Author Topic: Cold stages, air jets and other anticontamination devices  (Read 2944 times)

Anette von der Handt

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Cold stages, air jets and other anticontamination devices
« on: December 11, 2015, 02:24:36 pm »
Dear all,

I am interested to hear about everyones opinions and experiences regarding anti-contamination devices available to microprobes.

From what I have seen/heard/talked to people, there are various lines of defense available:

Keeping the vacuum clean: Scroll pumps, cryo-baffles around diffusion pumps
Keeping the sample clean: Cold stage, Airjet, LN trap.
Starting out with a clean sample: Plasma cleaner, possibly attached to the air lock.

We have a LN trap right now but we never really use as it is awkward to use and you have to keep it going with Liquid Nitrogen or it dumps all the carbon back on. I am currently dreaming up the perfect new microprobe for light element work and beam sensitive samples, so I want to hear about successful or failed attempts (both over-the-counter solutions (by the vendors) as well as home made contraptions) of keeping it clean!

Thanks,
Anette


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Probeman

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Re: Cold stages, air jets and other anticontamination devices
« Reply #1 on: December 14, 2015, 12:35:09 pm »
I am interested to hear about everyones opinions and experiences regarding anti-contamination devices available to microprobes.

From what I have seen/heard/talked to people, there are various lines of defense available:

Keeping the vacuum clean: Scroll pumps, cryo-baffles around diffusion pumps
Keeping the sample clean: Cold stage, Airjet, LN trap.
Starting out with a clean sample: Plasma cleaner, possibly attached to the air lock.

We have a LN trap right now but we never really use as it is awkward to use and you have to keep it going with Liquid Nitrogen or it dumps all the carbon back on. I am currently dreaming up the perfect new microprobe for light element work and beam sensitive samples, so I want to hear about successful or failed attempts (both over-the-counter solutions (by the vendors) as well as home made contraptions) of keeping it clean!

Hi Anette,
Here is a cost effective and sustainable solution Cameca came up with for my SX100 bought in 2006 or so (see attached photos below- you'll need to be logged in to see them):

Basically, it's an air cooled "freon" compressor with the expansion valve attached to a set of vanes just above the diffusion pump. 

The unit is sold by PolyCold in Petaluma California, and Cameca engineered it to fit the instrument. I think they charged me about $28K for the entire system without the temperature readout and warmup/cooldown controller which I bought separately for another $4.5K.  This is significantly cheaper than the oxygen jet-cold finger solution usually offered by Cameca.

I find that not only does it just run without any LN2, but it also provides about the same level of hydrocarbon decontamination as the oxygen jet and cold finger system usually provided by Cameca. I had that oxygen jet and cold finger system on my SX51 at UC Berkeley and it was hard to regulate the oxygen flow and required refilling every 8 hours with LN2.  See here for some data from my instrument using a diffusion pump and another using a dry turbo pumped system:

http://probesoftware.com/smf/index.php?topic=140.0

One nice thing about diffusion pumps is that they don't fail in a spectacular fashion as turbos sometimes do, and they are much cheaper.  Though the newest turbo pumps are getting more reliable and less expensive.

The cold vanes run at around 100 degrees Kelvin and so not only pump hydrocarbons, but also water vapor!  About once a year we let it warm up with the gun valve closed and let the roughing pumps pump out the accumulated condensates.

That said, we recently did a complete column rebuild on the SX100 and found they were quite coated with a very tough film that was not affected by solvents, but cleaned off pretty easily using detergent.  We suspect it was vaporized epoxy from beam damage.
john
« Last Edit: December 14, 2015, 12:38:16 pm by Probeman »
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Probeman

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Re: Cold stages, air jets and other anticontamination devices
« Reply #2 on: December 14, 2015, 03:32:03 pm »
Our engineer also makes the following observations for what they are worth:

Quote
What's amazing is that 100K baffle temp can be reached without any IR shielding from the hot DP zone below! Possible improvements for this design:

1) Slice that SS rod which supports the baffle and bridge the gaps with KEL-F couplings (that SS rod is acting as a thermal short, so good thing is that SS is a somewhat poor heat conductor)

2) Install a simple annular IR shield around the baffle to block IR from room temp metal nearest the baffle

3) Install an IR shield to block IR from DP zone below (most complicated but you yield best possible reduction in baffle temperature).
The only stupid question is the one not asked!

Anette von der Handt

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Re: Cold stages, air jets and other anticontamination devices
« Reply #3 on: December 15, 2015, 08:14:09 pm »
John,

thanks for all the excellent information!
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Ben Buse

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Re: Cold stages, air jets and other anticontamination devices
« Reply #4 on: December 16, 2015, 11:42:57 pm »
Hi Anette,

Strange enough we were just discussing this at the UK Jeol Users meeting yesterday in Cornwall. As you point out the LN cold finger is a pain when it runs out. What we have been investigating and looking to developing is a pelter cooled cold finger. Hirsh et a. 1994 showed that only a small reduction in temperature is required on a cold finger. We ran a test in only SEM making a crude pelter cold finger and it showed a reduction in the contamination. I can send you the pictures and slides if you like.

We regularly use a LN cold finger when doing carbon in steels and low voltage analysis (where overvoltage very small). The LN finger greatly reducing amount of carbon depositing in vaccum. But as you say its a right pain when it warms up. With steels we find although we can prevent carbon intensity changing during analysis - the levels of contamination seem to vary from sample to sample (=/- 1000ppm) - despite using same method of polishing and cleaning. Suggusting need for plasma cleaning of samples prior to analysis - would work for steels - maybe not for carbon coated geological samples though!

Ben
« Last Edit: December 17, 2015, 12:48:40 am by Ben Buse »