I'm extrapolating the results we have to answer your question. Firstly, we
haven't tried spinning up to 60 um (I hope you meant 60 microns and not 60 m).
The max we do by spinning is about 15-20 um. For higher thicknesses we cast
PMMA (I believe that the original recipe was developed at FzK). Without the
prebaking the samples crack.

The prebake conditions that we use are:

Ramp up:Room Temperature to 135 C @ 20C/hr.
Hold: @ 135 C for 2 hrs
Ramp down: to Room Temp. @ 10 C/hr

Samples after the pre-baking showed no signs of cracking. We use the same
recipe for cast PMMA too; it works just fine. Try spinning 5 um of PMMA at a
time than the 20 um.

I have also observed that excess development leads to cracking too. Try
reducing the time in the GG.

Hope this helps.

Sandeep
-------
Sandeep Akkaraju                           Ph: +1 504/388-6487 (Office)
Graduate Research Assistant                    +1 504/769-9487 (Home)
Microsystems Engineering Team,LSU          Fax:+1 504/388-5924
                                         WWW:http://www.lsu.edu/~muset/sandeep