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Electron probe microanalysis
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Acquiring Images in the SEM
Modified 9/18/09
Modified 2/17/2014
What’s the point?
“A picture is worth a thousand words”.
The more we know about how images are
acquired, the better we can present
research results graphically.
High Vacuum: Need Conductivity!
Above left: uncoated. A charge builds up causes
oversaturation (white) and horizontal streaking from
beam
Above right: what same area would appear with
conductive coating
Right: High vacuum carbon coater (“evaporator” not
sputterer)
SEM Resolution
Tradition: insert an object
with a sharp edge that
produces high contrast
relative to the background,
crank up the mag and
measure the distance (red)
where the SE (secondary
electron) signal changes from
10 - 90% maximum contrast
difference. (or 14-86%)
Lyman et al, 1990, “Lehigh Lab Workbook”, Fig 2.2, p. 11
SEM Resolution
The above 2 scans show the technique (though apparently
the proofreader didn’t catch the inverted signal on the left
image). The only way to increase resolution is to turn
DOWN the beam current, as the plot on the right shows
dramatically -- the current is a few tens of picoamps, not
nanoamps.
Lyman et al, 1990, “Lehigh Lab Workbook”, Figs A2.3, A2.4, p. 191
SEM Resolution: K409
We found a “failed” NIST-standard that makes a great
resolution standard for FE-microprobes. Peter Sobol wrote a
program that automatically calculates the beam resolution.
SEM Resolution
However, the approach
apparently used today
(e.g. our Hitachi field
service engineer) is to
take his “test sample”
(gold sputtered on
graphite substrate) and
with optimized
contrast,
find the narrowest spacing between 2 gold blobs and
define that as the “resolution”.
Depth of Field
A strength of the SEM is the enhanced depth of field compared
to optical microscopy as shown above for the radiolarian
Trochodiscus longispinus. Optical image has only a few micron
depth of field (=plane in focus), whereas SEM images can be
made to be in focus for hundreds of microns (e.g. increasing
working distance)
Goldstein et al 2003 Fig 1.3
Stigmatism
Imperfect magnetic lenses (metal
machining, electrical windings,
dirty apertures) can cause the beam
to be not exactly round, but
“astigmatic”. This can be
corrected using a “stigmator”, a set
of 8 electromagnetic coils (bottom
image).
Top left: original “poor” image
Top right: underfocus with stig
Bottom left: overfocus with stig
Bottom right: image corrected for
astigmatism. Marker = 200 nm
Goldstein et al 2003 Fig 2.24
Edge Effects
Edges of objects can appear to be brighter in SE and BSE
images, because electrons can be emitted not only from the
top but the side, artifically making that part of the image
brighter. This can lead to some incorrect conclusions for
BSE images.
Reed 2005 Fig 4.3
Secondary electron images
SE imaging: the signal is
from the top 5 nm in metals,
and the top 50 nm in
insulators. Thus, fine scale
surface features are imaged.
The detector is located to
one side, so there is a
shadow effect – one side is
brighter than the opposite.
Everhart-Thornley detector: low-energy
secondary electrons are attracted by +200 V
on grid and accelerated onto scintillator by
+10 kV bias; light produced by scintillator
(phosphor surface) passes along light pipe to
external photomultiplier (PM) which converts
light to electric signal. Back scattered
electrons also detected but less efficiently
because they have higher energy and are not
significantly deflected by grid potential.
(image and text from Reed, 1996, p. 37)
SE1 and SE2
The picture gets a little more complicated … secondary electrons in fact
can be generated and detected from more than just the “landing point”
of the E0 beam (SE1’s). As the electron scatters away from the impact
spot, if it stays near the surface, a second generation (SE2’s) can be
generated and detected -- these will cause the SE image to be less sharp.
Solution: go to lower E0 (5 kV or less is mentioned by Goldstein)
Goldstein et al 2003 Fig 3.20
SE1 and SE2…and SE3!
And in the real world it may be even more complicated … backscattered
electrons may bounce off the chamber walls, or the bottom of the
column, generating SE3’s. Because the E-T detector has a positive bias
to attract the low energy SEs, these extraneous signals can add more
noise to the image. This is especially problematic at high E0s and is a
reason not to expect high resolution SE images at high kV values.
Goldstein et al 2003 Fig 4.20
BSE images
There are several different types of detectors
used to acquire BSE images:
(1) Everhart-Thornley detector can have a -50
ev bias put on the grid to reject secondary
electrons, so only BSEs get thru -- however
this is not useful at fast, TV scanning rates,
i.e. moving the stage.
BSE imaging: the signal
comes from the top ~.1 um
surface). Above, 5 phases
stand out in a volcanic ash
fragment
(2) Robinson detector — a modern version of
the E-T for BSE at TV rates. Must be
inserted and retracted.
(3) Solid state detector — which is most
commonly used today on electron
microprobes and many SEMs. Permanently
mounted below polepiece.
BSE images
BSE imaging: the signal
comes from the top ~.1 um
surface; solid-state detector is
sensitive to light (and red
LEDs).
A solid-state (semi-conductor) backscattered
electron detector (a) is energized by incident high
energy electrons (~90% E0), wherein electron-hole
pairs are generated and swept to opposite poles by
an applied bias voltage. This charge is collected
and input into an amplifier (gain of ~1000). (b) It
is positioned directly above the specimen,
surrounding the opening through the polepiece. In
our SX51 BSE detector, we can modify the
amplifier gain: BSE GMIN or BSE GMAX.
Goldstein et al, 1992, Fig 4.24, p. 184
Variations on a theme
There are several alternative type SEM images sometimes found in BSE or SE
imaging: (left) channeling (BSE) and (right) magnetic contrast (SE). I have found
BSE images of single phase metals with crystalline structure shown by the first
effect, and suspect the second effect may be the cause of problems with some MnNi phases.
Crystal lattice shown above, with 2
beam-crystal orientations: (a) nonchanneling, and (b) channelling. Less
BS electrons get out in B, so darker.
From Newbury et al, 1986, Advanced Scanning Electron
Microscopy and X-ray Microanalysis, Plenum, p. 88 and 159.
BSE and SE Detectors
on our SX51
Anticontamination
air jet
Annular
BSE
detectors
Plates for
+voltage
for SE
detector
View from inside, looking up obliquely
(image taken by handheld digital camera)
Hitachi SEM Detectors
Annular
BSE
detector
EDS
detector
ESED
detector
E-T SE
detector
View from inside, looking up obliquely
(image taken by handheld digital camera)
IR
Chamber
scope
Mosaic Images
There are occasions where the feature you
wish to image is larger than the field of view
acquirable by the rastered beam. A complete
thin section (24x48 mm) can have a mosaic
BSE image acquired in < 1 hour (though an
X-ray map could take a week, so only smaller
areas are typically X-ray mapped.) This is
achieved by tiling or mosaicing smaller
images together. The software calculates how
many smaller images are needed based upon
the field of view at the magnification used,
drives to the center of each rectangle, and then
seemlessly stitches the images into one whole.
The false colored BSE image of a cm-sized
zoned garnet to the right was made by many
(>100) 63x scans (each scan 1.9 mm max
width).
From research of Cory Clechenko and John Valley.