The bubbles you see are probably nitrogen bubbles. If you have an automated
resist dispenser and use nitrogen to pressurize the line you may have it
turned up too high. I've been told that nitrogen is a product of exposure,
which combined with any excess dissolved nitrogen in the resist, will
produce bubbles.

Try waiting a couple of hours between softbake and exposure. It'll give it
time for the nitrogen to come out of solution. Just like a scuba diver. You
have a pretty thick layer of resist, so you may even want to give it a day
or two.

I've also been told that softbake-develop-expose-develop also works. But I
don't actually believe this myself.

Gabriel


---------- Forwarded message ----------
From: kris 
To: mems-talk@memsnet.org
Date: Fri, 4 Nov 2005 14:50:37 -0800 (PST)
Subject: [mems-talk] Bubbles in Photoresist
Hello All,

I am following the following steps for AZP4903 recipe.
I have bubbles problems

[1]. Dehyrate Sio2 wafer at 150C.
[2] Spin HMDS for 4000 r.p.m for 40 sec.
[3] spin AZP4903 to shoot for the thickness of 15
microns. Spin is usually done for longer periods of
time to evaporate all the solvent in the photoresist.
[4} Softbake the wafer from room temperature to 110C
on contact hot plate. This takes almost 10 mins.

when i expose the photoresist to the light(25mW/cm2).
I have seen bubbles in the PR which were not observed
during the softbaking step.

Can anyone suggest me on this.

Thanks,
Kris