Ask the Staff

john.d.watson.2 2013-01-21 02:09 PM
Roughing of evaporators
I have a question/suggestion regarding the evaporators in the cleanroom. My impression is that it is not uncommon for the evaporators to be contaminated by oil from the roughing pump back-streaming into the chamber (I say this is not uncommon because there have been two times I know of where the CHA went down due to back-streaming the day before I was scheduled to use it ). Contamination issues like this obviously waste a lot of staff time and source material as well as interfere with user processing runs (my samples for instance can't sit around oxidizing for days once I start processing them). My suggestion is to switch all of the evaporators over to rough pumping with LN2 sorption pumping systems like the Varian uses. The sorption pumps have zero chance of contaminating the chamber (this is why we use them for the MBE's), no moving parts/maintenance, and are almost impossible to break. There would of course be an initial cost to buying new pumps and LN2 lines (though the dewars can also be manually filled quite easily as we do with the MBE's), but I don't think it would take too many contaminated gold crucibles or days of staff maintenance time to make up the cost. Just a suggestion...

John

david.a.lubelski.1 2013-01-24 03:59 PM
Roughing Pumps on Evaporators
John,
There are a couple of reasons we do not have sorption pumps on all the evaporators. First and foremost is the cost associated with the use of sorption pumps. Both the cost of purchasing the pumps, running the LN2 lines and the cost of the LN2 itself, and the high cost of replacing the oft broken thermocouples, repair to the cleanroom floor and subfloor, which causes particulate counts to skyrocket as the LN2 damages the concrete floor. These costs are very high as opposed to the cost of a mechanical pumping system.
In most MBE labs, users are limited to just a few individuals whereas in the cleanroom we have upwards of 250 users and cannot control the proper use and sometimes misuse in the evenings and on weekends.
The Varian is the last system to have sorption pumps and no new system will have them in the future.
Dave