SEM and EPMA

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SEM and EPMA

• Some Advanced Topics

Revised 2/17/2013
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Beyond the Basics

• Stereo SEM
• EDS of particles beware!
• TTL Detectors
• X-ray mapping
• Feature Sizing, Chemical Typing
• EBSD
• FIB
• and on and on

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Stereo SEM

• Why? Graphical demonstration of 3D shapes of
  rough or complex objects
• How? Very generally ..
• Acquire two SEM images of the same object at the
  same magnification but by tilting the stage
  slightly.
• Color each image red or green in Photoshop.
• Then in Photoshop overlay the two images, while
  wearing red-green stereo glasses, shift them
  slightly apart until you have the correct 3D
  effect.
• Show downstairs in 2nd floor visualization lab
  for maximum effect

Exact detailed instructions on wall in room 308
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Example porous sandstone
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Stereo SEM Details
For rough objects, use 3-7 tilt For smooth
objects, use more, 7-15 Use less tilt if higher
magnification If you need to refocus, do NOT
change objective lense setting, rather move
sample stage Z up or down. Cited reference in
literature Heuser, 1989, Protocol for 3-D
visualization of molecules on mica via the
quick-freeze, deep-etch technique, Journal of
Electron Microscopy Technique, vol 13, p. 244-263.
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EDS of particles
is easy. Maybe too easy? --gt And easy to make
mistakes!
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Particles - 1

• Mass effect/error electrons escape from sides
  of small particles if E0 is large enough, so
  quantitative analysis will be in error because
  different elements (with different binding
  energies) will be affected differently

Goldstein et al, 1992, p. 479, 481
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Particles - 2

• Absorption effect of non-flat upper surface
  different path length from normal flat geometry
  and the normal way we do quantitative analysis
  is to use FLAT polished standards for calibration
  -- so we could have too much x-ray intensity
  for particles

Goldstein et al, 1992, p. 479, 481
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Particles - 3

• Variable effect of geometry of trajectory between
  beam impact area on non-uniform surface and the
  location of the detector -- so we get different
  results from the same material, depending upon
  where we place the electron beam.

Goldstein et al, 1992, p. 479, 481
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Results from 2006 777 student project on EDS of
rough samples with VP-SEM
The first column shows the actual chemical
composition, followed by the average composition
of 20 points on different grains, followed by the
variation (standard deviation) of those
individual measurements
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EDS X-ray Mapping-1
BSE imaging provides rapid distinction of
some/many/most chemically distinct phases in a
multiphase sample. However, in many cases,
having explicit 2D chemical information is value.
This is where x-ray mapping comes into
play. X-ray mapping has changed tremendously
since it was first introduced in 1956 as a
combination of WDS (crystal diffraction
spectrometry) and SEM in those days it was
called dot mapping as literally dots would be
painted onto the CRT. To capture it, a photograph
would have to be snapped of the screen. It
generally was pretty grainy. And only 1 or 2 or 3
elements could be acquired simultaneously
(depending on how many spectrometers were on the
electron probe).
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EDS X-ray Mapping-2
EDS was developed in the late 1960s and soon
began to be used for X-ray mapping. For the next
30 years or so, users would select regions of
interest (ROIs), essentially the peak areas of
particular elements -- sometimes limited to a
finite value like 8 or 12 -- which would then be
used to color in a 2D area over which the beam
would scan, either one very long and slow scan,
or many faster scans that would be averaged.
Note To this point, all the above maps included
both the characteristic X-ray being chosen AND
the background/ continuum under the
characteristic peak.
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EDS X-ray Mapping-3

• Things have changed in the 40 years since EDS
  came on the scene
• Digital pulse processing have taken over from the
  older analog processing, making shorter time
  constants and thus higher count rates possible
  (e.g. 30,000 cps vs 3,000 cps before) which
  create many more opportunities -- larger areas,
  shorter times.
• And then the development of SDD (Si-Drift
  Detectors) have boosted the count rates to the
  hundreds of thousands of cps.

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X-ray Mapping and the clock
Reed, 1996, Fig 6.1, p. 102

• Due to the low count rate of detected X-rays,
  dwell times in the past generally needed to be
  hundreds of milli-seconds. A 512x512 X-ray map at
  100 msecs took 8 hours to acquire. The
  improvements to EDS systems with improved digital
  processing throughput allows 1-10 msec dwell
  times, dropping the x-ray mapping times down to
  the hour or less range, in many cases

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Continuum Artifact
Here is an example of false compositional
contrast, an artifact of the background being a
function of Z (MAN). Specimen is Al-Cu eutectic
X-ray maps are (a) Al, (b) Cu, (c) Sc. The
contrast in (c) suggests Sc is present in the
Cu-rich phase. However, there is no Sc, only the
background in the Cu-rich phase is elevated
relative to the background in the Al-rich phase.
Thus one needs to be aware whether the background
is or is not subtracted from X-ray maps, esp.
when looking at minor elements where the
continuum is a major component. Many published
maps do not state if bkg-subtracted, so assume
they arent.
Goldstein et al, 1992, Fig 10.6, p. 535
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However

• X-ray mapping of predefined elements is now
  pretty much a thing of the past (or of EDS
  systems purchased before 2000 or so).

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Data Cube

• With the large increases in computing power,
  memory, and storage, it is now possible to
  acquire a complete spectrum at each pixel --
  within the time frame of the acquisition
  time.Therefore it is possible to
• subtract the background
• find elements that were not known to be present
  beforehand

Kotula et al, 2003, p. 2
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Spectral (hyperspectral) Imaging
Here any and all x-rays detected are mapped to
each pixel over which the beam is scanning. This
is both very powerful (see elements not known to
be present before starting), but also loses
something relative to other slower
old-fashioned maps--lower counts in peak
channels.
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One of the issues with SEMs
Notice the smearing of the spectal image for
LONG maps, the SEM is not meant as a high
stability platform (images take seconds to
acquire). Some software (cost extra ) use a
reference point for each frame and if there is
drift, correct for it.
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Multivariate statistical analysis
Paul Kotula et al demonstrated the usefulness of
applying principal component analysis to
spectra images. They developed a procedure for
converting from abstract principal components to
physically meaningful pure components.
Kotula et al, 2003, p. 3
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Multivariate statistical analysis
Here is an example of the MSA to a braze between
copper and alumina, where 7 distinct chemical
components were detected with the automated
spectral image analysis described by Kotula and
coworkers.
Kotula et al, 2003, p. 6
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Needle in a haystack
Because the entire spectrum is acquired at each
spectrum, the software can sum up the entire
extracted spectrum for the whole image. If
there is just one pixel with a rare element
(and enough passes have been acquired to get
enough counts) it is possible to locate the
needle in the haystack using these spectrum
images.
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X-ray Mapping
This is an old-fashioned map using older EDS
software WDS channels with the SX51 electron
probe, slowly moving across the sample, then
applying colors to elements. This instrument is
built to be more stable
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.
than the SEM so there is no smearing as in the
previous image.
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Feature Sizing, Chemical Typing

• This is a valuable feature of the SEM for
  locating and identifying a particular mineral in
  a mixed population (e.g. K-rich minerals mixed
  with K-poor minerals.
• Acquire BSE image with good contrast so the
  K-rich mineral is distinct
• Set a desired brightness level as the test for
  the mineral in question (if brighter than, then
  acquire short EDS spectrum of center of grain)
• Set elements to be evaluated set short (2
  second) EDS acquisition time
• Set the stage coordinates of the corners of the
  area Run the program
• Return the next day and look at the list of
  grains it acquired EDS spectra order from high
  to low K then drive to each grain to verify it
  is the grain you want.

The next 3 slides go through the process
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1. Set the BSE intensity and then set the range
to select one phase
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2. Select the elements needed to find the phase
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3. After SEM done, check the grains the software
suggests are the ones you want
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What will they think of next?
The veritable scanning electron microscope keeps
providing more and more opportunities for
improvement and innovation
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TTLThrough-the-lens Detectors
Zeiss (Leo)s Gemini field emission SEM has a
neat detector TTL. The objective lens projects a
magnetic field downward which traps SE1,2 and
draws them up thru the lens where they are
detected by a scintillator-light
guide-photomultiplier. This enhances SE images,
in eliminating both SE3s and most BSEs. Sharper
images
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Energy filtering of SEs
At the August 2010 Microscopy and Microanalysis
meeting in Portland OR, I found one talk to be
very interesting Someone working with FEI, and
operating a low E0s, has found a way to energy
filter the very low energy secondary electrons,
producing a technique to see through the common
surface contamination on materials. The slightly
higher energy SEs (say 10 eV) result from the
impact of the E0 electrons with the surface
contamination layer, whereas those SEs coming
from the material under the contamination have
lower energy (say 2 eV).
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Electron Backscatter Diffraction
Over the past decade, EBSD has rapidly become a
desirable attachment to the SEM. The SEM permits
easy imaging of micron-sized domains, and the
EBSD detector permits grabbing -- and analyzing
-- the diffraction patterns. Thus in addition to
images, EDS spectra, now there is
crystallographic and orientation information
available. We will spend the last lecture of
this class going into detail on EBSD.
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Focused Ion Beam Instruments

• The past decade has seen the rapid acceptance of
  SEMs built to hold FIB sources. A gas injection
  source of liquid gallium is aimed precisely at a
  region imaged by the SEM, and can precisely
  sputter away sample material
• to expose material below the surface that was
  otherwise unaccessible for SEM imaging or EDS
  characterization, or
• to dissect the sample into very small
  (sub-nanometer) slices for TEM study

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Focused Ion Beam Instruments
From JAMES D. SCHIFFBAUER and SHUHAI XIAO, 2009
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FIB-EBSD
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FIB-EBSD
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Personal SEM
From ASPEX literature

• ASPEX has developed this small portable SEM for
  use in the field mainly for quality
  control/maintanence work
• runs on 110 or 220 volts
• puts out 2-20 kV
• BSE images and EDS
• portable, easily fits in the back of a van or SUV