Kris,

When I have observed bubbles in the photoresist after exposure that had
not been there before, there were only two real explanations I have
observed (although there may be others).

1) Insufficient removal of solvent. I realize from your description that
the bake seems pretty extensive, but the two steps you perform may
actually be conflicting.  The first step (long spin) causes solvent to
evaporate preferentially from the outer surface of the resist.  This
means that the outer surface will dry more rapidly, causing it to become
an anti-diffusion layer, trapping additional solvent within the lower
layers of the film, potentially even during the bake.  You may instead
wish to stop the spin after the film thickness has been established,
followed by a ramped proximity bake (i.e., place the wafer onto
proximity pins that will suspend it above the plate at a fixed distance
for a certain period of time, and then ramp that gap over time until
contact is made). This will cause the primary driving force for solvent
removal to occur from the bottom, thus causing it to continuously
migrate upwards.  You may also want to consider raising the bake
temperature.

2) To aggressive of an exposure - the exposure is heating up the resist
or causing nitrogen to evolve.  There are a couple solutions to this.
One would be to reduce the exposure intensity.  The second would be to
allow for cool down or diffusion by performing an interval exposure.
This is where the exposure occurs in several partial exposure steps (or
intervals).  Some aligner systems (such as the EVG620) can do this in an
automated manner, allowing programming of both the interval dose/time,
and the "rest" time between exposures, all while maintaining a specified
contact mode.

Best Regards,
Chad Brubaker


-----Original Message-----
From: kris
Sent: Friday, November 04, 2005 3:51 PM
To: mems-talk@memsnet.org
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.