[labnetwork] N2 pump purge source of chamber contamination?

[John Shott]

Vito:

While I'm sure that folks more knowledgeable than I will chime in, let 
me get the ball rolling with some of my thoughts.

First, a modern turbomolecular drag pump should have a very high, but 
not infinite, compression ratio for nitrogen on the order of 10^12.  To 
me that means that there should be very little, but probably not zero, 
nitrogen flowing back through the turbo pump.  Knowing more about your 
system flows, pressure and nitrogen partial pressure in the foreline, 
etc. would probably be required to provide a more detailed analysis.

Do you have an RGA on this system ... or at least the ability to add one 
temporarily?  I would think that this would be a useful tool.   
Particularly, if I had an RGA, I'd be inclined to replace the nitrogen 
pump purge with argon ... at least as a quick experiment ... to see if 
that made a difference.  Actually, I'd probably run experiments using 
argon as a pump purge even if I didn't have an RGA on the tool ...

Do you have SIMS (or similar) data that shows constant nitrogen 
background levels or does the nitrogen level show a spike at the start 
of the deposition or at other times during the process?

How confident are you that nitrogen and/or air aren't able to enter the 
chamber through the load lock or through leakage in the main chamber?  
What is your pumpdown/purge procedure in the load lock?  Is that using 
argon to purge and backfill? What's the rate of rise in the chamber 
itself?  Has the chamber been hard baked so that you are more confident 
that you aren't seeing evolution of adsorbed gas, contributions from 
virtual leaks, etc?  Depending on what nitrogen background levels you 
are seeing, it doesn't take much of a leak in the chamber to result in a 
significant partial pressure of air in the system?  Are there any rotary 
or linear seals in the system that could result in unwanted partial 
pressures of air/nitrogen?

My first guess is that back flow of pump purge nitrogen is not your 
primary source of nitrogen in the chamber ... but that should be fairly 
easily tested using clean argon for your pump purge for a bit.  Although 
I haven't used one, I know that EBARA (and maybe other dry pump 
manufacturers) offers the ESR20N that claims to require no nitrogen 
purge in clean applications.  If backflow of nitrogen purge gas IS your 
problem, this may be a way of reducing that source ... and maybe at a 
lower total cost than using high-cost argon continuously as a pump purge.

Finding the source of low-partial pressure contaminants in these type of 
systems is often tricky and typically requires a significant number of 
experiments to fully resolve.  We have been working hard to find the 
source of low levels of oxygen contamination in some SiGe epitaxial 
processes ... and thus far have a hefty stack of receipts from SIMS 
analysis but less-than-complete resolution of the problem.

Good luck,

John

On 1/28/2013 2:23 PM, Vito Logiudice wrote:
> Dear Colleagues,
>
> We are depositing single crystal diamond via PECVD in a newly 
> purchased tool dedicated to this process. Initial deposition runs have 
> revealed the presence of film defects caused by stray nitrogen. The 
> system and process gas lines have been helium leak-tested. The 
> load-locked process chamber is pumped down by a turbo pump backed by a 
> nitrogen-purged dry pump (the process makes use of methane and 
> hydrogen). We're wondering if the N2 purge on the roughing pump might 
> somehow be contributing to the problem.
>
> I'd appreciate hearing the community's thoughts on the possibility of 
> nitrogen back flow from an N2-purged roughing pump back to the process 
> chamber.
>
> Many thanks,
> Vito
> -- 
>
> Vito Logiudice  M.A.Sc., P.Eng.
>
> Director of Operations, Quantum NanoFab
>
> University of Waterloo
>
> 200 University Avenue West
>
> Waterloo, ON      Canada   N2L 3G1
>
> Tel:  1-519-888-4567  ext. 38703
>
> Email: vlogiudi at uwaterloo.ca <mailto:vlogiudi at uwaterloo.ca>
>
> Website: https://qncfab.uwaterloo.ca/
>
>

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