Transcript 30_Electron-optic_Column_old

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Electron Probe Microanalysis
EPMA
Electron
Optical
Column
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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; 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
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
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 workfunction 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 filament:biased Wehnelt Cap
First
electron
cross-over
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.
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 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
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
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
Similar to light optics
(though inverted: reducing
image size):
Light Optics
Electron Optics
1/f = 1/p + 1/q
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
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 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)
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
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
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).
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
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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