Dear Bin:
        The problem may cause by the CTE (coefficient of thermal
expansion ) mismatch between silicon wafer and SU8 film. To reduce
thermal stress, you could try softbake/PEB/hardbake at lower temperature
lasting longer time.     Or you could bake the SU8 film with gradually
increasing temperature to avoid thermal shock. When you cool down the
SU8, try to just turn off the hotplate and let SU8 remain on the
hotplate in atmosphere.  Using natural cooling may improve your SU8
process.

And many researchers mention that DO NOT use HMDS.  It don't work with
SU8.

Good Luck~
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Department of Mechanical Engineering, NCTU.
1001 TA HSUEH ROAD, HSINCHU, TAIWAN 30056,ROC
Chun-Wei Chang