Electron Probe Microanalysis EPMA

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Electron Probe MicroanalysisEPMA
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• Electron Optical Column

Updated 1/19/13
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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
  (thermoionic and field emission). 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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• Two electron sources thermoionic and field
  emission.
• Thermoionic electric current passes thru bent
  W wire (or sharpened LaB6 crystal tip), heating
  it and adding thermal energy which permits
  electrons to overcome the work-function energy
  barrier of the material and to leave the wire. A
  high voltage potential then can aim the
  electrons at nearby anode. Common in electron
  microprobes and many SEMs.
• Field emission a single crystal, shaped to a
  very sharp point, and a high voltage potential is
  placed between it and nearby anode. Generally no
  heating occurs (one exception). Because of the
  electric field, electrons can jump the energy
  barrier to the nearby anode. Requires higher
  vacuum and is more complicated to use. Common in
  high resolution SEMs (and more )

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Electron Gun source
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W and LaB6 sources
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Most common in e-probes and many SEMs is the W
filament, a 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 (each tip is 5-10x
cost of W filament) and fragile (sensitive to
vacuum problems). (Also CeB6). Both have very
good (1) beam stability, compared the field
emission (FE) guns, which are brighter and have
much smaller beams (great for high resolution SEM
images) but lower beam stability and require
ultra high vacuum. And for FE, add 400K to the
price of the SEM or electron probe.
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Thermoionic vs Field Emission
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For the ultimate in high resolution imaging, FE
is tops -- if you have the money (currently NSF
is not funding FE electron probes). For imaging,
you do not need a reliably stable current over
minutes-hours-days, as you acquire an image in
seconds. However, for quantitative chemical
microanalysis (EPMA) you must have a stable
current over minutes-hours and hopefully days --
as you must normalize your x-ray counts to beam
current.
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Resolution Comparison-1
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For the ultimate in high resolution imaging, FE
is tops -- if you have the money. Note the
importance of reduced beam current--good for
imaging, though not good for EPMA. Side note
beam diameter in electron
microprobes is incorrectly assumed to be
represented by the diameter of the bright CL spot
on fluorescent samples. It is not!
From Reed
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Resolution Comparison-2
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• To right is another version, from a JEOL sales
  brochure.
• I have two comments
• effective range for analysis by FE is wildly
  incorrect no one show believe you can do epma at
  100 pA! Normal currents are 10-20 nA
• Reeds figure showing the cross over above 1 nA
  needs explanation....

From Reed
Above from JEOL 8530F brochure 2012
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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 lose
W, thinning down to ultimate failureleft, from
our SX51. With care/luck, a filament may last 6-9
months, though 1-2 month life is not uncommon.
Top 3 images Goldstein Fig 2.8, p. 33
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W filament failure closeup
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Recent closeup images of the probes W filament,
imaged with the Hitachi SEM note the
crystallinity that is accentuated, and the
hollowness of the zone where the filament failed.
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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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Emission current vs probe (Faraday cup) current
This shows filament output on the S3400 emission
current IE, which flows from cathode to anode, is
high (to 10-4 A). However, what escapes through
the hole in the anode and reaches down the
column, is much lower, only 10-8 A. Most SEMs can
only read IE, lacking Faraday cups.
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Saturation on the SX51
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 unit-less values
(0-255 scale)
(left) Goldstein et al Fig 2.5, p. 278
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Saturation on the Hitachi S3400
Since gt99 of SEMs do not have Faraday cups, they
provide black box saturation buttons. But that
is not optimal for high resolution imaging, where
the you need a tight beam. Thus
you set the instrument in filament image mode
and optimize settings to get the tight spot
(right), not the donut on the left.
Goldstein, 2003, p.32
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Producing minimum beam diameter
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Similar geometry 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.
(opposite of magnification,here showing the
object size shrunk)
1/f 1/p 1/q
(Goldstein et al, 1992, p. 49)
(f focal distance)
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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.
WD
WD
Note we cannot change the working distance on
the SX51 however, this is a critical adjustable
parameter on the SEM.
(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 the aperture. Thus higher
current with larger probe (d2) and decreased
spatial resolution (a2). b) is converse case, for
low current situation.
With quant. EPMA we are shooting for high
currents, so the left case holds. For SEM work,
it depends for CL and EBSD, the same holds, but
for high resolution SE imaging, the right case
holds, i.e. drop the current as low as you can
get away with.
(Goldstein et al, 1992, p. 52)
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Beam Diameter and Imaging Resolution

• The electron microprobe and the SEM have
  significantly different beam diameters and
  imaging resolutions, because
• The probes main job is cranking out x-ray
  counts, and to optimize that, you need lots of
  beam current (tens of nA, up to hundreds for
  trace elements). Also, because the interaction
  volume is 2-3 microns anyway, it makes little
  sense to worry about beam size and resolution
  in EPMA. But
• The SEMs goal is to produce sharp images, and
  you can utilize several features to do that
• (a) Introduce small apertures to tighten up the
  beam diameter (150, 80, 50, 30 microns)
• (b) Turn down the probe current (pA) which
  minimizes scattering
• (c) Go to high saturation so filament image is
  tight and centered
• (d) Go to the shortest working distance possible
  (e.g. 6 mm)
• (e) Play with kV, to find best setting (could be
  high, could be low)

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Beam/Probe Diameter and Imaging Resolution

• The electron microprobe and the SEM have
  significantly different beam diameters and
  imaging resolutions
• The SX51, at its best resolution (set up as SEM,
  not microprobe), has 70 Å resolution. As set up
  as a microprobe, its resolution is much worse,
  maybe 1000 Å
• The S3400, at its best resolution (upon
  installation/performance tests, using evaporated
  Gold on Carbon, a common material for such tests)
  is rated at
• Secondary electron image at 30 kV 30 Å
• Secondary electron image at 10 kV 100 Å
• Backscattered electron image at 30 kV 40 Å

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Resolution Tests
One common SEM resolution is defined as point to
point resolution and is the smallest separation
of adjacent particles that can be detected in an
image. The manufacturers cheat by using optimal
images, sputtered gold balls on carbon (image to
right), where there is good secondary electron
generation and high contrast. It is optimized in
being a 3D image where the
secondary electrons show surface well. A flat
polished rock thin section would not show such
fine scale resolution. Also note that a 3 nm (30
A) resolution does not mean you can see 3 nm gold
balls most of the balls are 50 nm in
size. Another resolution test is scanning across
a very sharp edge (e.g. razor blade) and
determining the distance between 90 to 10 of
the intensity drops off.
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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 Faraday) 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). Later, with the development
of the SEM as a separate tool, scanning was
essential.
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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Scanning --gt 2 D Image
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The electron probe, a fine point (lt1 um) is
rapidly scanned across the sample, and the signal
from each (x,y) coordinate is mapped onto the
screen or a file.
Goldstein et al, 3rd Edition, Fig 4.4
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SX50/1 specs
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Fixed Working Distance ?
Rowland Circle Radius?
Optical Microscope Mag ?
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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 tightenings by
operators.)
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Go to Vacuum Module
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