[Fwd: Re: Chemical exposure]

Mary Tang mtang at snf.stanford.edu
Tue Aug 17 08:26:15 PDT 2004

Hi all --

Here's some more detail from Scott Andrews about how he got a good whiff
of SPR-220.  I'm just about to ask Maureen to make up some colored,
laminated signs which say "Avoid Exposure to Fumes:  Open Slowly" which
I'd like to post on the 90 C and 110 C ovens (maybe the little blue M
too?), the flammables cabinets and the flammables refrigerator.


-------- Original Message --------
Subject: Re: Chemical exposure
Date: Mon, 16 Aug 2004 21:17:38 -0700 (PDT)
From: "Scott D. Andrews" <sandrew at stanford.edu>
Reply-To: Scott Andrews <scott.andrews at stanford.edu>
To: Mary Tang <mtang at snf.stanford.edu>

Hi Mary,

> When transferring wafers straight from spin (when the resist is still wet) to
> either hot plates or the post-spin-bake oven, wafers should be placed inside
> an enclosed box.

When transferring the wafers, I was using a single wafer box to carry
over. I still smelled a little, but I assume that was contamination on
tweezers or gloves.  As I said, this was really minor, and I wouldn't be
too concerned about any exposure here.  Perhaps you can answer one more
question, though.  I have wondered about the best procedure when
trasfering full wafers.  Can they be put back in the vertical holders
main boxes that hold 25 wafers) after spinning?  I was always concerned
about screwing up the spin at the edge where it touches the box and also
contaminating my box which would then spread vapors for days.  I have a
few reservations too about using the single holders -- although that is
usually what I do.  Since occassionally, small amounts of resist go onto
the backside of the wafer either due to contaminated tweezers or a bad
spin, putting it in a box can cause it to become sticky and again vent
days.  Do you have any good suggestions on how to handle this?

> Was your exposure during the post-spin-bake?  If so, which stations were you
> using and what was your procedure?

All the exposure I had was during post-spin baking.  I was spinning very
thick resist (SPR-220, 1 krpm, 30 sec) as a protection layer.  I was
using the headway, hot plates, and the white ovens next to it. 
Initially, I
was using 90 degrees and then transferred to the 115 degree oven for the
final bake.  My exact procedure was:

1) Put blue tape on back of my wafer (needed because it had free
membranes that would have been destroyed by the vacuum)
2) Spin SPR-220, 1 krpm, 30 sec
3) Remove tape
4) Move to hot plate, bake for 100 seconds at 90 degrees
5) Spin SPR-220, 1 krpm, 30 sec on backside
6) Move to 90 degree oven for ~30-45 minutes (Since this was a
layer, I didn't care if I had the proper time, I just needed it dry)
7) Move to 115 degree oven for another 30 minutes or so

> Another observations I've had is how many of us open the doors on the
> flammables cabinets (and may apply here as well)...  The exhaust is static and
> when a door is opened quickly, it counters the exhaust, potentially drawing
> fumes across the person opening the door...

I TOTALLY agree about the flammables cabinets.  I've have always been
concerned about that.  I didn't realize that the speed of opening the
cabinets makes a difference.  I always assumed the exhaust simply wasn't

> I'll try this out in the litho area a few times, and if this helps, I'll
> put some signs up ("open the door slooowwwwllly") and add it to our
> safety training info.

Please let me know what you find.  One recommendation on the sign is to
indicate why one should open it slowly too.  Since the processes people
use in the cleanroom are complex often information seems to get lost as
why certain thing need to be done.  For example sometimes I thought the
order of certain operations such as opening certain valves were critical
to get a machine to operate safely/properly.  Only later when I
the process more, did I realize it was just the order the trainer had
happened to use during training.  I think knowing why helps us all
the equipment and ourselves if we know why certain things need to be
For example, if the sign only said "open the door slowly," I would
that the door was fragile and watch for something wrong near the hinge.
Thus, if I still smelled fumes, I would not realize I had opened it too
fast.  Also, with regards to the furnaces, this may very well be part of
the problem.  After getting several whiffs of the fumes, I tended to
the door as quickly as possible, insert the wafers, and close it quickly
with the thought that the less time the door was open, the less time for
fumes to escape.

> I'm really sorry this happened, and truly appreciate you letting us
> know...

I realize thing like this happen.  I just appreciate you taking this so
seriously and helping make the lab a safe place for us to work.

> Knowing that you are a conscientious person in the lab, I'm concerned that
> there is something wrong in our procedures and/or our equipment setup.  Again,
> if you could provide a little more detail (which stations, etc.) and insight
> as to what you feel might help, it would be much appreciated...

I really think I was doing everything generally right.  Especially after
breathing the fumes the first time, I tried to be very careful because I
knew it wasn't healthy.  As I mentioned, the only stations were the
headway, the nearest hot plates, and the two white furnaces.

Thanks for your help.  Let me know if there is any other info I can


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