Electron probe microanalysis
Low Voltage
SEM Operation
Modified 9/23/10
What’s 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.
“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!
Goldstein et al 2003
Figure 5.1
As seen in the figure SE1’s (and BSE1’s) occur immediately at
the beam impact point. Of course, electrons continue to scatter
within the sample and SE2’s (and BSE2’s) will emerge over a
range of distances from that central point.
Dropping the E0 therefore should produce better resolution
electron images.
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.
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.
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
Additionally, as the surface layers become more important
(that’s 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
I’ve 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 nonunique solutions to the question: is there trace amounts of
oxygen present in this metal?
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
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.
2003 Fig. 5.2
Figure caption: SE1 has FWHM
of 2 nm and SE2 has FWHM of
~10 um. 30 keV probe, 1 nm
beam, on silicon.
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!
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 Auspheres on graphite, 30 kV is better….but for “real” samples,
maybe lower kV will produce a better image…
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/RESOURCES/ElectronOptics/DocumentsDownloads/tabid/320/Default.aspx