Do you bake the wafer before spinning the SU-8?  If not, there may be
adsorbed water which will evaporate when you pre-bake.  Also you can try
different temperature profiles.  With the postbake I have used a ramp from
50 to 95C, or a bake at 50C for a longer time.
Glen

At 07:38 PM 12/13/2003, you wrote:
>Dear all,
>i am using SU-8 25 to fabricate a 27 micron feature on a 3 inch
>glass wafer (Ti seed layer).After the completion of this process, i am
>fabricating a 200 microns thick channel as a second layer using SU-8
>50.The process parameters for the 200 microns are the following:
>200 process parameters (7ml SU-8 50)
>
>?
>Spin: 500rpm for 10sec (100rpm/s ramp)
>     1500rpm for 30 sec (300rpm/s ramp)
>i.e 500.100.10 and 1500.300.30
>
>?
>Leave for 1 min
>
>?
>Prebake : 65C for 6 minutes and 95C for 22 minutes
>?
>Exposure: 160microJ/cm2
>?
>Postbake: 2min at 65C and 95C for 7 minutes
>?
>Development: 8-10 min with EC Solvent, Clean with IPA afterwards
>
>The problem is that during the prebake phase,the su-8 wrinkles
>irreversibly.as a result the structures are very rough
>I would very much appreciate any help and/or recommendations on this
>problem.Thank you in advance for your help.I am looking forward to your
>reply.