I have done similar processes and was able to get roughly 250um on a
single spin.  My spin speed was about the same as yours.  My suggestion
would be to decrease your spin time.  With the thicker resist spin time
has a dramatic effect on film thickness.  I would try a spin time of
40-60 seconds. This was done on 8" substrates so there may be some
adjustments to spin speeds and time.  Having the relax step to even out
the surface is also a very good step to improving your uniformity.

In addition I'm not sure what effect it will have but the recommended
developer for SU8 is PGMEA, not Acetone.

Best Regards,
Mike Sexton

EV Group
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Process Technology Engineer - Direct: +1 (480) 305 2450, Main: +1 (480)
305 2400 Fax: +1 (480) 305 2401
Cell: +1 (602) 625 1275
E-Mail: [email protected], Web: www.EVGroup.com

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Owen The
Sent: Sunday, March 23, 2008 10:16 PM
To: [email protected]
Subject: [mems-talk] SU8 Processing

Hi all, I've been having some trouble with my SU8 Processing.
Currently I'm using SU8 2100. I'm trying to get a postbaked thickness
of 260um, but I've been consistently getting half the quoted thickness
(130um in this case) for any given speed.

Here's my processing steps:

1) Wafer prep
2) Dispense 4ml of SU8 on a slowly spinning wafer (to center the blob)
3) Ramp to the set speed (in this case 1000 RPM), hold for a few minutes
and then stop
4) Rest in a covered petri dish on top of a small filter with a little
acetone (to even out the surface)
5) Softbake (ramping to set temps)
6) Expose and postbake (ramps to temps again)
7) Develop in acetone, rinse with isopropanol and water

Not sure what I'm missing, but I've done this process with other
grades of SU8 and have always seen 50% of the quoted film thickness.