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Iulian,<br>
<br>
I was dragged kicking and screaming to dry pumps because of their
high cost. A lot of early dry pumps had reliability issues. In the
UC Berkeley Microlab we settled on the Edwards DP series eventually
moving to later series such as the QDP, then iQDP and iH series. We
have a few other brands that we consider reliable, i.e Ebara and
Kashiyama. Both have been good. The Kashiyama has thus far, three
year's on, been flawless. We are about to test a new pump from
Hanbell. <br>
<br>
Mostly standardizing on a single pump manufacturer gave us the
advantage that when we replace pumps we have a standard form-factor.
This means our manifold connections match as well as the pump/tool
electrical interface. <br>
<br>
My experience with wet pumps is mostly good. Here are some some
ideas. <br>
<br>
For hivac systems and load locks: backstreaming is not too much of
an issue when backing a cryo or turbo pump as long as roughing and
crossover pressures stay above 50 mTorr (a running turbo rejects oil
passing through it). Having said this, accidents can happen and
contamination can be an issue. I see no advantage to using Fomblin
or a perfluorinated oil for this type of pumping with one
exception. "White oil", a high grade hydrocarbon oil, will deposit
in the fume exhaust of a lab. From experience, this oil load is a
fuel source and fire hazard. Fomblin is not flammable; however, it
has the disadvantage of high cost and in an e-beam system its
decomposition products are dielectric which can bias an e-beam and
have bad effects.<br>
<br>
For etch: not many issues with etch pumps and Freon etching, at
least in the volume of etching done in a university fab. However, I
would not use a wetpump with acid gases, even with Fomblin. This is
one place dry pumps excel. Wetpumps pumping acids will fail at the
shaft seals.<br>
<br>
For lpcvd and pecvd: we got away with wet pump using oil filtration
units and changing the oil filters after "x" number of hours. Moving
to Fomblin on low-stress nitride was tried and not a success. The
sight glass showed a snow-storm inside the oil. with this process.
Given pyrophoric are being pumped the already stated flammable
hydrocarbons/lab's fume exhaust load fire is of serious concern. <br>
<br>
Our dry pumps get rebuilt several times. They seem to last as long
as new ones as long as the vendor doing the work knows their stuff.
<br>
<br>
We routinely buy rebuilt pumps and if cheap enough used pumps and
have them rebuilt. Our cost of rebuild is typically in the $4k-5k
range depending on the size and vintage of the pump. The more modern
dry pumps have better, in-board bearing systems, better coatings and
better service life. Having said this, given the harsh conditions of
a fab, I would not want to reincarnate as as dry pump.<br>
<br>
Regards,<br>
Bob Hamilton<br>
<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 10/2/2013 2:45 PM, Iulian Codreanu
wrote:<br>
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<div class="moz-cite-prefix">Bob - thank you very much for the
detailed analysis.<br>
<br>
I am also writing to ask my esteemed colleagues for advice on
the following two related items:<br>
- My limited experience seems to indicate that dry pumps cost
significantly more both upfront and in terms of maintenance.
Are wet pumps that bad (in terms of oil backstreaming) to
justify the increased cost of dry pumps? Are there some type of
processes where dry pumps are a must and other processes where
wet pumps are just fine? Are there other advantages of dry
pumps I am not aware of?<br>
- Are there dry pumps that have standby N2 purge modes (less N2
used when process gases are not flown in the chamber) or do all
makes/models need constant N2 purge flow (I heard that some of
them will shut down if they do not "see" enough purge N2).<br>
<br>
Thank you very much!<br>
<br>
Iulian<br>
<pre class="moz-signature" cols="72">iulian Codreanu, Ph.D.
Director of Operations, UD NanoFab
University of Delaware
149 Evans Hall
Newark, DE 19716
302-831-2784</pre>
On 9/19/2013 6:17 PM, Bob Hamilton wrote:<br>
</div>
<blockquote cite="mid:523B7813.1050508@berkeley.edu" type="cite">
<p class="MsoNormal">Lab Network Colleagues,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">In response to a labnetwork posting a few
months ago, proposing the use of compressed dry air (CDA) in
lieu of N2 for some drypump purging, the UC Berkeley NanoLab
undertook a review of our dry-pumps. A total of 73 mechanical
pumps are in use in the NanoLab. Thirty six or ~ 50% of these
are drypumps which require N2 purge. <br>
</p>
<p class="MsoNormal"><br>
The NanoLab nitrogen supply is derived from liquid nitrogen.
The N2 resource is a major expense for our operation. A rough
calculation shows our N2 cost to be ~$100/yr/slpm (bulk N2
costs plus cryogenic vessel support). Our average dry pumps
consume ~35 slpm of N2 for purging (note: some vendor-designed
purge circuits are process-driven meaning N2 is used at high
flow rates only during process).</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Our first effort was to review CDA vs. N2
with our pump manufacturers and with our pump rebuilders. Both
gave us positive reports about the use of CDA in some
applications. For obvious reasons the 19 pumps used to pump
flammables or pyrophoric gases were excluded from
consideration. This left the pumps that support etchers,
load-locks and high-vacuum systems. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Following a review of the dewpoint of the
NanoLab CDA (-75F or ~ 6.5 ppm H2O weight/volume) a decision
was made to further exclude pumps that pumped the “acid gases”
(more specifically Cl2, BF3, HBr, HCl, HF, SiCl4, etc.). While
the NanoLab CDA dryer can produce air at dewpoints around -95F
the dryer’s shuttle-valve and check-valves must work
significantly harder to achieve this value thus requiring more
frequent maintenance and rebuilds. We have set our CDA
standard at -75F.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Eighteen 18 pumps were identified and
converted to CDA-purge. Our initial results look good. A
review of our N2 flow rates shows a saving of about 23%;
average N2 flows decreased from 2200 slpm to 1700 slpm saving
us ~$50k per annum. So far, we have seen no negatives from
this change. Our decision remains open to future review.</p>
<p class="MsoNormal"><span> </span></p>
<p class="MsoNormal">As a footnote, we’ve also decided to add 25
psi check valves to the 90 psi N2 supply for the pumps that
remain on N2-purge. The reason for this is we’ve found dry
pumps will pump their N2 supply to sub-ambient pressure if the
N2 supply is inadvertently interrupted. In some cases this can
have negative repercussions.<span> </span></p>
<p class="MsoNormal"> <br>
On behalf of the NanoLab equipment staff, regards,<br>
Bob Hamilton<br>
</p>
<p class="MsoNormal"> </p>
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<pre class="moz-signature" cols="72">--
Robert Hamilton
University of California at Berkeley
Marvell NanoLab
Equipment Eng. Mgr.
Room 520 Sutardja Dai Hall
Berkeley, CA 94720-1754
<a moz-do-not-send="true" class="moz-txt-link-abbreviated" href="mailto:bob@eecs.berkeley.edu">bob@eecs.berkeley.edu</a>
Phone: 510-809-8600
Mobile: 510-325-7557
e-mail preferred
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<pre class="moz-signature" cols="72">--
Robert Hamilton
University of California at Berkeley
Marvell NanoLab
Equipment Eng. Mgr.
Room 520 Sutardja Dai Hall
Berkeley, CA 94720-1754
<a class="moz-txt-link-abbreviated" href="mailto:bob@eecs.berkeley.edu">bob@eecs.berkeley.edu</a>
Phone: 510-809-8600
Mobile: 510-325-7557
e-mail preferred
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