplifier

fast amplifier

charge staircase

analog-to-digital

converter

spectrum

energy binscollects e-h as charge


14Lyman - 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


Counting is linear up to 3000 cps (20-30% dead time)

fast amplifier

slow amplifier

slower amplifier


15Lyman - 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


16Lyman - 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

€

FWHM ≈ C2E + N 2( )

1/ 2

C = 2.35(Fε)1/ 2

E = x-ray energyN = electronic noiseF = 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.


17Lyman - 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



18Lyman - 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


19Lyman - 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 7.478 keV – Ni L (low energy line) at 0.852 keV

» Cu specimen– Cu K (high energy line) at 8.046 keV– Cu L (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



20Lyman - 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


21Lyman - EDS Qual

Expand Vertically to See EDS Artifacts

Escape peaks

Si internal fluor.

Sum peaks

System peaks



22Lyman - 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


23Lyman - 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



24Lyman - 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

EDSdetector

Rim

Thinned

Detector

Top view of disc

Edge of hole furthest from detector



25Lyman - 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


26Lyman - 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*

% hole count = I on NiO film

MoK

I on NiO filmNiK

x100%

( 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


27Lyman - 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


28Lyman - 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



29Lyman - 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 largecorrect C2 aperture size

“witch’s hat” beam tail excites x-rays


30Lyman - 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?


31Lyman - 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


32Lyman - EDS Qual


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


33Lyman - EDS Qual


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


34Lyman - 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 3B1/2


35Lyman - 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


36Lyman - 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


37Lyman - EDS Qual

Know X-ray Family Fingerprints


38Lyman - 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 @ 2.013 keV Z = 40 (Zr) L @ 2.042 Z = 77 (Ir) M @ 1.977

M-series

L-series

K-seriesZ

after Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, 2003


39Lyman - EDS Qual

Unknown #1

Energy (keV)


40Lyman - EDS Qual

Data Analysis for Unknown #1

26 6.4 Fe(26) K

7.0 Fe(26) K

4.5

4.9


41Lyman - EDS Qual

Unknown #1

from xray.optics.rochester.edu/.../spr04/pavel/


42Lyman - EDS Qual

Unknown #2

after www.pentrace.com/ nib030601003.html


43Lyman - EDS Qual

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) L escape (0.86) Ni(28) La (0.8)

3 1.4 w, sym W(74) M (1.4)

4 1.8 s, sym W(74) M (1.8) No possible K line

5 2.6 m, asym Ru(44) L (2.6)

6 6.4 w, sym Fe(26) K (6.4)

7 7.1 vw Fe(26) K (7.1)

8 7.4 m, sym W(74) Ll (7.4) Ni(28) Ka (7.5)

9 8.0 vw Cu(29) K (8.0) Ni(28) Kb (8.3)

10 8.3 m, sym W(74) L (8.4) No possible K line

11 9.6 m, overlap W(74) L1 (9.7)

12 10.0 m, overlap W(74) L2 (10.0)

13 11.3 w, sym W(74) L1 (11.3)

14 11.6 vw W(74) L3 (11.6)

Start


44Lyman - EDS Qual

Qualitative Analysis

W M

W L

W L1

W L1

W LlW M Fe K

Cu

Ru L

C

Elements present major: W, Ru trace: Fe, Cu, C


45Lyman - 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 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


46Lyman - 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) 545-561


47Lyman - 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)

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