Transcript Variable Pressure ("Environmental") SEM work

Electron probe microanalysis
Variable
Pressure /
Environmental
SEM Operation
Revised 2-21-12
What’s the point?
Traditional SEMs and microprobes operate at high vacuum.
Today, however many/most SEMs being sold are versatile in
being able to operate both at high vacuum, and a lower
vacuum, where any gas including water vapor may be present.
This presents new opportunities for examining specimens that
otherwise would be difficult to examine.
HOWEVER, there are serious difficulties when attempting to
use EDS in the variable pressure mode.
Not to belabor “the point”
But …
Compared to electron probes, SEMs are very easy to use
…. Which means that people will stick anything and
everything into them!
…. And people with little or no training will push buttons
to generate data (=EDS) which they have no business
doing!
… And possibly generating data that is not correct.
Vacuum
Traditional SEMs and electron probes have operated with
vacuums in the range of 10-3 Pa (10-5 Torr).
This was for several reasons:
• protecting the electron gun (subject to oxidation and ruin)
• keeping the beam of electrons tight to reach good spatial
resolution (the gas molecules would interact and cause
scattering--both elastic and inelastic)
Why use a poorer vacuum?
Sometimes you want to look at things that are naturally
moist or wet -- biological specimens or clays -- and under
the normal high vacuum SEM they become dried out and
lose important features you want to view and document.
There has been an experimental undercurrent from 1960,
attempting to image wet material, by modifying in an SEM,
and
• sandwiching the liquid between 2 thin carbon film, or
• using a special substage, a closed box enclosing the
sample and BSE detector, with a tiny aperture for the
electron beam to enter, and a valve where water could be
leaked in
Another benefit of poorer vacuum
One critical feature of high vacuum SEM and EPMA was
that samples either had to be conductive or conductively
coated.
However, if there is a poorer vacuum, there are enough gas
molecules in the chamber that interact with both incident
electrons and (mainly) emerging backscattered and
secondary electrons, that become ionized, and produce a
cloud of cations that neutralize any charging (bleed the
electrons to the ground).
Thus, having a poorer vacuum can become a tool to
examine insulating specimens…though there are some
serious drawbacks regarding EDS.
VPSEM and ESEM
Today there are two distinct varieties of SEM operating in
this manner:
• the “Environmental SEM” (trademark of Philips/FEI/
Electroscan), which can operate up to 2700 Pa in chamber
and has a propriety secondary electron detector. For many
years it was the only one that could operate with liquid water
stable.
• the variable pressure SEM sold by the other SEM
companies, which are limited to a maximum pressure of ~266
Pa and may not have a special secondary electron detector
(instead using BSE detector).
But BOTH have same general features for the most part.
The “skirt” and its importance
Newbury, 2002
In a traditional SEM, 100% of the incident electrons land at a
very tiny spot. But with VPSEM, they scatter as they come
down and create a wide circular region called the “skirt”
The “skirt” and its importance
Newbury, 2002
• For imaging, the skirt acts as a source of noise and degrades
slightly the image -- not a big problem
• However, for X-ray microanalysis (EDS), there are
significant problems because spurious x-rays can easily be
generated up to tens to hundreds of microns away, depending
on the gas pressure -- a big problem -- as discussed by Dale
Newbury in his 2002 article
The “skirt”
This image shows
graphically the “skirt” in
the form of luminescent
gas inside a VPSEM. The
upper image is cross
sectional view, whereas
the bottom is looking
normal to the beam. Note
the small very bright
center spot, a ~75 um
bright diameter halo, and
a ~250 um dimmer halo.
Goldstein et al, 2003
The “skirt”
Analogy (bottom image):
You are on a foggy road
and looking at the beam
of an oncoming car
headlight, diffused/
scattered thru the fog.
Now consider yourself
the specimen and the
light coming at you is the
package of electrons
coming down the column
at you…
Goldstein et al, 2003
The “skirt”
The skirt is extremely important to understand; its radius rs
can be estimated from a simple analytical model (Danilatos,
1988):
364Z
p
3/2
rs =
´
´ GPL
E
T
where GPL is the gas path length (=working distance) in
meters; p is pressure in Pa; T is in degrees K, Z is atomic
number of gas, and E is E0 in volts.
rs is in meters…
The “skirt”
Fig 5.20. Plot of beam broadening
(skirt radius) as function of gas
species and pressure. The gas path
length is 5 mm and beam energy is
20 keV
Fig 5.21. Plot of beam broadening
(skirt radius) as function of gas
path length (GPL) and beam energy
in helium.
Goldstein et al (2003)
EDS errors due to the skirt-1
Consider a standard block with Fe metal in a brass holder and a piece of
Zr nearby (above left image). Under normal high vacuum, if the 20 kV
electron beam is focused on the Fe about 50 microns in from the edge,
the EDS pattern is as expected (above right), showing essentially only Fe
Ka-Kb and Fe La
Goldstein et al 2003, Fig 5.22
EDS errors due to the skirt-2
Now bleed in 20 Pa of air (above left) and even though we haven’t
moved the beam, we are now seeing Cu and Zn from the brass holder in
the EDS spectrum.
Increase gas (air) pressure to 50-110 Pa and we now see also the Zr La
from the Zr metal 500 microns away!
Goldstein et al 2003, Fig 5.22
Detectors for VPSEM-ESEM
Having gas present inside the vacuum chamber presents a
major problem for the traditional E-T (Everhart-Thornley) SE
detector. That detector has a high (e.g. +10 kV) bias on it, and
would immediately arc over, blowing out a fuse (if you are
lucky).
So a VPSEM-ESEM can instead use its BSE detector without
that problem -- but the image is somewhat washed out. OK
but not great, as the fine detail of the surface is muted.
Detectors for VPSEM-ESEM
Goldstein et al, 2003
A better detector is above, where the collisions of the emitted
secondary (and backscattered) electrons with the gas molecules is
used advantageously, producing an avalanche effect (=gas
amplification) with electrons being attracted to a + bias electrode.
This yields a secondary electron image. The detector is called a
GSED or ESED.
ESED Detector for Hitachi VPSEM-1
BSE
In 2007, we upgraded our SEM
to a 2nd generation Hitachi
Environment Secondary
Electron Detector (ESED) and
have different/improved images
acquired in VPSEM mode.
ESED
BSE: little/no shadowing, can lose topo
ESED: directional (shadowing), topo enhanced
ESED Detector for Hitachi VPSEM-2
To improve the image, you can modify your conditions:
• Lower voltage (dropped from 15 to 5 keV)
• Drop the stage (=increase the working distance) from 10
mm to 25 mm, to give a greater depth of field
• Increase the gas pressure from 25 to 40 Pa
Wet Samples
• In normal high vac SEM,
water present at low
pressure and room T (*) is
strongly out of equilibrium
and will evaporate (and be
pumped out) quickly.
• However, for a “true”
ESEM, you can set the
pressure to 2600 Pa (@)
and stabilize liquid water
in your sample, so there is
no evaporation and change
in the materials properties
or features you are
imaging.
@
*
• However, a VPSEM (like our Hitachi)
cannot reach 2600 Pa, so the only
alternative would be to install a cold stage
that would keep liquid water meta- stably,
slowing the evaporation (<-------)
Particles!
While we’re discussing Hitachi SEM operation, it is
important to point out a major difference with the
electron probe
• In the e- probe we examine flat polished samples
because they must be, so there is constant path length for
the absorption correction.
• But the SEM is so flexible you can put anything in it,
e.g. lots of particles of different sizes and shapes
• Q: What would be the effect on the path length for a
jagged homogenous particle?
• A: You would get different compositions depending
upon where you put the beam. Light elements would be
particularly absorbed and incorrect.
Low Voltage
Where imaging is concerned, sometimes it pays to
experiment with E0… WHY?
The issue is spatial resolution. Recall that the electrons
scatter, and with higher E0, they have more energy and will
scatter further from the “landing point”.
It is thus a balancing act between having a detector that is
sensitive to low energy electrons (BSE detector, right?) and
enough E0 to give a strong enough signal to give a “good
enough” image
(Along similar lines, lowering the beam current also helps to
give less beam electron scattering…but the signal will be
weaker.)
But lowering E0 of course creates complications with EDS…
VP SEM Applications in Weeks Hall
• Imaging forams, some which may have carbon isotope
measurements (Clay Kelly group)
• Imaging Zebra mussels prior to carbon isotope analysis
(Dana Geary group)
• Evaluating synthetic nanoparticles (size mainly) (Nita
Sahai group)
• locating K-feldspars in a population of grains on
carbon tape, for Ar-Ar dating (Brad Singer group)
• EBSD (need there to be no coating on surface for
optimal signals) (Laurel Goodwin and Basil Tikoff
groups)
VP SEM Applications in Weeks Hall
• Rapid identification of minerals in uncoated mounts
(everyone!)
• Imaging slightly wet samples: fault gauge from underwater
zone, from California (Laurel Goodwin group)
• Imaging sheared sediment samples” (Harold Tobin group)
• Imaging very wet samples: microbial cultures (Eric Roden
group) -- would need a cold stage to do this
Operational Issues
• Use the least amount of air you can get away with; start with
~20-25 Pa but increase if still have charging; if using ESED
detector, experiment with higher pressures
• Have exposed the least amount of non-conductive surface,
especially glass; surround area of interest with conductive tape
if at all possible. Give the electrons an easy path!
• Be very wary of EDS data acquired on particles, and
particularly in VP mode! Only consider such data to be very
qualitative. If you wish to be more quantitative, you MUST
acquire standards of known composition similar to the
unknowns and spend time studying them FIRST.
Thanks to Ken Severin, UAF probe & SEM labs