Acquiring Images in the SEM

1
Electron probe microanalysisEPMA

• Acquiring Images in the SEM

Modified 9/18/09
2
Whats 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.

3
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)

4
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.

Lyman et al, 1990, Lehigh Lab Workbook, Fig
2.2, p. 11
5
SEM Resolution
The above 2 scans show the technique (though
apparently the proofreader didnt 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
6
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.
7
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
8
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
9
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
10
Secondary electron images
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)
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.
11
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 (SE1s). As the electron scatters
away from the impact spot, if it stays near the
surface, a second generation (SE2s) 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
12
SE1 and SE2and 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 SE3s. 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
13
BSE images

• There are several different types of detectors
  used to acquire BSE images
• 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.
• Robinson detector a modern version of the E-T
  for BSE at TV rates. Must be inserted and
  retracted.
• Solid state detector which is most commonly
  used today on electron microprobes and many SEMs.
  Permanently mounted below polepiece.

BSE imaging the signal comes from the top .1 um
surface). Above, 5 phases stand out in a volcanic
ash fragment
14
BSE images
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.
BSE imaging the signal comes from the top .1 um
surface solid-state detector is sensitive to
light (and red LEDs).
Goldstein et al, 1992, Fig 4.24, p. 184
15
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 Mn-Ni phases.
Crystal lattice shown above, with 2 beam-crystal
orientations (a) non-channeling, 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.
16
EBSD
Electron backscatter diffraction is a relatively
new and specialized application whereby a
specimen (single crystal or more commonly a
polished section) is tilted acutely (70) in an
SEM with a special detector (camera). The
electron beam interacts with the crystal lattice
and the lattice planes will diffract the beam,
with the backscattered electrons striking the
detector, yielding sets of intersecting lines,
which then can be indexed and crystallographic
data deduced.
Also referred to as Kikuchi pat-terns. There is
a similar phenom-enon, of internally generated
x-rays diffracting on the internal structure
Kossel X-ray diffraction.
(Left) EBSD pattern from marcasite (FeS2)
crystal. (Right) Diagram showing formation of
cone of diffracted electrons formed from a
divergent point source within a specimen.
Dingley and Baba-Kishi, 1990, Electron
backscatter diffraction in the scanning electron
microscope, Microscopy and Analysis, May.
17
Forescattered Image
EBSD cameras may have a pair of Si-chip BSE
detectors up high, and a pair down low. These
lower ones detect high energy electrons that are
scattered in a forward direction, with a
component of diffraction. They thus give
crystallographic orientation information to the
image. Here is an image (courtesy Jason Huberty)
of a banded iron formation rock from Australia.
The normal BSE image would only show the thin
brighter folded bands of magnetite, with the
uniformly dark grey quartz. However in the above
image, you see each discrete quartz crystal with
the grey level indicating a particular
orientation. And a careful look at the magnetite
show both separate MT domains, plus small lines
of pits down the center of each row, whose
meaning is being investigated.
18
BSE and SE Detectors on our SX51
Annular BSE detectors
Anti-contamination air jet
Plates for voltage for SE detector
View from inside, looking up obliquely (image
taken by handheld digital camera)
19
Hitachi SEM Detectors
Annular BSE detector
EDS detector
ESED detector
E-T SE detector
IR Chamberscope
View from inside, looking up obliquely (image
taken by handheld digital camera)
20
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 lt 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 (gt100) 63x
  scans (each scan 1.9 mm max width).

From research of Cory Clechenko and John Valley.
21
X-ray maps . And the Clock
3 X-ray maps combined each element set to a
color, and then all merged together in Photoshop.
The maps took 8 hours to collect.
Reed, 1996, Fig 6.1, p. 102

• X-ray maps can provide useful information as well
  as attractive eye candy. However, due to the
  low count rate of detected X-rays, dwell times
  generally need to be hundreds of milli-seconds. A
  512x512 X-ray map at 100 msecs takes 8 hours to
  acquire. Large area maps that combine beam and
  stage movement require additional overhead
  (1-10) for stage activity. The recent
  improvements to our EDS system give us more
  leeway, as the larger solid angle of EDS and
  improved digital processing throughput lets us
  use 1-10 msec dwell times, as well as allowing
  low mag images (no need to worry about Rowland
  circle defocusing).

22
Image Acquisition

• Consider
• What Image depth? 8 bit (SE,BSE,CL) or 16 bit
  (X-ray) -- which translates into 256 vs 65536
  intensity (gray) levels
• Image size
• mm in x and y (rectangular vs square depends on
  machine/software)
• pixels in x and y
• Image resolution-- is it sufficient for the
  need? mm/pixel total pixels final printed
  size gt will determine whether or not it is
  pixelated
• Image size total kb or mb. Storage can become
  an issue when you have lots of large (gt 2 mb)
  images, but with todays storage options, this
  is less difficult, as students can afford 250 mb
  Zip disks.
• Time for acquisition SE,BSE,CL is rapid X-rays
  require much longer time
• EDS spectra sometimes a picture of two
  contrasting spectra is useful.
• Adjust conditions (brightness, contrast) for
  optimal image quality BEFORE you acquire. Be sure
  not to oversaturate the brightest phases.
• Record conditions (keV, nA, dwell time, mag) in
  your lab notebook (scale bar may NOT be
  necessarily saved on image)

23
Image Storage

• Use clear, descriptive names for your images
• Pick a format for your needs for plain old
  images, nothing fancy planned JPEG is OK (image
  size is small)
• If you know you will be doing some image
  processing, particularly quantitative, then TIFF
  is preferable (though images can be very large)
• Transfer your images to your own computer in a
  timely manner (they will remain on probe lab
  computer for a limited period (1-3 years?)
• Always fiddle with copies, not originals

24
Some Image Formats

• JPEG Name refers to a compression method that
  is Lossy there is some loss of exact pixel
  values square subregions are processed with
  cosine transform operation compression of 101
  to 1001 is possible (Joint Photography Experts
  Group)
• TIFF Lossless compression (image does not
  degrade with repeated opening/closing). Photoshop
  gives option of LZW compression, best not used.
  (Tagged Information File Format)
• Photoshop (psd) layered image must flatten if
  to be used elsewhere.
• Adobe Acrobat (pdf) non-Lossy compression

Graphic Converter (Mac) is a Swiss Army tool
program that can open about any format you can
think of, and save to anything else.
(Share/cheapware)
Lossy compression throws away some data to better
compress the image size different schemes focus
on different features, i.e. JPEG is based on fact
that human eye is more sensitive to changes in
brightness than in color, and more sensitive to
gradations of color than to rapid variations
within that gradation. JPEG keeps most brightness
info and drops some color info.