Kris,

You may not be spinning at the wrong speed.  Instead, you may have some issue
with solvent evaporation during the spin-off.  SPR220 (I am guessing the 7.0µm
version) is classified as a thick resist material.  As such, it has a relatively
high solids content (~40%), which means it has a correspondingly low solvent
content.

When looking at a chart of viscosity vs. solids content, materials such as
SPR220 are beginning to enter a region where small changes in solids content
results in a large change in viscosity.  This becomes a problem during spin-off,
especially at high speeds. Basically, the upper region of the resist on the
wafer begins to dry rapidly, at which point the viscosity of this "skin"
increases, slowing the flow.  However, the material underneath still retains its
lower viscosity, allowing it to flow freely.

The interaction between these two layers can often lead to wrinkling and
streaking of the resist surface.

The solution to this is to create a solvent-rich environment within the coat
module.  This is achieved by a combination of an integrated cover for the
module, coupled with exhaust flow control within the chamber (as is found on the
EVG150 resist processing system).

Best Regards,
Chad Brubaker

-----Original Message-----
From: kris
Subject: [mems-talk] SPR 220 on AU

I was trying to spin SPR220 on sputtered AU. The
thickness is around 5.5u. I was using AP140 as an
adhesion promoter. The substrate was dehydrated before
spinning AP140.

The PR on the substrate appears to be kind of wavy. I
could see streaks shooting at the edges from the
center. After exposing the PR i could observe that PR
shrinks at some places and results in kind of bubbles.

I was wondering if i am spinning at wrong speeds??