Re: Which SEM??

On Mon, 10 Nov 2008 09:33:55 -0800 (PST), birerozgur@xxxxxxxxx wrote:



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

VP imaging is not very good. At 25KV, Zeiss is about 40nm and that is
in reality limited to less than about 20KX. Big disconnect. The
other negative factor is that the column is much taller than non-VP.
While this does add a second ion pump, the alignment procedure and
long term stability is an issue. The Supra and Ultra have none of
these issues. For charging issues, run at 500V or 1KV. No EDS of
course but good imaging. With high current on, DOF is large even at
short WD.

ESEMs are usually W and are made to handle bio/messy specimens. The
Quantomix holder is a good option for bio specimens and high vacuum.
No contamination.

Field-free in FEI usually means magnetic immersion is off. That also
means resolution and S/N are lousy. The purpose is to do EDS. If you
can't image anything at say >25K, how can you EDS an area of interest?
Highly problematic. Zeiss uses electrostatic optics so there is no
magnetic field issue. Zeiss also has Optiprobe option where one sets
the desired probe current and the SEM will automatically adjust
aperture size and extractor current.

Regarding UPS--dual conversion is the best way to go. I use two
Liebert Nfinity internally redundant 8KVA units. Each will keep their
assigned SEM systems running for up to 280 minutes. Being dual
conversion, they are running all the time. If input power fails, they
just switch to their batteries in about 5mS. Plus, the power is
stable to about +-2% and very low harmonics. One unit runs chiller
and air compressor while the other runs SEM and detectors. You do not
need any pneumatic valves with Zeiss--they are already built in for
failsafe operation. When power fails, system keeps running. The
decision is to guess when to turn gun off...before whole system shuts
down. shutting down to red is not good. All vacuum goes off and gun
chamber vacuum will degrade. This necessitates a bakeout at least and
possible tip change at worse. With UPS, there is minimal worry.

Hot and cold tips do not like crud in the gun chamber. Normally,
these will flash off. On some ocassions, a very small piece of crud
on the ceramic holder for the tip will get dirty. Uninformed persons
clean it (destroy it) with Isopropanol. Service techs will use gloves
in the critical vacuum areas.

There is an optical sensor to aid in protecting the pole piece.
However, this is the CCD camera and one's eyeball. The Specimen
Current Monitor (SCM) is a very useful option. It detects when the
specimen has contacted the pole piece or final lens. Usually, it is
an advisory that you need to get out your check book. For Supra and
Ultras, the camera is at center rear of chamber looking towards the
chamber door. Anything in about a 180 degree viewing angle can be
seen. But with a holder with differing height specimens, it is easy
to whack a high specimen on the front side of the chamber where it is
hidden from CCD view. Thus, prudence dictates that tall specimens are
towards the rear while short ones are towards the front. Or, don't
mix them on the same holder.

Two good specimens are Amphipleura diatoms which you can get on a
round microscope cover slip from Klaus Kemp in UK. The second is a
good Au on C like from Agar or Ted Pella. These should be on 12mm
diameter pin stubs. Highest resolution will be at 2-3mm WD. If the
operator or tech knows how, the Zeiss system will directly allow
measurement of resolution. Published specs can be met or exceeded.

Another interesting specimen is a small piece of styrofoam from a
coffee cup. This will need to be coated as will most of the diatoms.
The diatoms are a challenge due to the very small features and low
contrast (not much Z contrast). They also will challenge at the ends
where they are cupped. The styrofoam will challenge for detail of a
wide range of DOF.



Kiss French. Drink California.

gary at gaugler dot com
.

Relevant Pages

  • Re: Which SEM??
    ... For un-fixed and wet bio specimens, high resolution is ... The Zeiss Gemini column uses an ... multi-hole aperture in the gun chamber. ...
    (sci.techniques.microscopy)
  • Re: Which SEM??
    ... high vacuum model does plus the ability to work on non-conducting ... ESEMs than the high vacuum models. ... Quantomix holder is a good option for bio specimens and high vacuum. ...
    (sci.techniques.microscopy)
  • Re: Which SEM??
    ... acoustic noise into the chamber and column. ... wet specimens, get an ESEM. ... Considering resolution specs, what is the mag needed to measure 1nm of ... Note that there is no WD stated for high vacuum or VP. ...
    (sci.techniques.microscopy)
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