Re: Which SEM??

On Nov 7, 11:44 pm, Gary G <see.signature@bottom> wrote:
On Fri, 7 Nov 2008 01:53:33 -0800 (PST), bireroz...@xxxxxxxxx wrote:

Thank you Gary for your message.

Yes, I have read the catalogs and compared the high end products from
FEI, Zeiss and JEOL. "High end"=Field emission gun+high pressure models
+3rd party attachments i.e. EDX, EBSD. They all have comparable
resolutions (1.0 nm v. 1.2 nm, yeah I know, 20% on paper  BUT...is it
worth it?), low kV imaging capability, attachments and detectors. They
claim they have "special" detectors, but it is the same modified
detector with comparable capacity. The only significant difference I
realized was the cold v. hot field emission gun among the competitors
and extra charge compensation, which may not be essential since
satisfactory imaging of non-conducting specimen is possible without
this option as well. After having thought the products  LOOK very
similar, I decided to ask for personal experiences hoping that someone
point out one model of one brand is actually under-performing etc. or
one particular model is really a work horse. Different models of the
same company may even perform differently.

As for the  range of specimen, it is difficult to say. There are
material scientists and engineers in the facility so, polymers, thin
films, nanoparticles, magnetic nanoparticles, metals, are all
possible. There will be visitors, who knows what they will want to
bring? One thing we know for sure that we will not allow biological
specimen into a 700,000 Euro microscope.

So as you can see, wide range of specimen and -at least on paper-
similar products was my motivation to write here to ask for personal
experience, of course keeping in mind that they would be "personal".

Ozgur

High end does tend to mean FEGSEM/FESEM.  It also strongly suggests
high vacuum.  For a huge range of specimens, this is perfectly
complementary.  For un-fixed and wet bio specimens, high resolution is
not possible in an ESEM but is possible using a Quantomix specimen
holder.  But this holder is fundamentally for BSE imaging and requires
contrast enhancement to get any useful images, IMO.

One facet of FESEMs that you are not likely to read about is the way
high resolution final lenses work.  These are magnetic immersion
lenses where an intense magnetic field surrounding the pole piece is
part of the final lens.  This field fundamentally negates the ability
to do high resolution imaging on Ferrous materials.  it also distorts
the field and requires a different operating mode for EDS.  Basically,
it turns off the field.  As a result, resolution dives.  Also, the
EDS, some SEMs have restrictive WD.  This is in conflict with the WD
range for high resolution imaging.   Ferrous material near a magnetic
immersion lens is a serious and usually fatal event.  The field can
rip the specimen from its mount and/or physically bend the column such
that it cannot be repaired.  The Zeiss Gemini column uses an
electrostatic lens and thus has no problem with Ferrous specimens.

Highest resolution imaging is at short WD and using the in-lens
detector (TLD for FEI). FESEMs come in two flavors--one is cold FE
like Hitachi while the others are all Schottky FE.  The Hitachi cold
FESEMs produce excellent image quality but suffer from lack of beam
stability.  They further require flashing about twice a day.  An
upside to cold FE is long tip life and lower tip cost.  In contrast,
thermal FE (Schottky) uses a W filament which has a very sharp tip
attached to it. Towards the end of the tip is an encirclement of ZrO2
which is the electron reservoir.  heating the filament causes
electrons to boil off.  The extractor then pulls the electrons off and
they then travel down the column.  The huge advantages of thermal FE
are high brightness (high probe current) and highly stable beam
(essential for EDS mapping and EBSD scans).  Downside is higher
maintenance contract cost for either flavor of FESEM.  When extractor
current starts to drop (almost like off a cliff), the tip needs
replacing.  It can also become erratic as a sign of imminent failure.

Thermal FESEMs use either a Denka or FEI TFE "tip."  Denka tips
typically always are easy to start and are stable.  FEI tips are
difficult to start and perhaps three out of five fail to start.  Once
started, they are very stable as well.  typical life for either of
these tips is almost two years of constant operation (gun ON).  for
these thermal FE tips, it is essential to have ultra high gun chamber
vacuum (E-10Torr) and always on.

Modern FESEMs will offer standard E-T detector plus in in-lens or
through the lens detector.  The E-T is good for a wide range of
specimens, KV, WD and tilt.  The in-lens types are not real good for
tilted imaging unless one increases probe current.  The "look and
feel" of E-T SE images versus in-lens is usually profound.  for some
cases, mixing the two produces a good compromise.  Along with this
issue is the design of the column itself.  All but Zeiss use a
traditional design which uses selectable final apertures.  The Zeiss
does not have these in their Gemini column.  Rather, they use a
multi-hole aperture in the gun chamber.  This aperture has a 30u hole
dead center with other sizes of holes surrounding the center hole.
The beam is electronically shifted to any of the aperture holes.  Once
aligned, this is a very stable system.  The SEMs with final apertures
are typically heated strips with holes of varying size.  The heat
keeps junk from building up.  However, small shifts is aperture
alignment requires frequent re-alignment (operator does this) to keep
stigmation down.  Furthermore, these columns have a beam cross over
point that diffuses the electrons and makes correction more difficult.
In contrast, the Zeiss Gemini column does not have a cross over.

Depending on the sizes of your specimens, that will dictate the size
of the SEM chamber.  My FESEM is a Zeiss Supra 55VP which is the
largest chamber for normal work below the Supra 60.  To facilitate
specimen changes, a Fjeld M-100 specimen interchange load lock is
installed and uses a small mechanical and turbo pump to evacuate the
specimen interchange chamber.  Then, one opens the gate valve between
the main chamber and the interchange chamber and inserts the specimen
onto the dovetail specimen holder wedge on the top of the stage.  some
Hitachi and JEOL units have a similar provision.  It is a very useful
feature not only for rapid specimen exchanges but for minimizing
contamination of the chamber and everything in it.

I think that the Supra has been replaced by the Ultra.  It has a
similar YAG in-lens detector.  Everything is PC controlled.  There are
many options and most are software licenses.  find out what you need
up front no matter which maker you choose.  Zeiss has been very good
and easy to use.  Remember that resolution is quoted on specific KVs
and usually do not say that it is with the in-lens detector at zero
tilt.  Max KV for Zeiss in-lens is 20KV (1nm resolution--that is real)
due to the 8KV on the column liner tube.  E-T can go up to 30KV but
resolution is not 1nm.  other makers will quote 1nm or so but mostly
at 30KV.  Zeiss will do about 1.4nm at 5KV and 1.8nm at 500V.  VP is
not worth it, IMO.

useful addition is SDD EDS (Si(Li) is history, IMO) and EBSD if you
want to look at crystaline materials.  With Zeiss, high current option
is very useful.  If you want more info about Zeiss, let me know.

Hope this helps.

Kiss French.  Drink California.

gary at gaugler dot com


Yes, thanks a million. This was exactly what I was looking for. I have
a couple questions/comments if you do not mind.

First off, why do you say VP is not worth it? Isn’t it everything the
high vacuum model does plus the ability to work on non-conducting
samples? It seemed to me the vendors are trying to sell more VP and/or
ESEMs than the high vacuum models. Or do you prefer to process those
samples before?

What do you think about the low voltage surface sensitive imaging
capabilities using beam deceleration? There are several models: FEI
Nova NanoSEM model also comes with a two-mode final lens; for
immersion and field free operation. Ziess Ultra Plus is improved
version of the Ultra, which differs from the Supra by the filtering
grid. Also, JEOL calls it “Gentle Beam Mode” and the 7600F model has
“aperture angle control lens” for real time compensation of the probe
current.

Frankly, I have been slightly leaning towards Zeiss and JEOL, not
because others are bad instruments but after phone conversions with
all the local reps, which will also provide technical support, I felt
those 2 really knew what they were talking about. I did some UHV
work, and claiming that bio-samples will not contaminate the column at
all was just not convincing for me. In this part of the world, running
a high-end instrument can be a very difficult task especially if you
think about the time you need to wait for parts and the limited number
of highly qualified technical service personnel available.

I also have plans to have UPS, pneumatic valves and possibly a
generator installed against sudden power outages, which are
unfortunately too frequent. Do you have any information as to the
damage threshold/robustness of different hot tip source models in the
case of power interruptions? I imagine if the outage happens when it
is on, it would need replacement but how does the dirt and
contamination from the vacuum system affect the cold tip? Do they tend
to partially regenerate as they get hot? How sensitive are the lens’
field for vacuum contamination? That, I imagine, would require
complete breakdown and cleaning of the column. In that case I guess
Zeiss has the advantage.

You have mentioned bumping onto the pole several times by now. Why is
it difficult to place an optical sensor to avoid this?

I do not want to test the equipment with gold nanoparticles or
anything like that. I do not want the catalog pictures. What would you
say a good (difficult) test sample would be to test the limits of the
equipment we’d like to purchase?

Thanks again for your help..

Ozgur
.

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