Electron Beam Analysis (EPMA, SEM-EDS)

Warren Straszheim, PhDEPMA, Ames Lab, 227 Wilhelm

SEM-EDS, MARL, 23 Town Engineeringwesaia@iastate.edu 515-294-8187

With acknowledgements to John Donovan of the University of Oregon

Instrumental Techniques

• Excite• measure characteristic response• quantify by comparison to standards

Bulk or microanalysis

• Can excitation be focused?• Can detector be focused?

Electron beam microanalysis

Excitation: focused electron beamSample interactions secondary electrons backscattered electrons auger electrons cathodoluminescence absorbed current X-rays

•Precise x-ray intensities

•High spectral resolution

•Sub-micron spatial resolution

•Matrix/standard independent

•Accurate quantitative chemistry

Electron-Sample Interactions

• characteristic emissions

• Be and heavier elements

• background (bremsstrahlung)

X-rays

X-ray Lines - K, L, MKa X-ray is produced due to removal of K shell electron, with L shell electron taking its place. Kb occurs in the case where K shell electron is replaced by electron from the M shell.

La X-ray is produced due to removal of L shell electron, replaced by M shell electron.

Ma X-ray is produced due to removal of M shell electron, replaced by N shell electron.

Ranges and interaction

volumes

It is useful to have an understanding of the distance traveled by the beam electrons, or the depth of X-ray generation, i.e. specific ranges. For example: If you had a 1 um thick layer of compound AB atop substrate BC, is EPMA of AB possible?

Differences between SEM and EPMA

Many shared componentsResulting from intent - imaging vs. analysisStability (higher for EPMA)Current capability (higher for EPMA)Spatial resolution (higher for SEM) via smaller spot and limited aberration correctionattached analyzer (EDS vs. WDS)

EDS vs. WDS

• technology – solid state crystal vs. wavelength spectrometer

• Resolution~126 eV vs 20eV• P/B ratio • Detection limit• count rate limitations

500 kcps in total vs. 70 kcps/element• parallel vs. serial operation

Spectral Resolution

WDS provides roughly an order of magnitude higher spectral resolution (sharper peaks) compared with EDS. Plotted here are resolutions of the 3 commonly used crystals, with the x-axis being the characteristic energy of detectable elements.

Note that for elements that are detectable by two spectrometers (e.g., Y La by TAP and PET, V Ka by PET and LIF), one of the two crystals will have superior resolution (but lower count rate).

Reed, 1995, Fig 13.11, in Williams, Goldstein and Newbury (Fiori volume)

Spectrometer Efficiency

The intensity of a WDS spectrometer is a function of the solid angle subtended by the crystal, reflection efficiency, and detector efficiency.

Reed (right) compared empirically the efficiency of various crystals vs EDS. However, the curves represent generation efficiency (recall overvoltage) and detection efficiency.

Reed, 1996, Fig 4.19, p. 63Reed suggests that the WDS spectrometer has ~10% the collection efficiency relative to the EDS detector.

How to explain the curvature of each crystal’s intensity function? At high Z, the overvoltage is presumably minimized (assuming Reed is using 15 or 20 keV). Low Z equates larger wavelength, and thus higher sinq, and thus the crystal is further away from the sample, with a smaller solid angle.

Effect of voltage

• Excitation volume goes as V1.7

• Available X-ray lines

25kV 5um

15kV 2.5um

10kV 1.3um

5kV 0.4um

Typical steel spectrum, 15 kV

Lines available at low kV

Note overlap of V, Cr, Mn, and Fe. Also, O has its line at 0.53 keV.

Effect of current

spatial resolution reduced with high currentsgreater sensitivity with high currents• detectability• precision/repeatability

Overlap considerations

• Smaller issue for WDS – effects background choices

• Deconvolution option for EDS if statistics permit

• Statistics become problematic if trace element on major element background

EDS Overlap: S, Mo, Hg

HgS std Line Type Wt% Wt% Sigma Atomic %

S K series 13.38 0.14 49.15

Hg M series 86.62 0.14 50.85

Total 100.00 100.00

Stoichiometry is on-the-mark - in this case.

WDS “overlap”: HgS, PbS, Mo

Note that signal drops to background in between most peaks. Mo tail interferes with S.

Rare earths by EDS and WDS

Pr peak fits between Ce La and Lb peaks.

Er

DyTb

EDS Atomic fractionCompound Fe Y Ce Pr Nd Gd Tb Dy Ho Er Lu

D5 Y2Fe17 88.49 11.51

B4 Ce2Fe17 89.05 10.95

B5 Pr2Fe17 88.39 11.61

C1 Nd2Fe17 88.81 11.19

C2 Gd2Fe17 90.12 9.88

C5 Tb2Fe17 86.21 13.79

D1 Dy2Fe17 88.43 11.57

D2 Ho2Fe17 87.59 12.41

D3 Er2Fe17 84.69 15.31

E2 Lu2Fe17 89.77 10.23

2/19 = 10.53%

Suitable samples

• solid/rigid• stable under beam• conductive (while under beam)• nonconductive samples can be coated with C

or metal (e.g., Au, Pt, Ir)(coating obscures features and elements but only a little)

Samples include

• Metals• Geologic samples• Ceramics• Polymers• Experimental materials

Quantitative Considerations• Homogeneous (within excitation volume)• Thick (enclosing interaction volume);

therefore, problems with layered samples• Known geometry (preferably “flat” compared

to excitation volume; thus, polished); therefore problems with rough samples

• Be smart with construction (e.g., glass vs. Si)• Standards collected each time vs.

Standardless and normalization

Matrix effectsZ-A-F or Phi-Rho-Z corrections accounting for penetration depth, absorption, secondary fluorescence

Accuracy depends on well known curvature.

Alternatively, need standard in region for better results.

Range of Quantitation

100% down to 0.05% (500 ppm) EDS, 0.001% (10s of ppm) WDS

Limited by statistics, differentiation from background

More counts help!

Mapping and Line-scansPoint analysis are most sensitive to concentration differences (30s/point)

Line scans are next (500 ms/pixel)

Mapping is least sensitive (12 ms/pixel)

Graphics convey much information quickly(i.e., a picture is worth a thousand words)

Digital image showing regions of analysis and line-scan

Mg portion of overlapped peak

Ge portion of overlapped peak

Line-scan using typical windowsGe-Mg overlap causes problems

Line-scan using deconvolutionGe contribution is stripped from Mg profile

Mapping using deconvolution

EDS-WDS comparisonCharacteristic EDS WDS

Geometric collection efficiency (solid angle)

<3% <0.2%

Spectral resolution (FWHM) <130 eV 2-10 eV

Instantaneous X-ray detection entire spectrum (0.2 keV thru E0) single wavelength (a few eV)

Maximum count rate100s of thousands cps over entire spectrum

tens of thousands cps (single wavelength)

Artifacts sum peaks, Si escape peaks, Si fluor. peakhigher order peaks,

Ar escape peaks

Low-Z limit = Be With thin window detector With synthetic "crystals"

Detection Limits 0.05 wt% (500 ppm) 0.001 wt% (10 ppm)

Bottom LineCheaper, quicker but some elements are too

close to resolve, e.g., S-Ka, Mo-La, Pb-Ma

Slower, more expensive, but with better resolution and higher

peak/bkgd ratios giving lower detection limits

“Harper’s Index” of EPMA1 nA of beam electrons = 10-9 coulomb/sec

1 electron’s charge = 1.6x 10-19 coulomb

ergo, 1 nA = 1010 electrons/sec

Probability that an electron will cause an ionization: 1 in 1000 to 1 in 10,000

ergo, 1 nA of electrons in one second will yield 106 ionizations/sec

Probability that ionization will yield characteristic X-ray (not Auger electron):

1 in 10 to 4 in 10.

ergo, our 1 nA of electrons in 1 second will yield 105 x-rays.

Probability of detection: for EDS, solid angle < 0.03 (1 in 30). WDS, <0.001

ergo 3000 x-rays/sec detected by EDS, and 100 by WDS. These are for pure

elements. For EDS, 10 wt% = 300 X-rays; 1 wt% = 30 x-rays; 0.1 wt % = 3 x-ray/sec.

ergo, counting statistics are very important, and we need to get as high count rates

as possible within good operating practices.

From Lehigh Microscopy Summer School

Raw data needs correctionThis plot of Fe Ka X-ray intensity data demonstrates why we must correct for matrix effects. Here 3 Fe alloys show distinct variations. Consider the 3 alloys at 40% Fe. X-ray intensity of the Fe-Ni alloy is ~5% higher than for the Fe-Mn, and the Fe-Cr is ~5% lower than the Fe-Mn. Thus, we cannot use the raw X-ray intensity to determine the compositions of the Fe-Ni and Fe-Cr alloys. (Note the hyperbolic functionality of the upper and lower curves)

n l s m j number ofelectrons

Sub shell X-raynotation

1 0 ½ 0 ½ 2 1s K

2 0 ½ 0 ½ 2 2s LI

2 1 LII

2 1 ½ -1, 0, 1 ½ ½ ½

6 2p LIII

3 0 ½ 0 ½ 2 3s MI

3 1 MII

3 1

½ -1, 0, 1 ½ ½ ½

6 3pMIII

3 2 MIV

3 2

½ -2, -1, 0,1, 2

½ ½ ½ ½ ½

10 3dMV

n = principal quantum number and indicates the electron shell or orbit (n=1=K, n=2=L, n=3=M, n=4=N) of the Bohr model. Number of electrons per shell = 2n2

l = orbital quantum number of each shell, or orbital angular momentum, values from 0 to n –1

Electrons have spin denoted by the letter s, angular momentum axis spin, restricted to +/- ½ due to magnetic coupling between spin and orbital angular momentum, the total angular momentum is described by j = l + s

In a magnetic field the angular momentum takes on specific directions denoted by the quantum number m <= ABS(j) or m = -l… -2, -1, 0, 1, 2 … +l

Rules for Allowable Combinations of Quantum Numbers:· The three quantum numbers (n, l, and m) that describe an orbital must be integers. · "n" cannot be zero. "n" = 1, 2, 3, 4... · "l" can be any integer between zero and (n-1), e.g. If n = 4, l can be 0, 1, 2, or 3. · "m" can be any integer between -l and +l. e.g. If l = 2, m can be -2, -1, 0, 1, or 2. · "s" is arbitrarily assigned as +1/2 or –1/2, but for any one subshell (n, l, m combination), there can only

be one of each. (1 photon = 1 unit of angular momentum and must be conserved, that is no ½ units, hence “forbidden transitions)

No two electrons in an atom can have the same exact set of quantum numbers and therefore the same energy. (Of course if they did, we couldn’t observably differentiate them but that’s how the model works.)

One slide Schrödinger Model of the Atom

Origin of X-ray Lines for K and L Transitions