[labnetwork] N2 grade for blow guns

[Hughes, John S]

We're in a similar place as John. We have a 24 year-old cleanroom design with high-quality (99.995% purity; Mil. Spec. 27401-C) distributed "house" nitrogen. In our case, there are two separate LN2 tanks being used. The one distributes at low pressure (~40 psi) through copper pipe, the other at high pressure (~120 psi) through stainless lines.

We use 0.2 um filters on all the blow guns and some of the process chamber vent lines. Any nitrogen process gas comes from UHP cylinders, but we haven't had any problems using our house nitrogen for everything else.

 -- John

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John S. Hughes                         Office: (217) 333-4674
Associate Director                        FAX: (217) 244-6375
Laboratory Operations                     hughes at illinois.edu<mailto:hughes at illinois.edu>
Micro and Nanotechnology Laboratory
University of Illinois at Urbana-Champaign
2000E Micro and Nanotechnology Laboratory
208 North Wright Street
Urbana, Illinois  61801              http://mntl.illinois.edu
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On Jan 12, 2013, at 12:51 PM, John Shott <shott at stanford.edu<mailto:shott at stanford.edu>>
 wrote:

Iulian:

First class is better than coach.  A Mercedes is better than what I drive.

But, I don't fly first class and I don't drive a Mercedes, so I guess that I'm going to voice an opinion that is somewhat contrary to that expressed by most of my esteemed colleagues who have weighed in.  You've heard from a number of the best university lab/facility managers representing a number of the best university research facilities and the best fab design people and the great majority say use UHP nitrogen.  Also, many of those folks are in newer facilities whereas ours has been use for 25+ years.

However, in addition to some of the cost/quality/convenience issues that Tom Britton raised, I think it is important to ask what is the quality difference between your utility nitrogen and your UHP nitrogen.  If utility nitrogen has a standard meaning, I'm not sure that I know what it is.

In our case, both our utility (AKA 'house') nitrogen and our UHP nitrogen come out of the same (non-stainless) 9000 gallon cryogenic tank.  So, I believe that our utility nitrogen is "pretty good" quality.  Although we don't monitor it, I suspect that it is likely less than 1 ppm moisture content most of the time ... it certainly has been every time that we have had it tested, but we don't have either online or routine monitoring of our moisture levels.  The utility system also has high-quality inline particle filters just downstream of the vaporizers, but is routed in copper piping.  Our UHP nitrogen, however, goes through an additional set of Entegris Gatekeeper cartridge filters and then is routed in stainless.  In short, our utility nitrogen is, in my view, very good, but our UHP nitrogen is better.s

Like everyone, I suspect, we use UHP nitrogen for all of our process gases to furnaces, etchers, deposition tools, etc.
Utility nitrogen is used for pump purges and all of the other things that others have outlined.

However, all of our wet benches are only supplied with utility nitrogen.  Not only does that mean that our blow guns use utility nitrogen, but so do our SRDs ... the last thing that wafers will see before a furnace step.

While that likely sends shivers down the spines of many, that's the way that our facility is set up and it would now be expensive to change. Over the years, we've built a wide range of high-efficiency solar cells and high-energy physics particle detectors with finished carrier lifetime in the msec, if not 10s of msec, range.

So, while our utility nitrogen is still of pretty high quality, except for the fact that it is distributed in copper piping, that is what we use for blow guns and for our SRDs.  There are a lot of things to lay awake at night and worry about in operating facilities of this type, but in my case, using our utility nitrogen for these purposes is not one of them.  Of course, if utility nitrogen means something of greatly lower quality ... such as a nitrogen generator of the type used to fill tires with nitrogen at Costco ... then that is an entirely different matter.

Have a good weekend all,

John




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