Electron Probe Microanalysis EPMA

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Electron Probe MicroanalysisEPMA
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• Electron Optical Column

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Whats the point?
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We need to create a focused column of electrons
to impact our specimen, to create the signals we
want to measure. This process is identical for
both scanning electron microscope (SEM) and
electron microprobe (EMP). We use conventional
terminology, from light optics, to describe many
similar features here.
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Key points
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• Source of electrons various electron guns W in
  particular. We want high, stable current with
  small beam diameter.
• Lenses are used to focus the beam and adjust the
  current
• Current regulation and measurement essential
• Beam can be either fixed (point for quant.
  analysis) or scanning (for images)
• Optical microscope essential to position sample
  (stage) height, Z axis ( X-ray focus)
• Vacuum system essential

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Generic EMP/SEM
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Electron gun
Column/ Electron optics
Optical microscope
Scanning coils
EDS detector
SE,BSE detectors
WDS spectrometers
Vacuum pumps
Faraday current measurement
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Electron Guns
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Several possible electron sources most common is
the W filament, thermoionic type. A W wire is
heated by 2 amps of current, emitting electrons
at 2700 K the thermal energy permits electrons
to overcome the work-function energy barrier of
the material. Another thermoionic source is
LaB6, which has added benefits (brighter,
smaller beam) but it is more expensive and
fragile. Both have very good (1) beam
stability, compared to a different variety of
sources, the field emission guns, which are
brighter and have much smaller beams (great for
high resolution SEM images) but lower beam
stability and require ultra high vacuum.
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W filamentbiased Wehnelt Cap
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Current (2 A) flows thru the thin W filament,
releasing electrons by thermoionic emission.
There is an HV potential (E0) between the
filament (cathode) and the anode below it, e.g.
15 keV. The electrons are focused by the Wehnelt
or grid cap, which has a negative potential (
-400 V), producing the first electron cross over.
First electron cross-over
Goldstein Fig 2.4, p. 27
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SX50 Gun and Wehnelt
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Wehnelt diameter (below) is 20 mm
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W filament
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W filament is 125 mm diameter wire, bent into
hairpin, spotwelded to posts. W has low work
function (4.5 eV) and high melting T (3643 K),
permitting high working temperature. Accidental
overheating will cause quick failure (top right).
Under normal usage, the filament will slowly
ablate W, thinning down to ultimate failure
(uncertain why offset). With care/luck, a
filament may last 6-9 months, though 1-2 month
life is not uncommon.
Goldstein Fig 2.8, p. 33
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Some electron units/values
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Brightness is a measure of the current
emitted/unit area of source/unit solid area of
beam (not used in daily activities)
High voltage and Current - Analogies
Baseball HV speed of the ball curr size of the
ball
Water through hose HV water pressure curr size
of the stream of water
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Saturation
Saturation is the optimization of 1) current
stability (on the plateau) and 2) filament life
(minimal heating). The Operating or Saturation
point is at the knee of the plot. On the SX51,
HEAT is the variable, with saturation usually
between 228 and 200, with new filaments at the
upper value, and gradually declining as the
filament ages (thins). These are unitless values
(0-255 scale)
Goldstein et al Fig 2.5, p. 278
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Producing minimum beam diameter
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Similar to light optics (though inverted
reducing image size) d0 is the demagnified gun
(filament) crossover--typically 10-50 um, then
after first condenser lens, it is further
demagnified to crossover d1. After C2 and
objective lens, the final spot is 1 nm-1um.
1/f 1/p 1/q
(Goldstein et al, 1992, p. 49)
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Column focusing the electrons
Simple iron electromagnet a current through a
coil induces a magnetic field, which causes a
response in the direction of electrons passing
through the field.
Rotationally symmetric electron lens beam
electrons are focused, as they are imparted with
radial forces by the magnetic field, causing them
to curve toward the optic axis and cross it.
(Goldstein et al, 1992, p. 44)
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Condenser Objective Lensesworking distance
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Left shorter working distance (q2), greater
convergence (a2) smaller depth of field, smaller
spot (d2), thus higher spatial resolution. Right
longer WD, smaller convergence larger depth of
field, larger spot, decreased resolution.
(Goldstein et al, 1992, p. 51)
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Condenser Lensesadjusting beam current
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Probe current (Faraday cup current, e.g. 20 nA)
is adjusted by increasing or decreasing the
strength of the condenser lens(es) a) weaker
condenser lens gives smaller convergence a1 so
more electrons go thru aperture. Thus higher
current with larger probe (d2) and decreased
spatial resolution (a2). b) is converse case, for
low current situation.
(Goldstein et al, 1992, p. 52)
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Probe diameter
What is the beam/probe size? I would suggest
this is a philosophical question what is the
theoretical size of the beam--before it enters
the specimen?-- a question of limited importance
in EPMA For that hypothetical question, Reed
provides a ballpark estimate (for 5 nA of beam
current, the minimum diameter is 0.2 mm.
(D is demagnification, a is beam semi-angle).
However, in the real world, the actual
interaction volume (due to electron scattering)
and thus size of analysis volume is larger, as
you can appreciate from your Monte Carlo
simulations.
Reed 1993, Fig 4.11, p. 46
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Probe current monitoring and stabilization
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EPMA requires precise measurement of X-ray
counts. X-ray count intensity is a function of
many things, but here we focus on electron
dosage. If we get 100 counts for 10 nA of probe
(or beam or Farady) current, then we get 200
counts for 20 nA, etc. Therefore, it is
essential that we 1) measure precisely the
electron dosage for each and every measurement,
and 2) attempt to minimize any drift in electron
dosage over the period of our analytical session.
The first relates to monitoring, the second to
beam regulation.
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Probe current monitoring
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Electron beam intensity must be measured, to be
able to relate each measurement to those before
and after (i.e. to the standards and other
unknowns). This is done with a Faraday cup, where
the beam is focused tightly within the center of
a small aperture over a drilled out piece of
graphite (or metal painted with carbon). Current
flowing out is measured.
Why graphite? Because it absorbs almost all of
the incident electrons, with no backscattered
electrons lost.
Goldstein et al 1992, Fig 2.25, p. 65
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Probe current monitoring
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Modern electron microprobes have built in,
automatic, Faraday cups. This is a small cup that
sits just outside of the central axis of the
column, and can be swung in to intercept the beam
upon automated control. This is typically done at
the end of each measurement on both standards and
unknowns, and using these values, the measured
X-ray counts are normalized to a nominal value
(e.g. 1 nA, or actual nominal value like 20
nA) In older instruments, this automation was not
implemented. An alternative solution would be to
create a homemade Faraday cup and mount it with
samples, and move the stage to it to do the
measurement, or measure absorbed current on
another reference material (e.g. brass).
Goldstein et al 1992, Fig 2.25, p. 65
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Probe current regulation
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Optimally, the beam current should remain as
constant as possible, particularly over the
duration of each measurement (depends upon number
of elements, etc, but most are 45-120 seconds).
This is accomplished in a feedback loop with the
condenser lenses, where a beam regulation
aperture measures the electrons captured on a
well defined area (red area on bottom aperture),
where larger
aperture above it provides shading and
eliminates excess electrons (green).
Reed 1993, Fig 4.12, p. 47
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Scanning Coils
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The primary mission of the electron microprobe is
to focus the beam on a spot and measure X-rays
there. However, it was early recognized that
being able to scan (deflect) the beam had two
advantages X-rays could be produced without
moving the stage, and electron images could be
used to both identify spots for quantification,
and for documentation (e.g. BSE images of
multiphase samples).
Scanning requires 1) deflection coils and 2)
display system (CRT) with preferably 3) digital
capture ability.
Reed 1993, Fig 2.3, p. 18
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SX51 specs
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Optical Microscope
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An essential part of an electron microprobe is an
optical microscope. The reason is that we need to
consistently verify that all standards and
specimens sit at the precise same height (Z
position). This is because they must all be in
X-ray spectrometer focus, which shortly you
will find described as the Rowland circle.
Mounting of specimens relative to an absolute
height is problematic, for a variety of reasons
(difficult to mount samples perfectly flat, and
the fact that we use different holders and
shuttles manufactured to different tolerances,
together with different screw tightings by
operators.)
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Go to Vacuum Module
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