[labnetwork] N2 pump purge source of chamber contamination?

[Robert M. Hamilton]

Vito Logiudice,

The UC Berkeley Marvell NanoLab supports a CVD diamond tool 
made by sp3. It is CVD and not PECVD; however, I'll take a 
stab at a way of ruling out your pump-stack as a source of 
back-diffused N2.
It is unlikely N2 could back diffuse through a turbo-pumped 
system from its foreline roughing pump. By chance, does this 
turbopump use N2 as a bearing-purge, another source of N2? 
If so, is the flow for this purge under control?

One way to rule out N2 from the pump stack would be to 
substitute Ar as the pump-purge gas and do a run. If your 
runs are long and a cylinder of Ar does not contain enough 
gas (~6500 standard liters/cylinder) to support a full run 
consider rental of a liquid Ar Dewar. Depending on the size 
of your dry pump it likely uses 35-50 slpm of N2 purge and 
the turbo bearing purge would be much less than the mech 
pumps N2 purge gas consumption.

An RGA would be a terrific tool to review effects. Appended 
is a graph from Edwards on the RGA analysis of the ratio of 
residual gases from their turbos. Given the base pressure of 
turbos and the throughput of your process gas the N2 partial 
pressure should be nil:

  * Edwards STP magnetically levitated turbomolecular pumps


Regards,
Bob Hamilton

Bob Hamilton
Marvel NanoLab
University of CA at Berkeley4
Rm 520 Sutardja Dai Hall
Berkeley, CA 94720-1754
bob at eecs.berkeley.edu (e-mail preferred)
510-809-8600 510-325-7557 (mobile - emergencies)

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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