Re: Which SEM??

On Fri, 14 Nov 2008 02:06:11 -0800 (PST), birerozgur@xxxxxxxxx wrote:



Thanks for the tips Gary,

I am a bit confused about the resolution at VP mode. Zeiss' spec for
Supra 55VP is 2.0 nm@30kV (VP Mode) and you say it is much worse. ?!

As to cleaning the crud, of course one should have gloves when working
with high vacuum parts but how do you clean the crud if not with a
"safe" solvent?

Ozgur


That is the spec. Don't believe it. I don't and have never been able
to produce that resolution. the high vac mode specs use in-lens
detector at 2-3mm WD. The VP detector is a glass rod pointed at the
specimen and depends on light being generated by the electrons hitting
chamber gas (N2). This light is then amplified by a PMT as is done
with E-T. S/N is really poor as mag increases so given that, the
extra cost and the tricky column, I don't see the value. The way the
VP is designed, it introduces a serious path for mechanical and
acoustic noise into the chamber and column. If one needs to look at
wet specimens, get an ESEM.

Considering resolution specs, what is the mag needed to measure 1nm of
resolution? For Zeiss, it is typically done at 300KX-400KX. So, for
2nm in VP mode, 150KX-200KX? Good luck getting any useful image.
Note that there is no WD stated for high vacuum or VP. VP is
typically done at longer WD to find the optimum WD along with chamber
pressure value. For specimens that need VP or have charging issues,
just lower the KV. The other option is the Zeiss Ultra Plus. This
uses a small jet of N2 to neutralize charge at the specimen. It is
akin to a flood gun in an ion implanter.

Cleaning takes on two facets. One is cleaning the gun chamber and
components. Best approach is not to have to do this. it is a
nightmare path. particles can be blown out with canned air but no
liquids should be used. On the otherhand, the chamber and column can
be cleaned with lint-free (Kimwipes) cloth and methanol--no
isopropanol. It will still cause out gassing but not as bad as a
messed up gun chamber. Unlike thermionic SEMs, never use Pol anywhere
in the system.

Look at the Ultra and Ultra Plus. These have multiple in-lens
detectors and can do really neat imaging.



Kiss French. Drink California.

gary at gaugler dot com
.

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