2/6/97
piece of archeological glass by SEM. When examining the sample at 10, 5
and 3 kV, I saw a distorted reflection of the secondary electron detector
at the surface of the sample. The sample was attached to an aluminum stub
using double-sided adhesive tape, and was not coated. The SEM is a
Cambridge stereoscan 100. I did not observe any reflected image at higher
kV.
Any thoughts?
James Martin
Williamstown Art Conservation Center
James.S.Martin@williams.edu
You have encountered one of the neater artifacts you can accomplish
with a SEM. What happened was that while imaging at 10 keV you
established a fairly uniform charge on the surface of the glass. Then
when you dropped down in energy, the incident electrons were elastically
reflected by this uniform charge field and bounced backwards without
ever hitting the specimen. In other words, the uniform charge field
created a very nice "mirror" which reflected the electron beam towards
your secondary detector so that's what you got an image of. It is also
very typical to get a nice image of your final lens pole piece. The
field created by this type of charging tends to be hemispherical in
shape, so you get a kind of "fisheye" lens effect which at low mag
allows you to get a "panoramic" image of the inside of your specimen
chamber.
It is very easy to duplicate this phenomenon. I have found that a piece
of smooth polysterene works quite nicely (I used a divider from a
plastic parts bin). Simply charge the surface by scanning at low mag
for a while at a high voltage and fairly high spot size and then switch
to a lower keV and you will see the mirroring effect. A saphire bead
also works very well. The better the insulator, the longer the effect
lasts. You can actually get very nice images of the inards of your SEM.
Take a few pictures and see if your microscopist friends can figure out
how you did this!
This is really quite a fascinating effect which can really "blow your
mind" if you stumble across it accidentally without knowing that such a
thing is possible. This is such a neat effect that it just seems that
there SHOULD be some good use for it -- Alas -- I don't know of any,
other than to amuse yourself and your friends.
Fred Schamber
fhscham@sgi.net
with uncoated styrofoam without knowing why it worked. They first
bombarded the foam at 25 kV, then turned the acc. voltage down to
3kV or so. Here's my interpretation:
The glass, or styrofoam or whatever, charges up with electrons due to lack
of grounding. As more primary electron bombard the sample they begin to
be repelled by the like charge that has built up within the sample.
When you use low kV the primary electrons are not able to penetrate
the cloud of electrons around your sample, and are repelled by it. These
electrons begin to hit the detector (what you saw), the final
lens (what I saw with the styrofoam), or whatever else in the chamber,
eliciting secondary electrons, and forming an image. What you see
probably depends on the geometry of the chamber. We were able to look
right up the final lens and see the aperture. At higher kVs you
don't see the effect. Kinda cool, huh? Take a picture. Enjoy. Coat the
sample and remove the coating after viewing.
I thunk this up myself - does this sound right?
Aloha,
Tina
http://www.pbrc.hawaii.edu/bemf/microangela
Tina (Weatherby) Carvalho tina@pbrc.hawaii.edu
Biological Electron Microscope Facility (808) 956-6251
University of Hawaii at Manoa http://www.pbrc.hawaii.edu/bemf
poorly adfixed to the stub. I presumed that it was charging SO MUCH
that the Primary Electrons were being completely repelled from the
specimen back to the roof of the chamber. It looked great - a normal
background with a shell-shaped "mirror" showing the ceiling of the
chamber!
Geoff Avern
Microscopy Labs
Australian Museum
Sydney, Australia
geoffa@amsg.austmus.gov.au
Fred Schamber and Tina Carvalho described a little gizmo which we sell.
It's a lucite sphere mounted on carbon which will produce a reflected image.
We call it a "Lumisphere".
Best regards,
Steven E. Slap, Vice-President
Energy Beam Sciences, Inc.
Adding Brilliance To Your Vision
ebs@ebsciences.com
http://www.ebsciences.com/
useful application: if you suspect something wrong
inside the chamber, such as a loose wire, or a cold
stage tubing gone amuck, you can check it out this
way without having to open up the chamber to atmos.
That's the only use I've found; any others?
Damian Neuberger
Baxter International
neuberd@baxter.com
electrons. Although I've never observed these "reflection" images (we
religiously coat our glass samples, although I understand why one may not want
to coat an art relic), I think I have a better explanation. The surface of the
non-conducting sample is acting as part of a capacitor... the other part being
the inside of the chamber. What you are imaging is a surface charge set up by
this capacitance, and not reflected electrons. Electrons deflected completely
away from the sample are unlikely to image much of anything (try running the
EDS simultaneously with this effect...if the electrons are reflected there
should be a huge bremstraalung peak, and nothing else.
Jeff Fortner
jeff_fortner@qmgate.anl.gov
Got it in one. Somebody (SPI? EDS?) used to sell a "specimen
chamber inspection" stub that was basically half a marble that did this.
Charged some outrageous price for it.
Phil
P.S. I hope my "not the obvious" was taken as it was meant.
Philip Oshel
Station A
PO Box 5037
Champaign, IL 61825-5037
(217)244-3145 days
(217)355-3145 evenings
oshel@ux1.cso.uiuc.edu
One can also go to any well stocked hardware store and buy a nylon
lock nut, the type that has a hemispherical surface closing off one
side.
Damian Neuberger
neuberd@baxter.com
The idea of a surface image charge is interesting, but doesn't
correspond to the characteristics of the phenomenon. Let me state
several distinctive characteristics:
1. You can focus these images just like an ordinary specimen image.
2. The topography of the reflecting "mirror" specimen disappears.
3. One can image and differentiate objects which are at a uniform
ground potential.
4. One can image objects which are quite some distance away.
5. The effect requires one to first "charge up" the surface at a higher
beam voltage.
Let me illustrate by my first encounter with this effect. I was imaging
on a swatch of nylon fabric at 1 keV when a local thunderstorm knocked
the power out. When power came back on, the SEM came back online at 30
keV and after tuning up my filament settings, I attempted to return to
imaging my nylon material at 1 keV (not very bright in retrospect, but
hey, it was 3 a.m. and I wasn't too coherent). Instead of seeing the
fibrous structures I had seen earlier (at the same relatively low mag) I
was seeing unexpected unfocused contours -- some rounded, some linear --
vaguely familiar, but I couldn't put my finger on it. Fiddling with the
controls, I noticed that by focusing to longer working distances, the
shapes became sharper -- but still in no way recognizable as the
material under the beam (I had meanwhile peeked through the glass
viewport and verified that the nylon sample really WAS what was under
the beam). As the image came into sharper focus, I suddenly realized
that what I was seeing was a "fisheye" image of the entire inside of the
SEM chamber -- polepiece, detectors, stage, and other internal
mechanisms. For a moment I felt like I had entered some sort of
"Twilight Zone"!
After I figured out what was going on I became enamored with the effect
for a time and perfected my technique -- graduating to a saphire bead
with which I could make truly detailed and relatively distortion-free
images of the chamber interior. I could easily zoom in on and image
parts of the chamber which were 12-15 inches from the specimen (this was
a very large chamber).
The point is that although the nylon material had an obviously irregular
topography, it produced a quite uniform "mirroring" field. This is not
surprising, since the electric potential of an insulator charged up by
30 keV electrons will deflect a 1 keV electron at some distance from the
surface. At this distance the contributions from the various surface
charge sites integrate into a quite uniform equipotential surface which
acts as a nice smooth mirror for the incident electrons. The incident
electrons "bounce" off the equipotential surface according to the normal
laws of optical reflection and the electron trajectories behave exactly
as if one introduced a mirror in the path such that the scanning beam
now scans the objects visible in the mirror -- the interior of the
specimen chamber. It takes only a very weak field to attract the
produced secondaries to the detector so a usable image is produced
(though typically weaker, given the longer collection distances.)
If a capacitive "image charge" were responsible, one would need to have
a very regular capacitor surface to retain an intelligible image --
clearly not the case with the nylon swatch. Secondly, creation of such
an imaging charge would require that the features being imaged would
need to be producing strong variations of local field at the capacitor
surface -- not the case when I could image features of the chamber which
were all at ground potential and at considerable distance. Finally, an
image charge would not lend itself to focusing.
I can see the merits of your hypothesis, especially when the original
question involved seeing the secondary detector (which is at an elevated
potential) on a glass surface (presumably smooth). I can imagine a weak
image charge being produced on the glass via the proximity of the SED
field. But this would be evidenced by a rather subtle and smooth
modulation of the normal image contrast (remember that the SED
collection field at the specimen is necessarily weak and uniform, else
the incident beam would also be badly deflected). In the case of the
effect I have been talking about, the topography of the specimen is
REPLACED with a highly detailed reflected image -- exactly as if a
mirror were inserted above the specimen.
I don't recall ever attempting to look at the x-ray spectrum produced.
I wouldn't expect to see anything much since the electrons are striking
objects which are out of the line of sight of the EDS detector. A pure
brehmstrahlung spectrum should be produced, as you suggest, and this is
an interesting idea.
A final note -- in my earlier posting I commented that I knew of no good
use for this effect. In fact, I did once use it to locate a breakdown
across an interior insulator. It is also a good way of noting which
interior features of the chamber are most strongly producing secondary
electron "background" as would normally occur from electron
backscattering onto the chamber walls. But its best use IMHO is still
its considerable potential for amusement!
Fred Schamber
RJ Lee Instruments Ltd.
fhscham@sgi.net
detected and they are not all bremsstrahlung:
Unless fast electrons reach the atoms in the sample they cannot generate
bremsstrahlung in the sample. The electrons reflected by a strong enough
electrostatic field produced by an electrically isolated, electrically
charged specimen can generate characteristic x-rays as well as
bremsstrahlung from all specimen chamber materials which are visible in the
mirror image of the chamber.
If an abnormal background shape is observed in x-ray spectra recorded in the
"mirror" condition, a significant source is the reflected electrons
themselves -- either entering the x-ray collimator and generating
detectable x-rays there or even penetrating the detector window and
reaching the detector crystal itself (especially if there is a thin window
or if there is no magnetic "electron trap" in the collimator). BTW, this
same effect can be observed clearly in TEM or STEM if the collimator's
internal geometry is bad and the EM is operated with a very low or zero
magnetic lens field at the specimen, as is commonly found in low mag mode.
Best wishes,
Brian
Brian W Robertson Office 402 472 8308
Associate Professor Lab 402 472 8762
Department of Mechanical Engineering and FAX 402 472 1465
Center for Materials Research and Analysis,
University of Nebraska-Lincoln
255 Walter Scott Engineering Center
Lincoln NE 68588-0656 USA
bwr@unlinfo.unl.edu
To image the inside of my SEM, I first stick a piece of PTFE or a round glass
coverslip onto a stub and then glue a small ball bearing (3mm) onto the centre.
This gives a much better view of the chamber. You can try using larger ball
bearings, also to give a weird effect try sticking two small ball bearings together.
Also try switching on your back scatter detector once you have 'charged' up the
stub, you can visualise the sectors - (if + the sector is white and if - the sector is
black)
Kevin Mackenzie
Tillydrone E.M. Unit
University of Aberdeen
Tillydrone Avenue
Aberdeen
AB9 2NT
SCOTLAND
Tel 01224-272847
Fax 01224-272396
web site: http://www.abdn.ac.uk/~nhi691/
nhi691@abdn.ac.uk