Transcript No Slide Title

UofO- Geology 619
Modified from Fournelle, 2006
Electron Beam MicroAnalysisTheory and Application
Electron Probe MicroAnalysis (EPMA)
Energy Dispersive
Spectrometry (EDS)
Review of Wavelength Dispersive Spectrometry (WDS)
What is EDS?
Using X-rays to produce electron-hole pairs (total summed
charge which is proportional to incident x-ray energy),
which are amplified and then “digitized” by voltage,
displayed as a histogram of the number of x-rays pulses (y
axis) versus x-ray energy (x axis). A solid state technique
with unique artifacts.
EDS spectrum for
NIST glass K309
(Goldstein et al, Fig. 6.12, p. 356)
Summary
• X-rays cause small electric pulses in a solid state detector.
Associated electronics produce ‘instantaneously’ a spectrum,
i.e. a histogram of count (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
Generic EMP/SEM
Electron gun
Column/ Electron
optics
Optical microscope
EDS detector
Scanning coils
SE,BSE detectors
Vacuum
pumps
WDS
spectrometers
Faraday current
measurement
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), vacuum, cold finger, preamp, amp and MDA
electronics (“multi channel analyzer”).
EDS Windows
Windows allow X-rays to pass and
protect detector from light and gases.
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 thin enough
that low energy X-rays pass through.
This plots shows the transmittance of X-rays
thru difference types of window material.
(Quantum [BN] 0.25 um, diamond 0.4 um).
The higher the 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.
Goldstein Fig. 5.41, p. 318
Energy Dispersive System Schematic
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); 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 it works: inside the detector
X-rays are absorbed by Si, with K
shell (photo)electron ejected with hv Ec (Ec=1.84 keV) energy. 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
The Monolithic Semiconductor
Energy Dispersive X-ray Spectrometer
Al reflective coating,
20 - 50 nm
Ice? (pathological defect)
Active silicon (intrinsic), 3 mm
Area
10- 60 mm2
Au electrode, ~20 nm
Si 'dead' layer (p-type),
~100 nm
X-rays
Electrons
Holes
Rear Au electrode, ~20 nm
Inactive silicon (n-type), ~100nm
Window:
Be, BN,
C (diamond),
or polymer
0.1 - 7
mm
- 1000 V
X-ray photons with E from ~100 eV to
~ 40 keV can be detected, one at a time!
X-ray detection
Silicon Drift Detector (SDD)
X-rays
SDDs are thin!
300 mm (400 um for TEM)
SDD Backsurface
SDDs have a complex
back surface electrode
structure.
Ring electrodes
Resistor bridge
The anode of an SDD
Central anode,
is ~ 0.005 mm2 for a
50 mm2 detector, about
80 mm diameter
Active area 5 mm2 to 100 mm2 1/10,000 the area of EDS
X
X-- RAY DETECTOR
Back
R ladder
Steering
electrodes
Anode
l
High resistivity (n-type) silicon
l
p+ electrodes on front and back reverse biased
l
Radial applied bias on back-side tilts the valley toward n+ anode
Internal field efficiently collects
over entire detector area and
brings charge to central anode
produces potential valley
Fe
Al
Raney Nickel Alloy
E0 = 20 keV 10 nA
TC = 500 ns (188 eV
at MnKa)
128x128
10 ms per pixel
Mapping 185 sec
20 mm
Ni
Phases
Al 99.5 Ni 0.5
Al 71.2 Ni 24.6 Fe 4.2
Al 60 Ni 40 “I”
Al 46.5 Ni 53.5 “H”
Al Fe Ni
I
H
Al
Artifacts: Si-escape peak;
Si internal fluorescence peak
There are 2 exceptions to the previous
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.
Fig 5.22 Goldstein et al
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
Question: Do all characteristic Xrays 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 will 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?
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!)
We need low noise, high
gain amplification. Best is
multi-stage, with a preamp
(FET, field effect transistor)
immediately adjacent to the
detector
crystal. The detector and FET are cooled to about 100K with liquid nitrogen (LN) to
prevent noise (and prevent diffusion of Li in detector). More signal gain provided then 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
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
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
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
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 30-35% range.
This optimizes both user/machine time
and moderate to high throughput of
counts.
Goldstein et al (1992), Fig. 5.25 and 5.29, p. 300 and 303
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
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.
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 Kabsorption 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
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”.
Background Modeling
The spectrum of Kakanui
hornblende (top left), with
superimposed calculated
(modeled) background, based
upon Kramers Law.
Bottom shows after the
background has been
subtracted. Cu is artifact (stray
X-rays). Mn is actually present
at <700 ppm.
Goldstein et al Fig 7.4, p. 372
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.
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
More Artifacts: Pulse Pile Up
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 “piles” it up
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.
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:
Sum Peak
Mg + Ca
Si + Ca
Mg + K
Al + Si
Ca + Ca
Element
V
Cr
Ti
K
Ni
eV separate
6
18
57
87
91
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.
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
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)
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.
Also fluorescent yield is less
for low Z elements and high Z
elements “share” more incident
electrons with outer shells.
Slope down from Si to Zn:
there are less and less X-rays
being produced because the
accelerating voltage is
constant (e.g. 10 keV) and the
overvoltage is lower.
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
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
EDS-WDS comparison
Characteristic
EDS
WDS
Geometric collection efficiency
<2%
<0.2%
Spectral resolution (FWHM)
160 eV and less
2-10 eV
Instantaneous X-ray detection
~ 1 keV thru E0 (window dependant)
a few eV
Maximum count rate
10s of thousands cps over whole spectrum
tens of thousands cps (one wavelength)
Artifacts
sum peaks, Si escape peaks, Si fluor. peak
n>1 peaks, Ar escape peaks
Light elements?
With windowless or thin window detector
With synthetic diffractors ("crystals")
Detection Limits
~1000-5000 ppm, 0.1-.5 wt%
<100-500 ppm, <0.01-.05 wt%
Bottom Line
Cheaper, quicker but some elements are
More expensive, but with much better
too close together to resolve
spectral resolution and higher Pk/Bkg,
(eg S Ka, Mo La, Pb Ma)
giving lower detection limits.
(solid angle)
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
<www.osinst.com/ANLPDP174.htm> has a nice
technical publication “EDS Hardware Explained”
available as a pdf.