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Version Last Revised: 2/4/2014
Electron Probe Microanalysis
EPMA
Energy Dispersive Spectrometry
(EDS)
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What’s the point?
Using X-rays to produce e-hole pairs (charges
proportional to X-ray intensity), which are amplified
and then “digitized”, put in a histogram of number
of X-rays counts (y axis) versus energy (x axis). A
solid state technique with unique artifacts.
EDS spectrum for
NIST glass K309
(Goldstein et al, Fig. 6.12, p. 356)
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Summary
• X-rays cause small electric pulses in a solid state
detector. Associated electronics produce
‘instantaneously’ a spectrum, i.e. a histogram of
counts (number=intensity) vs the energy of the X-ray
• Relatively inexpensive; there are probably 50-100
EDS detectors in the world for every 1 WDS
(electron microprobe)
• Operator should be aware of the limitations of EDS,
mainly the specific spectral artifacts, and the poor
spectral resolution for some pairs of elements, and
general lack of quality control of results
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Generic EMP/SEM
Electron gun
Column/ Electron
optics
Optical microscope
EDS detector
SE,BSE detectors
Vacuum
pumps
Scanning coils
WDS
spectrometers
Faraday current
measurement
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EDS assemblage
Goldstein et al fig 5.21
There are several types of solid state EDS
detectors, the most common (cheapest) being the
Si-Li detector. Components: thin window (Be, C,
B); SiLi crystal, FET (field effect transistor: initial
amp), cold finger, preamp, vacuum, amp and
electronics (“single channel analyzer”).
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EDS Windows
Windows allow X-rays to pass and
protect detector from light and oil/ice.
Be: The most common EDS detector
window has been made of Be foil ~7.6 mm
(0.3 mil) thick. It allows good transmission
of X-rays above ~ 1 keV. It is strong
enough to withstand venting to atmospheric
pressure, and opaque to optical photons.
Thin - Ultrathin: For transmission of light
element X-rays (<1 keV), windows ~0.25
mm thick of BN, SiN, diamond or polymer
are used. They must use supporting grids to
withstand pressure differentials; the grid
(e.g., Si or Ni) takes up about 15% of the
area, but the window material is thin
enough that low energy X-rays pass
through.
This plots shows the transmittance of X-rays
thru different types of window material.
(Quantum [BN] 0.25 um, diamond 0.4 um).
The higher the transmission number, the better
“Windowless”: Here there is no
film, and there is a turret that allows
swapping with a Be window.
Difficult to use as oil or ice can coat
the detector surface. Not used much.
Goldstein Fig. 5.41, p. 318
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EDS Windows
This plots shows the transmittance of X-rays thru different types of window material. (Quantum
[BN] 0.25 um, diamond 0.4 um). The higher the transmission number, the better
Goldstein Fig. 5.41, p. 318
How it works: energy gap
A semi-conductor like Si has
a fully occupied valence
band and largely unfilled
conduction band, separated
by an energy gap (1.1 eV).
Incident energy can raise
electrons from the valence to
the conduction band.
X-ray hits the SiLi crystal, producing a specific number of electron-hole
pairs proportional to X-ray energy; e.g. one pair for every 3.8* eV, so for
incident Fe Ka, 6404 eV, 1685 e-hole pairs are produced. With a bias**
applied across the crystal, the holes are swept to one side, the electrons to
the other, producing a weak charge. Boron is important acceptor
impurity in Si and degrades it (permits thermal excitation: bad); at the
factory, Li is drifted in (donor impurity) to counter its effects.
* 1.1 eV + energy wasted in lattice vibrations, etc
**bias: a voltage is applied between 2 points; e.g. one +1500 v, other -1500 v.
Goldstein et al, Fig 5.19
How they make the Si(Li)
For electronics, dope Si to change electronic structure:
Si + Grp 5 (P, As)  electrons majority carriers;
Si + Grp 3 (B, Ga) holes majority carriers, BUT
net effect is to allow current to flow in device in
presence of bias – not good
for an x-ray detector where
you ONLY want current to
flow when x-ray impacts.
WANT HIGH PURITY Si
(but Boron contaminant); so
diffuse in Li (n-type dopant).
Then drift with reverse bias.
Result: Li-rich surface ntype; center: no-charge
carriers (compensated);
Low-Li p-type. Remove the
Williams 1987
ends.
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How it works: inside the detector
X-rays are absorbed by Si, with
photoelectrons ejected. This
photoelectron then creates electronhole pairs as it scatters inelastically.
The Si atom is unstable and will
either emit a characteristic Auger
electron or Si ka X-ray. If Auger, it
scatters inelastically and produces
electron-hole pairs. If Si Ka X-ray,
it can be reabsorbed, in a similar
process, or it can be scattered
inelastically. In either case, the
energy will end up as electron-hole
pairs. The result, in sum, is the
conversion of all the X-ray’s
energy into electron-hole pairs -with 2 exceptions.
Fig 9.5 Reed; Fig 5.22 Goldstein
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Artifacts: Si-escape peak;
Si internal fluorescence peak
There are 2 exceptions to the neat
explanation of how the Si(Li) detector
works. Si-escape peaks are artifacts that
occur in a small % of cases, where the Si
ka X-ray generated in the capture of the
original X-ray escapes out of the
detector (red in figure). Since this X-ray
removes 1.74 keV of energy, the signal
generated (electron-hole pairs) by the
incident X-ray will be 1.74 keV LOW.
This will produce a small peak on the
EDS spectrum 1.74 keV below the
characteristic X-ray peak. Another
artifact is the Si internal fluorescence
peak, which occurs if an incident X-ray
is absorbed in the Si “dead” layer (green
region). This region is “dead” to
production of electron-hole pairs,
but Si ka X-rays can be produced
here which then end up in the
“live” part of the detector, and
result in a small Si ka EDS peak.
Consider Ti …
Fig 5.22 Goldstein et al
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Artifacts: Si-escape peaks;
Si internal fluorescence peak;
extraneous peaks
The figure shows a real spectrum of a
sample of pure Ti metal -- but there are
7 peaks besides the Ti Ka and Kb. At
1.74 keV below each, are the
respective escape peaks (blue arrows).
Also present is a Si internal
fluorescence peak (green arrow). The
Fe and Cu peaks are from excitation of
metal in chamber or sample holder by
BSE or Ti X-rays. Note the sharp drop
in the background intensity on the high
side of the Ti Kb peak (= Ti K
absorption edge, red arrow). (2 Ti Ka
and Ti Ka+Kb explained shortly.)
Note the scale of the spectrum: the Ti Ka
max is 1.3 million counts. These effects are
generally weak, but evident when you are
looking for minor elements.
Goldstein et al Fig 5.39,p. 316
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Question: Do all characteristic
X-rays have Si-escape peaks
in a Si(Li) detector?
Why or Why Not?
Hint 1: Sr La does not, but Os Ma does
Hint 2: Look up the characteristic energies of
each
Hint 3: Look up the absorption edge (critical
excitation) energy of Si Ka
Hint 4: Compare the numbers in 2 to number in 3.
Which one is greater than the one in #3?
Would a Si Ka x-ray produced in the sample,
which then makes its way thru the vacuum to the
EDS detector, have enough energy to knock out
the inner shell (K) electron of the Si detector
crystal?
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Signal processing
Si(Li) detector has no
internal gain*; for Ca Ka
photon with ~1000 e-hole
pairs, the charge is only
~10-16 Coulomb (weak!)
Directly coupled to the Si
crystal is a field effect
transistor (FET) that
converts charge to voltage,
followed by a preamp. We
need low noise, high gain
amplification, so the
Si detector and FET are cooled to about 100K with liquid nitrogen (LN) to
prevent noise (and prevent diffusion of Li in detector--at least in old ones). More
signal gain provided by main amplifier (signal now boosted to 1-10 volts) where
also RC (resistor-capacitor) circuits are used to shape the pulse, to maximize
signal/noise ratio and minimize pulse overlap at high count rates. Then ADC
(analog to digital converter) outputs data to the screen as a spectrum display.
*gain = electronic multiplication of signal intensity
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The first signals in the EDS detector
The set of electron-hole pairs
produced by the impact of the X-ray on
the Si(Li) detector produces a tiny
charge (~10-16 C), very quickly (~150x
10-9 sec). The FET(preamplifier)
changes the charge (capacitance) into a
tiny voltage (millivolts). These steps
are shown in the first half of (a) to the
right. The output of the FET is shown
below at (b) where the x axis is time
and y is voltage. The “jump” represents
the presence of a voltage proportional
to the number of electron-hole pairs
generated by each X-ray, so Photon
2’s jump is of a higher energy than
Photon 1’s jump which is higher than
Photon 3’s jump. At a certain point the
FET reaches the limit of the number of
“ramp”
charges it can hold, and then there is a
reset or zeroing back to some baseline
where it starts over. Following this are
electronics to shape the voltage into a
pulse that can be counted.
Goldstein et al (1992), p. 297
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Processing Time and
Pulse Pileup Rejection
The user can ‘tweak’ the time
constant (T.C.) which sets the time
allocated in the electronics to
process each pulse (=X-ray). In the
top figure, a short T.C. (1 ms)
permits each pulse to be counted
correctly. A longer T.C. (10 ms)
means the “gate” is open longer and
a second pulse can enter and be
incorrectly added; this is “pileup”
and causes distorted spectra.
Therefore, circuits are added (#4,
bottom figure) to sense when pileup
occurs and to ignore that pulse.
Goldstein et al (1992), Fig. 5.24 and 5.25, p. 300
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Dead Time
“Deadtime” is the period during which
the detector is “busy” and cannot
accept/process pulses. This can introduce
error unless it is accounted for, either by
extending counting time, or correcting for
it in the software. In most systems, the
user sets the “live time” which is the time
during which counts are actually counted,
and the “real time” is automatically
determined by the electronics or software.
40%
60%
80%
Optimal deadtime is in the 35-45% range.
This optimizes both user/machine time
and moderate to high throughput of
counts.
Goldstein et al (1992), Fig. 5.25 (p. 300) and
Goldstein et al (2003), Fig. 7.9 (p. 307)
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Detector performance: peak
resolution (FWHM)
The characteristic X-rays generated in
the specimens are very close to lines,
i.e. only a few eV wide at most.
However, the conversion of X-ray to a
pulse in the detector has several
variables (imperfections) that broaden
the peak to between maybe 135-200 eV,
depending upon the type of detector
and how well maintained it is. The
narrowness of the peak is measured by
the width of the peak at one half the
maximum intensity of the peak -- this is
what is termed the FWHM.
In EDS detectors, it is
usually measured at the Mn
Ka position, with values of
160 eV and below. Modern
(2005) one are quoted at
<130 eV.
Goldstein et al, Fig 5.34, p. 311
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Why Mn Ka
for EDS resolution?
EDS companies (their engineers mainly) do not want to
have to carry around an SEM or EMP to be able to test,
repair and calibrate an EDS system. Instead they carry
a small 1” diameter x 2” long tube that fits over the end
of the EDS “snout”. Inside it is an Fe-55 isotope source
(half life 2.7 yr) which emits an intense x-ray at 5.985
keV which is only a few eV different than Mn Ka.
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Time Constant+Beam Current--> Dead Time
Current
TC
DT
Cts
Res
Comment
Low
Short
10%
2000
150 eV
Takes a long time to
get a decent spectrum
Low
Long
20-30% 1000
135 eV
Optimal for phase ID
High
Short
40-60% 1020K
180 eV
Optimal for X-ray
mapping
High
Long
100%
___
Good for nothing
0
The above values are approximate, and meant only to
show the relation of the variables.
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Spectral processing:
background correction
The characteristic X-rays that we need to
quantify “ride” atop the continuum, and
the continuum contribution to the
characteristic counts must be subtracted.
(Top) Linear interpolation (B-D) will be
in error due to the abrupt drop of
continuum at the Cr K-absorption edge
(5.989 keV). B-C is possible but
critically dependent upon having good
spectral resolution (<160 eV). A-B
would be preferable. (Below) Doing
background fit of a complex stainless
steel.
Goldstein et al Fig. 7.1,2, p. 367
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Spectral processing:
background modeling or filtering
Correcting for the background is done by either of
2 methods: developing a physical model for the
continuum, or using signal/noise filtering.
Modeling is based upon Kramers Law: there is a
function describing the continuum at each energy
level, that is a function of mean atomic number,
and measured “detector response”.
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Background Modeling
The spectrum of Kakanui
hornblende (top left), with
superimposed calculated
(modeled) background, based
upon Kramers Law*. Bottom
figure shows after the
background has been
subtracted. Cu is artifact (stray
X-rays). Mn is actually
present at <700 ppm.
Ij = constant x Z (E0 - Ej) / Ej
at each energy channel j
Goldstein et al Fig 7.4, p. 372
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Background Filtering
Theoretically Fourier analysis will separate
out the low frequency continuum signal and
high frequency ‘noise’ from the medium
frequency characteristic peaks; however,
there is overlap and the result is a poor fit. A
better filter is the “top hat filter”, where no
assumptions are made about the spectrum,
and only the mathematical aspects of signal
vs noise are considered.
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Top Hat Filtering
This filter (top right) moves across the
EDS spectrum (with an optimally
defined window, ~ 2 FWHM* Mn
Ka;~320 eV), and assigns a new value
for the center channel based upon
subtracting the values in the left and
right channel from the center (value hk
chosen to total area =0). Thus, in the
simple spectrum (bottom right), the
center channel (+), when the left and
right channels are subtracted, leaves a
value ~0.
*FWHM: full width at half maximum.
Reed Fig 12.7 p. 174,Goldstein et al Fig 7.6, p. 374
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More Artifacts: Sum Peaks
There is a short period of time (t0)
during each X-ray capture by the
EDS detector, when the detector can
capture a second X-ray “by
mistake”. The electronics cannot
distinguish this “sum peak” from a
true single X-ray peak, and includes it
with all the other peaks from the
elements actually present. For 2 major
elements, could be 3 sum peaks; for 3, 6.
In reality,you only see 1 or 2 unless you
zoom in to the background level. Always
consider their possible presence.
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…Fools even the pros
Even the fanciest, slickest EDS setup can fool newcomers, not to mention
experienced users. Above, is a partial spectrum (major peaks) of a commercial
glass that has a lot of Si and O, plus Na and Al. Notice the S (Sulfur) label over
a peak around 2.3 keV … sure looks like it might be Sulfur, right? It is NOT,
rather it is a sum peak of O Ka (.525 keV)+ Si Ka (1.74 keV). Previous
experience with this “fake” peak had taught me to be skeptical
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Sum Peaks
In qualitative analysis of silicates, there are some
combinations of element Ka peaks that fall close to
Ka peaks of elements possibly present, as indicated
in the table below:
Some of more advanced EDS software now contain algorithms to
recognize and automatically remove sum peaks. But you must always be
on guard for them, particularly ones which “could really be there”. In
many of those cases, WDS is the solution.
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And More Artifacts
There is always a potential for ‘stray’ X-rays being
detected. It thus pays for the EDS operator to understand
what the path is for the electron beam and for the X-rays,
and know what ‘other’ elements might show up
unintentionally.
This is particularly true for EDS associated with TEM,
where specimens routinely sit on grids (Cu?) and the high
energy (200 keV?) electrons can go through the specimen
and hit a metal part of column or chamber, with the resulting
X-rays finding a way back to the detector.
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And More Artifacts
Another thing: many SEM labs use gold or palladium
coating on specimens. These very thin coats will produce
definite x-ray peaks!
Family of
Pd L lines
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…And beware of lazy peak IDs
Even the fanciest, slickest EDS setup can make misidentifications…so the
analyst cannot get lazy and assume just because the expensive software said
something was there, it was there. I knew Arsenic was possible (As La
identified), but unlikely, and rather Mg Ka was more likely. To confirm it was
NOT As, I cranked the accelerating voltage up to 20 keV (the K shell binding
energy is 11.9 keV) and found there was NO As Ka x-ray. Ergo, not As.
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…and trusting software
As Newbury (2005 and
2006 reply) pointed out,
an EDS operator is a fool
to believe that the
automatic peak ID will be
correct 99.9% of the time.
From Newbury (2006)
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Artifical EDS spectrum
Artificial: no background, no artifacts, and assumes EACH
element at 100% concentration. Why, then, the two slopes??
Peak intensities of elements from Si to Na decrease, and also
from Si to Zn -- why? (Hint: 2 physical phenomena)
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Artificial spectrum
The actual spectrum of pure
elements, as generated at the
point of impact, would be one
steady decreasing curve from
Na down to Zn, following the
red curve superimposed here.
Slope down from Si to Na: Xray energies are increasingly
weaker, and are absorbed both
within the specimen and by
the window.
Slope down from Si to Zn:
there are less and less X-rays
being produced because the
accelerating voltage is
constant (e.g. 20 keV) and the
overvoltage is lower.
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Evolution of EDS spectrum: from
the specimen to the monitor - 1
Simulation of element (say V) X-ray generation and display
The spectrum on our monitor (d)
is a result of many things
impacting the real spectrum
generated within the specimen
(a). At instant of generation
within the specimen, there is
only the Ka, Kb and continuum.
An instant later (b), as the Xrays leave the specimen, two
things can happen: some of the
continuum X-rays above 5.464
keV are absorbed, producing the
drop in the continuum there.
Goldstein et al Fig 5.53 (by R. Bolon) p. 330
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Evolution of EDS spectrum: from
the specimen to the monitor - 2
Also in (b) the lower energy
continuum is absorbed, causing
the dropoff in the spectrum
there. When the X-rays hit the
detector (c), Si fluorescence
peaks can result. And after
signal processing (d), the
display will show peak
broadening, sum peaks, Siescape peaks, further decrease
of intensity and low energy
noise.
Simulation of element (say V) X-ray generation and display
Goldstein et al Fig 5.53 (by R. Bolon) p. 330
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Comments about LN2 and EDS
That big tank of liquid nitrogen cools the SiLi crystal and
the FET, so the very low charge generated by the
electrons-holes can be detected with minimal noise. But
what about letting the thing warm up when you’re on
vacation? There is a lot of misunderstanding about this…
Modern systems “can” be allowed to warm up without
damage to the crystal (if the bias on it is turned off) -BUT that is not the only thing to be concerned about
when it warms up. Another important ingredient is the
vacuum within the snout that extends from the bottom of the dewar
to the end where the detector sits -- there is a “getter” (zeolites or Al
wool) inside that absorb yucky contaminants. But if the getter warms
up, they are released inside the snout, creating a poor vacuum, which
then means the LN usage increases significantly as the vacuum is
poor. Bottom line: keep it cold all the time.
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EDS-WDS comparison
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Recent Developments
Over the past 15-20 years, 2 new “spins” off the ‘old
school’ Si(Li) EDS detector have entered the
microanalysis world:
1. The microcalorimeter
2. The Silicon Drift Detector
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Microcalorimeter
The principal behind the microcalorimeter is that an x-ray
hitting a very sensitive thermal absorber will register a very
small temperature increase. However, this requires a very cold
absorber, with liquid helium cooling required. It would provide
the best of both EDS and WDS, with simultaneous capture of
all x-ray energies AND with very tight spectral resolution
(like with WDS).
However, there apparently have been major engineering
stumbling blocks and none have made it to the market.
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Silicon Drift Detector
The SDD is similar to SiLi Detector in that electron-hole
pairs are generated, but the physical design is radically
different. There is a lower capacitance, and also a lower
leakage current (high leakage current in SiLi is what requires
LN cooling). And because the SDD has the FET “built in”,
created during the lithography of the Si crystal, wires are
eliminated, reducing capacitance more.
Resulting advantages:
1. LN not needed (use a simple Peltier cooler)
2. Can handle high count rates >100,000 up to ~106 cps
3. Spectral resolution at 100,000 cps still good (~140-150 eV)
Image from Bruker web page
The SDD is created from a single Si crystal using micro-lithography.
“The major distinguishing feature of an SDD is the transversal
field generated by a series of ring electrodes that causes charge
carriers to 'drift' to a small collection electrode. The 'drift' concept
of the SDD (which was imported from particle physics) allows
significantly higher count rates.” - Wikipedia
Silicon Drift Detector Simulation
For the full simulation, go to
http://www.ketek.net/products/sdd-technology/working-principle/
url updated 2/3/14
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Further EDS details
There are several modern EDS companies, with
most producing very informative brochures that go
into the technical details of EDS hardware (and
software):
For example: Oxford Instruments
http://www.oxfordinstruments.com/products/microanalysis/energydispersive-x-ray-systems-eds-edx/eds-for-sem/sdd
has a nice technical publication explaining EDS
using the SDD as the detector. Well worth
downloading and reading.
url updated 2/4/14