[labnetwork] Update on CDA in lieu of N2 purging of drypumps

Wed Oct 2 18:50:08 EDT 2013

Iulian and esteemed colleagues,
    There has been more debate about this subject since diffusion pumps were challenged by those newfangled "jet age" turbo molecular pumps but with less of an argument.
Wet pumps can be used effectively but with caution. Valve sequencing must be precise in order to avoid backstreaming if the wet pump is to be used alone for processing, this means that you cannot pump on a chamber while at idle without some laminar gas flow to keep those pesky oil molecules in the pump where they belong. Vacuum pump oils must be selected correctly but what is important is that any system that runs O2 MUST have inert oil (Fomblin, Krytox) or risk explosion. Another consideration is with corrosive processes, it is generally accepted to run inert oil in pumps that will see corrosive etch gases but they tend not to "wet" the metal parts and protect them as hydrocarbon oils do which leads to premature pump and case failure. I would not recommend hydrocarbon oils for those applications since the oil traps acidic materials and doesn't offgas them as inerts do. I see no problem in using oil pumps for load lock applications but you must keep an eye on the oil levels since frequent pumping of these volumes from atmosphere tends to push vaporized oil up the exhaust.
With all that being said, I must say that whenever I have the opportunity to replace an oil pump with a dry pump, I do it. Dry pumps are cleaner both internally and externally. I do find them to be about twice the initial cost of wet pumps but I must disagree about the maintenance, my experience with dry pumps is that they give superior performance with an extremely long mean time to failure. PECVD and LPCVD processes are the best applications for dry pumps since they are dirty processes.  Most modern pumps have heat where its required to push out condensables and cooling where it's necessary to keep seals cool. In my rough estimation, considering the expense of pumps and the extra nitrogen needed  vs. the care and maintenance (and possible process/equipment contamination) dry pumps are a better value.
Regarding "standby mode" for N2 conservation, I am building a prototype unit that works in conjunction with the process gas pneumatic actuators. When any process gas is called for, a three way valve goes from supplying compressed air to N2. This design keeps those models of dry pumps that require a constant flow of N2 satisfied. I will interlock it with an N2  flowmeter that has a trip point to ensure that the pump sees only N2 when gases are on.
Best of luck

Steve Paolini
Equipment dood
Harvard University Center For Nanoscale Systems

From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Iulian Codreanu
Sent: Wednesday, October 02, 2013 5:45 PM
To: labnetwork at mtl.mit.edu
Subject: Re: [labnetwork] Update on CDA in lieu of N2 purging of drypumps

Bob - thank you very much for the detailed analysis.

I am also writing to ask my esteemed colleagues for advice on the following two related items:
- 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?
- 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).

Thank you very much!

Iulian

iulian Codreanu, Ph.D.

Director of Operations, UD NanoFab

University of Delaware

149 Evans Hall

Newark, DE 19716

302-831-2784
On 9/19/2013 6:17 PM, Bob Hamilton wrote:
Lab Network Colleagues,

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.

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).

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.

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.

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.
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.

On behalf of the NanoLab equipment staff, regards,
Bob Hamilton

--

Robert Hamilton

University of California at Berkeley

Marvell NanoLab

Equipment Eng. Mgr.

Room 520 Sutardja Dai Hall

Berkeley, CA 94720-1754

bob at eecs.berkeley.edu<mailto:bob at eecs.berkeley.edu>

Phone: 510-809-8600

Mobile: 510-325-7557

e-mail preferred

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