EPMA Vacuum Technology Discussion

[Probeman]

This is a topic for discussion of general vacuum issues in EPMA, both JEOL and Cameca instruments.

I'll start this thread by mentioning an excellent technology that Cameca provided for my SX100 instrument instead of the usual "cold trap finger" and "oxygen leak jet" hardware for minimizing carbon contamination.

When I actually set out to purchase the instrument about 5 or 6 years ago, the Euro was very unfavorable for the US, and although I had budgeted for a dry pumped turbo system, I could no longer afford it. So instead I suggested to Cameca that they provide a diffusion pumped system, with a cryo-trap over the diffusion pump and without the cold finger and oxygen jet.

The results were excellent as seen in our acceptance testing and the air cooled cryo-unit runs at 100 degrees kelvin! So it not only traps oil, but even water quite efficiently.  About once a year we warm it up to boil off all the contaminants with the secondary valve closed.

The link to the unit is here:

http://www.brooks.com/products/cryopumps-cryochillers/cryochillers/pcc-compact-coolers

The only trouble we've had over the last 6 years is to replace one cooling fan on the compressor. And zero LN2 and oxygen consumption...



Not too bad for an inexpensive and reliable oil pumped vacuum system!

Edit by John: There's another method which JEOL attaches to their airlock (see attached pdf image from Aachen), but I would prefer an "in-situ" cleaning method, so only the area of interest is cleaned as described below in the request for a UV laser for cleaning concurrently with the x-ray acquisition.

[Gseward]

Our SX100 has a turbo pumped system backed with scroll pumps. This test was done when the instrument was being installed. I have no idea what it is like today!

Description : test made on pure copper at 10kV 100nA.



Rate = 0.033% and is a similar result to John Donovan's Aqua Trap system. Not sure where the 'Limit without device' spec is from  - I'd be interested to see experimental results from a diffusion pumped system without any decontamination devices. It would also be interesting to compare numbers from somewhere that does a lot of Carbon analysis (Aachen 8530??).
Have you done a retest lately John? I have a plasma cleaner that I've been meaning to bolt onto the probe, so perhaps it is time to do a new test 'before and after'.

Our vacuum system hardware has been very reliable for 3.5 years. All I've had to do is replace the tip-seals in the scroll pumps a few times and clean the high vac gauge.

gareth

[Probeman]

This is a crazy idea, but I'd like to try a hydrocarbon experiment in my EPMA vacuum system and need to borrow or buy a small UV laser that I can mount just outside the instrument.  The specs my engineer suggests are as follows:

266 nm UV pulse laser (frequency quadrupled YAG), diode pumped solid state (DPSS)

Typically these devices are some 10 to 20 cm long. If anyone has one of these guys that we can borrow or buy (cheaply!) I would be very interested to hear from you.

Thanks.

[John Donovan]

Warren posted this recently on the list server and he said it was OK to re-post it here:

I presume your setup has three different purposes for those pumps and that each one is connected to pump a different part of the system. If so, I don't know that you will be able to combine them to a single vacuum manifold. They could be pumping against different loads at different vacuums. That is the case with our FEI Quanta variable pressure SEM. It has two roughing pumps because it needs to have two. One backs up the turbo pump at the best vacuum possible with whatever pumping load it has. The second pumps on the chamber when it is operating in variable pressure mode (10-160 Pa). In that mode, it would probably be more difficult (and expensive) to use a single pump. The second pump is tied in to help on roughing the chamber when first pumping down, but it is isolated and turned off before the turbo seriously comes into play. So, two pumps was by good design.

Now, if you have three pumps from three different instruments that are working at maximum vacuum all (or most) of the time, you might be able to design a system that could reduce the number of pumps. (Since parts came from different vendors, there was probably not an option to combine vacuum systems.) However, you would probably need extra valves and plumbing and a ballast tank to maintain high vacuum while you borrow a pump for roughing purposes. That would not be a trivial (or cheap) engineering exercise. My experience with engineering projects is that the cost could easily outrun the cost of three brand new vacuum pumps.

FWIW, it might be time to consider other aspects. For example, we swapped out our old, oil-sealed rotary pumps with oil-free dry scroll pumps. That reduced the contamination issues within our SEM. We have also been able to take advantage of manufacturers' occasional sales to get pumps at pretty serious discounts.

Warren Straszheim, Ph.D., manager
Materials Analysis and Research Lab
23 Town Engineering
515-294-8187