Transcript Electron probe microanalysis EPMA - UW

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Electron Probe Microanalysis
EPMA
Electron
Optical
Column
Updated 2/3/14
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What’s the point?
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
• 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
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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Electron Guns
Two electron sources: thermionic and field emission.
• Thermionic – 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.
Two types: cold and hot. Because of the electric field, electrons can
jump the energy barrier to the nearby anode. Requires higher
vacuum and much more expensive, but has longer life. Common in
high resolution SEMs and some newer electron microprobes.
Electron Gun source
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W and LaB6 sources
Most common in e-probes and many SEMs is the W filament, a
thermionic 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. $25-$100@
Another thermionic 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. Replacement emitter ~$8-12,000@
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Thermionic vs Field Emission
For the ultimate in high resolution imaging, FE is tops -- if
you have the money.
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
For the ultimate in high
resolution imaging, FE
is tops -- if you have
the money. Note the
importance of reduced
beam current (pA not
nA)--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
To right is another version, from a
JEOL sales brochure.
I have two comments:
(1)“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
(2)Reed’s figure showing the cross
over above 1 nA needs explanation,
maybe older cold FE?
From Reed
Above: from JEOL 8530F brochure 2012
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W filament:biased Wehnelt Cap
First
electron
cross-over
Current (~2 A) flows thru the
thin W filament, releasing
electrons by thermionic
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.
Goldstein Fig 2.4, p. 27
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SX50 Gun and Wehnelt
Wehnelt diameter
(below) is ~20 mm
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W filament:
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 failure–left, 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
Recent closeup images of the
probe’s 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
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.
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
Saturation on the Hitachi S3400
Since >99% 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
Similar geometry to light
optics (though inverted:
reducing image size):
(opposite of
magnification
,here showing
the object size
shrunk)
Light Optics
Electron Optics
1/f = 1/p + 1/q
(f = focal distance)
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.
(Goldstein et al, 1992, p. 49)
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 Lenses:
working distance
WD
WD
Note: we cannot change the working distance on the SX51;
however, this is a critical adjustable parameter on the SEM.
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 Lenses:
adjusting beam current
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)
Beam Diameter and Imaging Resolution
The electron microprobe and the SEM have significantly different beam
diameters and imaging resolutions, because
(1) The probe’s 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
(2) The SEM’s 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)
* Exception: now with FE-EPMA, tighter beams and operation at low keV, we do worry!
Beam/Probe Diameter and
Imaging Resolution
The electron microprobe and the SEM have significantly different beam
diameters and imaging resolutions
(1) 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 ~750 Å;
(2) The S3400, at its best resolution (upon installation/performance
tests, using evaporated Gold on Carbon, a common material for
such tests) is rated at:
(a) Secondary electron image at 30 kV: 30 Å
(b) Secondary electron image at 10 kV: 100 Å
(c) Backscattered electron image at 30 kV: 40 Å
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 85-90% to 15-10% of the intensity drops
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Probe current: monitoring and
stabilization
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
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
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
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 45120 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
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 --> 2 D Image
The electron probe, a fine point (<1 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
Fixed Working Distance 
Rowland Circle Radius
Optical Microscope Mag 
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Optical Microscope
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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