[labnetwork] LN2 vs N2 generation on site

Fri Mar 27 11:54:57 EDT 2015

Rick:

This is an excellent question and I thank Tom Tribble and John Nibarger 
for their initial contributions to this topic ... and others that will 
undoubtedly soon follow.

First, let me get my remedial class question out of the way:  Tom, 
you've got me feeling like a rank beginner.  I must confess to being 
unfamiliar with the 1δ, 3δ, and 6δ notation for gas purity.  Is this the 
equivalent of what I know as dot notation? In other words, so they 
correspond to 1-, 3-, and 6-nines purity that is 1.0, 3.0, and 6.0 in 
dot notation?  Where 99.5% purity is 2.5?  Or is δ notation something 
different?

I would like to add confirmation of your "unofficial audit" that found 
that 10% of your usage demands high purity.
We also have a 9000 gallon LN2 tank (which, sadly, gets filled every 
fourth day).  We have two sets of vaporizers on it: the big set supplies 
the copper-piped "house N2" and the smaller vaporizers that also go 
through a switchable set of purifiers and then feed our stainless-piped 
"UHP N2" system.  Nouse N2 usage is dominated by pump purge.  We have 
totalizing flow meters on each main line and typically observe that the 
UHP N2 consumption is 130-200 SLM and the house N2 consumption is 
2500-2800 SLM.  In other words our total nitrogen flow is very close to 
3000 SLM (or about 4.5M - 5.0M SCF per month!). In other words, our 
high-purity consumption is in the range of 5-7% of our total 
consumption.  Note: we actually use the house nitrogen for our SRDs ... 
if we moved that intermittent high-flow to UHP N2, it probably gets us 
close to, but no higher than, 10%.

My question to the labnetwork community is what level of nitrogen purity 
would you want for pump purges of some of our nastier processes?  For 
example, things like DCS and ammonia for LPCVD nitride?  Hydrogen, 
germane, silane, plus dopants for an epitaxial process?  is 1.0 nitrogen 
good enough?  The way that I look at it, that is only a 50% reduction of 
oxygen as compared to what you find in air.  How about 2.0 nitrogen 
(99%), good enough?

And one question to John Nibarger:  is the moisture content of your 
generated nitrogen controlled by the moisture content of your feed CDA?  
In other words, with your incoming CDA dew point of -40C is the dew 
point of your generated nitrogen also approximately -40C? If the 
incoming CDA were drier (say -55C to -60C) do you expect that your 
generated nitrogen would also be that much drier?  I ask because for 
pumps running corrosives such as HCl, Cl2, BCl3, and HBr it would seem 
that dryness of the purge gas would be more important than the 
background oxygen content.

Thank you all,

John

On 3/24/2015 1:31 PM, Tribble, Thomas wrote:
>
> Richard,
>
> We haven’t switched to onsite generation for the fab, but we (Harvard) 
> have looked at it.  The Harvard fab has a 9000 gallon LN2- 3δ storage 
> that gets re-filled approximately once a week. The storage facility 
> occupies approximately 300 GSF, and there  is no more real estate 
> available for this purpose. Based upon a much smaller on-site 
> generator recently completed, on-site generation to replace the fab 
> storage facility would require more real estate than is available. 
> Part of the demand for space is the self-imposed requirement for N+1 
> redundancy.  This also affects the capital budget.
>
> But if saving money is your focus, I suggest that your discussion 
> should be focused on the purity of your gas requirements. Locally 
> generated 1δ gas is inexpensive cheap to produce. Locally generated 3δ 
> gas is moderately expensive to produce.  Locally generated 6δ gas is 
> *very* expensive to produce.  From the refinery, 3δ gas is all that is 
> produced and transported.  What we at Harvard use is 6δ gas (3δ gas 
> run through a purifier).  An unofficial audit has shown that less than 
> 10% of our usage actually demands 6δ purity.
>
> Want to save money?  Figure out your demand by purity and segregate 
> the flows (piping).  Provide your pump and gas cabinet purges with 
> on-site generated 1δ gas.  Your facility will save.
>
> Tom Tribble
>
> *From:*labnetwork-bounces at mtl.mit.edu 
> [mailto:labnetwork-bounces at mtl.mit.edu] *On Behalf Of *Morrison, 
> Richard H., Jr.
> *Sent:* Tuesday, March 24, 2015 12:04 PM
> *To:* labnetwork at mtl.mit.edu
> *Subject:* [labnetwork] LN2 vs N2 generation on site
>
> Hi everyone,
>
> I am paying >$220K per year for LN2 to generate Nitrogen for my fab. 
> Have any of you guys switched to onsite Nitrogen generator?
>
> Looking for ideas on:
>
> Cost of system
>
> Maintenance cost of system
>
>  Quality of the Nitrogen.
>
> Operating cost per CuFt of Nitrogen
>
> Rick
>
> Draper Laboratory
>
> Principal  Member of the Technical Staff
>
> Group Leader Microfabrication Operations
>
> 555 Technology Square
>
> Cambridge Ma, 02139-3563
>
> www.draper.com <http://www.draper.com>
>
> rmorrison at draper.com <mailto:rmorrison at draper.com>
>
> W 617-258-3420
>
> C 508-930-3461
>
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