[labnetwork] N2 pump purge source of chamber contamination?

Tue Jan 29 18:54:53 EST 2013

I am not sure if the suggestion given below will work with a good small 
volume, load locked turbo pump system.  But it is worth considering 
after all the obvious causes of trace  nitrogen in the films have been 
eliminated.

One source of nitrogen could be the the backstreaming during the last 
stages of chamber  pump-down.  This is serious if the pre-process 
pumpdown goes on for a long time to attain a certain predetermined level 
of vacuum (or the operator leaves the system pumping for an extended 
period of time to get "good" vacuum and goes for a meeting or a break). 
Backstreaming is nearly absent when something is being pumped (i.e. 
flowing through the pumping line). Nitrogen purging suppresses 
backstreaming of species resident in the pump but can create the problem 
of that nitrogen itself backstreaming depending on the system and 
process design. Replacing nitrogen with another gas is one solution as 
described in Bob's e-mail.   Backstreaming of the pump purge gas can be 
suppressed by purging the chamber itself with a desirable gas (carrier 
gas used in the process) and thus pumping the chamber in a "carrier gas 
rinse" mode.    This is easy in sputtering or PECVD systems in which you 
can purge with a carrier gas, typically  Ar or He, at very low flow 
rates.  One may switch to the final process without cutting off the 
carrier gas purge  and without ever switching to the "full low vacuum 
state."  The purge also helps to reduce the effectiveness of virtual 
leaks by exhausting them and replacing them with carrier gas.   A 
Meissner coil or a water pump that is installed directly on the chamber 
further improves the vacuum quality if turned on after the chamber is  
pumped down.  Periodic system qualification, regular use of RGA, and 
qualification of the material on test substrates before committing 
valuable parts  may be routinely required.

You may want to check if the contamination varies as a function of the 
deposition number in a sequence of depositions. Does wafer (or run ) 
number 1 differ from wafer (or run) number 10?  You have to see if there 
is a virtual leak that you wear out as deposition progresses (as chamber 
components heat up).  One way to address this process quirk  without 
going into a large R&D project is to run simulated depositions before 
committing to a product wafer. Ignoring the first few depositions to 
stabilize the system is a no-no if you are running a product line but is 
OK in a research project.

This paper on effectiveness on N2 contamination might interest you:
R. Locher, C. Wild, N. Herres, D. Behr, and P. Koidl, "Nitrogen 
stabilized ?100? texture in chemical vapor deposited diamond films,"  
Appl. Phys. Lett. *65*, 34 (1994); http://dx.doi.org/10.1063/1.113064 
<http://link.aip.org/link/doi/10.1063/1.113064> (/3 pages/)

Problem mentioned in the original e-mail is very common when one 
deposits anything sensitive to trace impurities, such as 
superconductors.   In studies of superconducting films, deposition of 
ultra high purity films used to be very difficult in vacuum systems of 
the past.  Films that seemed to be OK at room temperature would exhibit 
very different (disappointing) properties when cooled to low 
temperatures.  Trace impurities were one of the culprits.  Implementing 
several "tricks" to improve purity of the films gave  researchers access 
to good superconducting films for device applications while using 
inexpensive systems.

Good luck.
Pramod Karulkar

Pramod C Karulkar Ph. D.
6024 33rd Street Ct NW
Gig Harbor WA 98335

On 1/29/2013 8:53 AM, Robert M. Hamilton wrote:
> 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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>
>
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