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Vito:<br>
<br>
While I'm sure that folks more knowledgeable than I will chime in,
let me get the ball rolling with some of my thoughts.<br>
<br>
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.<br>
<br>
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 ...<br>
<br>
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?<br>
<br>
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?<br>
<br>
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.<br>
<br>
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.<br>
<br>
Good luck,<br>
<br>
John<br>
<br>
On 1/28/2013 2:23 PM, Vito Logiudice wrote:
<blockquote
cite="mid:CD2C43E5.2104A%25vlogiudi@connect.uwaterloo.ca"
type="cite">
<div class="moz-text-html" lang="x-western">
<div>Dear Colleagues,</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Many thanks,</div>
<div>Vito</div>
<div>
<div>-- </div>
<div>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Vito Logiudice M.A.Sc., P.Eng.<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Director of Operations, Quantum NanoFab<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">University of Waterloo<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">200 University Avenue West<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Waterloo, ON Canada N2L 3G1<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Tel: 1-519-888-4567 ext. 38703<o:p></o:p></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Email: <a moz-do-not-send="true"
href="mailto:vlogiudi@uwaterloo.ca" style="color: blue;">vlogiudi@uwaterloo.ca</a></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;">Website:
<a moz-do-not-send="true"
href="https://qncfab.uwaterloo.ca">https://qncfab.uwaterloo.ca</a>/</p>
<p class="MsoNormal" style="margin: 0cm 0cm 0.0001pt;
font-size: 11pt;"><br>
</p>
</div>
</div>
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