Nitrogen and Venting UHV

1. It's cheap

2. It's readily available

3. The boil off from the liquid cylinder tends to be dry and will not cause condensation in your vacuum chamber

4. It is clean (i.e. no contamination/hydrocarbon), assuming that your venting lines and connectors are also clean.

Zz.

 

Common practice is to vent with dry nitrogen, e.g. boil-off from liquid, and to keep a constant flow of N2 through the chamber while you are working inside. Don't keep the chamber open any longer than necessary, and you will minimize contamination with water etc. from the ambient air. Less contamination is better than more.
There are also claims that once you've vented your chamber to dry nitrogen the surface is somehow passivated (at least partially) and will adsorb less water etc. It is not quite clear to me how this mechanism is supposed to work.
If you are going to keep your chamber open and exposed to air for a month or so you might as well vent it to air.
When venting with nitrogen from a gas cylinder be extra careful not to overpressurize the chamber as viewports cannot take a lot of internal pressure. In case of doubt always use a ballon or overpressure valve on the vent line and/or a burst disk on the actual chamber.

 

As I said, if and how the passivation thing works is not quite clear to me. Keeping a slow flow of N2 at least reduces the amount of water you get into the chamber. It is cheap, we just keep it flowing.

If you have time on your hand you can compare pump-down times and maybe RGA spectra after a fixed delay
for venting to air and to N2 with and without flow...

 

Don't use nitrogen if it's use is detrimental.

ie: for some processes, it poisons the stuff you're trying to make.

 

But you're VENTING the system! If the use of nitrogen gas "poisons" this stuff, then exposing it to air will be even worse! And that is what you are trying to do when you vent a vacuum system!

This doesn't make any sense.

Zz.

 

You're right, if the finished product will be exposed to air anyways. but my experience with R&D meant opening the chamber for adjustments and modifications only to continue the run later.

Nitrogen and oxygen were the two worst

 

You did read the original post of this thread, didn't you?

The OP wanted to know the reason why many of us who deal with UHV systems vent out those system using N2 gas. This is a common practice in MY R&D experience, both for accelerators and for thin film fabrications of air-sensitive material. Since most, if not all, of vacuum components are made of stainless steel or aluminum or copper, those are the types of material that you have to address when you are venting it with N2 and why.

Zz.

 

Yes, I was wondering why to use N2 myself too.

I saw that you had reasonably answered the question, and that naturally begs the question of "why not to use N2"

That's the question I attempted to answer! So that others can know of alternatives, and learn from my mistakes!





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My understanding is that during the pumping of a stainless UHV vessel, you will initially desorb more volatile compounds like water vapor, etc. Once these compounds have been desorbed from the surface, the process moves from strictly desorption to a diffusion/desorption scheme. Once you expose the system to atmosphere again, the material is essentially "clean" and will sorb these compounds again, and with enough time the compounds will diffuse throughout the material. Purging the system with dry N2 will prevent the lower surface energy compounds from sorbing to the surface, and can help slow diffusion, resulting in less pumping time to get the system back to UHV.

 

We don't use N2 in large chambers that people enter after venting. These machines are in a large semiconductor class clean room so they are vented using normal atmosphere.

 

But in that situation, you only require that the room and the vessel be kept very clean, rather than achieving ultra-high vacuum, don't you? What vacuum level do you get to when fabricating these semiconductors?

Achieving ultra-high vacuum (i.e. 10^-9 Torr or better, or in my case, very low 10^-10 Torr) in a reasonable amount of time and without having to buy many expensive ion pumps require several careful steps, and venting with an inert gas is one of them. N2 is the cheapest option.

Zz.

 

Typical base vacuum on a clean system is 10^-8 range using very large cryo pumps in the processing chambers and maglev turbo pumps in the beamlines. We normally use a neutral ion beam like argon (70kev 20mA) to bake the chambers after venting to drive out water. We have other systems like SEMs that require 10^-10 or better at the FE tip but the chambers are only single wafer size so N2 venting is used and getting to that range of vacuum requires sequenced bakes of the gun chambers and ion pumps.

Wafer chamber backside of processing disk (200mm) in a large vacuum chamber.
http://images.caeonline.com/im.php?id= 457724

 

If people enter the vessel, then venting with N2 presents a safety risk (lack of oxygen).

 

Nah, asphyxiation is so over-rated!

:)

Zz.

 

Arrived late to this thread, I wish to add a comment about reducing pump down time. Remember the UV lamp inside the chamber to speed up water desorption from inner walls. I never used it but remember readings about it.

 

We use UHV on a synchrotron x-ray beamline. We always find that the first time we get x-rays into a vessel there is a lot of outgassing, probably due to photo-desorption.

 

Nice observation!