pumping efficiency for a mixture of different gases

Question

hello! 
 we have a deposition system which operates at 100 mbar with a rotary pump. 
 inside this system we flow argon, but when we open the chamber we also expose it to air and so nitrogen enters. we are not bothered by oxygen, water vapor and carbon. 
 My question is: with our turbo pump and with our base pressure, should we expect to remove nitrogen and argon with the same efficiency? should we expect to have traces of nitrogen even if we purge the chamber several times with argon and we leave the chamber in argon flow some days? 
 would it be different if instead of argon we use Helium as main gas? 
 thanks a lot!

dahulett · Accepted Answer

Argon is a much larger molecule than helium and would do a better job of moving nitrogen from your system. Is the residual nitrogen causing problems with your application?

dahulett · Answer

Different gases have different pumping efficiencies based on the molecule size and mass. You may want to helium leak check your system if you cannot remove the traces of nitrogen.

bosone · Answer

thanks for your kind help! our system has a good vacuum, we tested once it with a turbo pump and it reached high vacuum, and we did a successful He leak test 
 We also connected a more powerful rotary pump and we reached 10-2 mbar. 
 we should not have contamination from outside. 
 we are right now trying to understand if nitrogen contamination is higher if we use argon OR helium, to assess some experimental results. 
 we thought about the pumping efficiency of gases with different masses. but, honestly, I cannot figure out a solution because of my lack of basic knowledge about vacuum systems.. any help would be appreciated

bosone · Answer

thanks again! that is helpful. we really do not know if N is the source of our problems. we are into semiconductor oxide deposition and our results were far better in the past with Helium. we changed a lot of things since then (including the deposition chamber itself) and we are using Argon right now, and unfortunately we are not getting the same results. 
 N contamination is one idea but now our hypothesis seems to not be valid...

dahulett · Answer

I worked on CVD chambers for decades in the semiconductor industry. Is it a particle, uniformity, or dielectric breakdown or thickness issue? I would go back to the same configuration and process set-up that worked initially.

bosone · Answer

you are really kind... unfortunately we changed completely the deposition chamber and so also the process, that has to be readaped. we will revert to He as carrier gas as see what will happen.

dahulett · Answer

Good Luck

pumping efficiency for a mixture of different gases

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