Electron probe microanalysis

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Electron probe microanalysis

• Low Voltage
• SEM Operation

Modified 9/23/10
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Whats the point?
Traditionally SEMs and microprobes operate at gun
voltages (E0) in the range from 15-20
kV. However, it is possible to operate at a wider
range of accelerating voltages down to around 1
kV as well as up to 30 kV. There are benefits
under certain conditions of operating at these
different (esp. lower) voltages.
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Thinking like an electron
You have seen with your Monte Carlo simulations
that for a constant material, dropping the
incident electron kV value will decrease the
scattering of the electrons in the sample. In
fact this decrease goes approximately as a 1.7
power, i.e. dropping from 15 to 1.5 kV will
reduce the electron range (scatter) by 101.7
which is a factor of 50!
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Goldstein et al 2003 Figure 5.1
As seen in the figure SE1s (and BSE1s) occur
immediately at the beam impact point. Of course,
electrons continue to scatter within the sample
and SE2s (and BSE2s) will emerge over a range
of distances from that central point. Dropping
the E0 therefore should produce better resolution
electron images.
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• There are potentially limits, however, to low
  voltage imaging
• Will the gun put out a bright enough beam at
  the particular lower voltage?
• Is the sample surface clean? Going to lower kV
  means that any junk on the surface will be
  preferentially enhanced in the image. -- On the
  other hand, if what you WANT to image is the junk
  on the surface, lower kV is definitely called
  for!
• SE detectors operate pretty well down to very
  low voltages (at or below 1 kV).
• However, many BSE detectors start to become less
  sensitive as you drop below 10 kV. However, our
  Hitachi S3400 BSE detector works very well below
  5 kV and even gives a weak image at 1 kV.

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EDS at Low Voltages
In many cases, you are not just collecting
images, but using EDS to qualitatively determine
the composition of some phase in your
sample. Operating at 15 or 20 kV gives access to
K lines of elements from B to Se, to L lines of
elements from Fe to Au or Pb, and M lines of
most of the rest of the periodic table. However,
operating at say 5 kV reduces the lines that are
available for EDS examination -- and they are all
crunched together, with potentially many
interferences and non-unique interpretations.
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Interferences at Low Voltages
As Newberry (2002) points out, EDS operation at
low kV is fraught with difficulties, as
demonstrated in his figure. If oxygen and/or
carbon are present (either intentionally or
not!), there are many important L and M lines
that are overlapped.
From Newberry 2002 Figure 6
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Additionally, as the surface layers become more
important (thats the region the electrons are
paying more attention to), then little details
like oxide skins (most metals will form some
oxide layer, even gold, according to one report
Ive seen). Therefore, a single low voltage EDS
spectrum can be a convolution of both the deeper
material composition plus the surface skin
contribution -- which makes for non-unique
solutions to the question is there trace amounts
of oxygen present in this metal?
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Why do some say use high kV for better images?
A lot of books and folks with years of experience
say that higher kV gives better images.
Goldstein 2003 Fig. 5.2
For example, Goldstein et al 2003, p. 197 go
thru an explanation why a 30 kV image of Silicon
would be sharp at 100,000 X -- for a 1 nm!
resolution beam on a 1024x1024 pixel image (1 nm
pixels) the 1 nm SE1 signal would have a high
signal/noise ratio, with the SE2 noise being
constant and therefore presumably vanishing.
Figure caption SE1 has FWHM of 2 nm and SE2 has
FWHM of 10 um. 30 keV probe, 1 nm beam, on
silicon.
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Why do some say use high kV for better images?
Nowhere in that text can I find an explanation
why this is not the case also at say 15 kV, and
why 30 is better than 15 kV. However, in the JEOL
booklet, they say theoretically the electron
probe diameter is smaller. My suggestion be
empirical try going to higher, then to lower kV,
and see what you think is better. and let me
know what you decide!
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From A Guide to Scanning Microscope Observation
by JEOL
When theoretically considering the electron
probe diameter alone, the higher the accelerating
voltage, the smaller the electron probe at
constant electron flux. However, there are some
unnegligible demerits in increasing the
accelerating voltage. They are mainly as
follows 1. Lack of detailed structures of
specimen surfaces 2. Remarkable edge effect 3.
Higher possibility of charge-up 4. Higher
possibility of specimen damage.
This suggests to me if you want to pass a test
with Au-spheres on graphite, 30 kV is better.but
for real samples, maybe lower kV will produce a
better image
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Suggested FREE references
A Guide to Scanning Microscope Observation by
JEOL and Invitation to the SEM World by
JEOL Are available on line as well as many other
worthwhile publications from JEOL www.jeolusa.com
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