UHV System Issues: Seeking Advice

[tigigi]

I got a problem with ultra high vacuum system, and would like to know some advice.

A UHV chamber just moved to the lab, and I cleaned it with aceton, put the ion pump indside and sealed all the ports, and I've been pumping the chamber for like 3-4 or 2-3 weeks. I leak tested it by a helium leak detector with sensitivity of 10^-10 cc/s, and found no leaks. It used to pump to 10^-8 torr before it moved here, and in theory it could go lower than it too, but right now the lowest it could go is 10^-5.

We leak tested it several times, and even used a plastic bag to wrap the system and insert helium inside, but still no leak reaction.

Currently, I pump it with a roughing pump, which could go to 10^-3 in theory, and a turbo pump, which could go to at least 10^-6 or even 10^-10 in theory. I checked the roughing pump, it could go to 10^-3, but I haven't check the turbo pump, and I doubt it may have a problem.

I analyze the problem this way. If there's no leak, it should go to 10^-8, so it might be that there's a leak but we didn't find it or it might be that there's no leak, but the pump has a problem.

Does anyone used to have this kind of problem before. I appreciate if there's any suggestions. Thank you.

 

[Gokul43201]

You need to check every section of vacuum lines and every single o-ring joint between the front end of the turbo and (including) the UHV chamber. If you've already done this, ask yourself if there's any possibility you let some water get into the chamber.

The standard cleaning procedure for vacuum parts is acetone + methanol + DI water + dry over several hours. If you clean with acetone, without immediately following up with methanol, you often leave behind a white residue.

What was the history of the chamber between the last time it worked and the first time it did not?

Without using the turbo, how do you know the pressure gets down to 10^-5 torr?

 

[mgb_phys]

Could be contamination or a virtual leak.
Do you have a heater source on the chamber to outgass any water vapour?
How well was it cleaned, is there any chance of dirt/oil/grease inside?

Have you fitted any new parts, do you have any non-vented bolts?

 

[tigigi]

Thank you for these replies. I guess it may be a problem that I just use acetone to clean it w/o methonal. Could I get rid of it using heater to heat it up ? I just guess that if I heat it, the particles might be pumped out ?

I'll go check what was the last time it worked. I don't know that cos both the chamber and I are new to the lab ~ Does it matter if it worked long time ago ?

We put heater outside of the chamber to heat it, and hope that we could get rid of possible water vapor inside. I think the chamber won't have any oil/grease inside since I cleaned it and double checked it.

I have a question that is virtual leak the same as outgassing ?

Thank you.

 

[mgb_phys]

Outgassing is gas absorbed on a surface or liquid evaporating.

A virtual leak is air trapped inside another void in the chamber that escapes slowly. Most common is in the bottom of a tapped hole under a bolt, for this reason you drill a small hole down the length of the bolt to let this gas escape. The same thing can happen with connectors or any mechanical systems in the vac chamber.
It's difficult to find all these places - especially if you have lots of bought-in mechanisms.

 

[tigigi]

Thank you for the explanation to the differeces of the 2 terms. I know better now.

We kept heating the chamber, rounding the chamber with heating tape for like 2-3 weeks or even more, like 4 more weeks (I don't remember exactly.). We turn the heater on several hours, and then turn it off, and then turn it on again. We repeated this process from time to time, and hope it could help.

I'm wondering if it's possible get rid of the gas hidden in the bolt and the residues. (I guess there's residues since I just used aceton w/o methanol. Besides, there's no leak according to leak detector, and seems only virtual leak, so I guess it's not that clean.)

I just think that it might make it worse to open it, let the air in, and clean it again, and it'd be a lot of work as well. So I'm thinking any possible way to get rid of the possible residues. What I could think of is just heating, but I'm not sure if it do work for the possible residue problem.

 

[Gokul43201]

Some questions to help make a diagnosis:

1. What is your pumping speed?
2. What is the lowest pressure you can get to with the turbo when you pump on the chamber?
3. What is the smallest leak that your leak checker can detect?
4. Does your leak checker use a He-4 mass spec?

Please include all relevant make and model names.

 

[tigigi]

1. the pumping speed of the turbo pump is 50 L/s
the pumping speed of the roughing pump is not sure. It doesn't show on the machine.
It's fisher scientific Maxima. I checked it online, and it's 45 gal/min, but I'm not 100% sure that they're exactly the same type.

2. the lowest could go is about 5x10^-5 torr

3. the smallest leak my leak detector can detect is 10^-10 cc/s

4. I'm not sure if my leak checker use a He-4 mass spec or not. I don't really understand what is He-4 mass spec. It's Veeco MS170, and we use gas helium to detect it.

We're right now turn off roughing and turbo pump, and just turn on the ion pump, but the pressure oscillate at around 2 ~ 8 x10^-5 torr.

I'm thinking that if I'm going to open it and clean it again, but I don't think it a good idea. I'm wondering if I continue to heat it then it'd help to get rid of white residues.

 

[jaap de vries]

Turbo pumps do not have infinite lifetimes. So you may want to check that. Vacuum leak testing is one of the most annoying things so good luck!