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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. <br>
<br>
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. <br>
<br>
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. <br>
<br>
This paper on effectiveness on N2 contamination might interest you:
<br>
<div class="authorList"
name="coden=APPLAB&volume=65&issue=1&pg=34&seqno=1">
<span class="looklikelink authorname aqslistener">R. Locher</span>,
<span class="looklikelink authorname aqslistener">C. Wild</span>,
<span class="looklikelink authorname aqslistener">N. Herres</span>,
<span class="looklikelink authorname aqslistener">D. Behr</span>,
and <span class="looklikelink authorname aqslistener">P. Koidl</span>,
"Nitrogen stabilized 〈100〉 texture in chemical vapor deposited
diamond films," Appl. Phys. Lett. <strong>65</strong>, 34
(1994); <a href="http://link.aip.org/link/doi/10.1063/1.113064"><font color="red"><b>The MTL Mail Server has detected a possible fraud attempt from "link.aip.org" claiming to be</b></font> http://dx.doi.org/10.1063/1.113064</a>
(<em>3 pages</em>) </div>
<div class="elementGroup"> </div>
<div class="abstitle"> <br>
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. <br>
</div>
<br>
Good luck.<br>
Pramod Karulkar<br>
<pre class="moz-signature" cols="72">Pramod C Karulkar Ph. D.
6024 33rd Street Ct NW
Gig Harbor WA 98335 </pre>
<br>
<br>
<div class="moz-cite-prefix">On 1/29/2013 8:53 AM, Robert M.
Hamilton wrote:<br>
</div>
<blockquote cite="mid:5107FE87.8010206@eecs.berkeley.edu"
type="cite">
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<div class="moz-cite-prefix">Vito Logiudice,<br>
<br>
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.<br>
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?<br>
<br>
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.<br>
<br>
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:<br>
<br>
<ul>
<li><img src="cid:part2.04010004.05030904@gmail.com"
alt="Edwards STP magnetically levitated turbomolecular
pumps"></li>
</ul>
<br>
Regards,<br>
Bob Hamilton<br>
<br>
Bob Hamilton <br>
Marvel NanoLab <br>
University of CA at Berkeley4 <br>
Rm 520 Sutardja Dai Hall <br>
Berkeley, CA 94720-1754 <br>
<a moz-do-not-send="true" class="moz-txt-link-abbreviated"
href="mailto:bob@eecs.berkeley.edu">bob@eecs.berkeley.edu</a>
(e-mail preferred) <br>
510-809-8600 510-325-7557 (mobile - emergencies) <br>
<br>
On 1/28/2013 2:23 PM, Vito Logiudice wrote:<br>
</div>
<blockquote
cite="mid:CD2C43E5.2104A%25vlogiudi@connect.uwaterloo.ca"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
<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>
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