There could be also a different reason for these islands.
What does the bake module look like?
Is the volume between wafer and bake module cover very low?
It can happen that the solvent condenses inside the bake module at cold spots
and create droplets. Then, these solvent droplets can drip down on the wafer and
cause the islands.
It seems the bake module can handle the solvent fume volume with 2" wafers. But
with 4" wafers, there is too much of it.

First approach to solve this increasing the bake module's exhaust rate.

Best regards,
Norbert Nodes

E-Mail: [email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of ???(Ruilin Zheng)
Sent: Mittwoch, 02. Juli 2008 16:48
To: General MEMS discussion
Subject: [mems-talk] Islands found during SU-8 soft-bake

Hello, everyone,

I've got a problem with SU-8 soft-bake as I shift from 2inch silicon wafer to
4inch: some islands form on the surface during soft bake ( start to appear
at 65 Celsius degree, and remain after 95 Celsius baking ). Does this
problem have something to do with contamination on wafer?
At first, I thought it was caused by uneven solvent content of SU-8 stored
for two or three weeks in covered beaker. But I also tried the fresh
photoresist, and got the same result.

Does anyone have some ideas with this?