Thank you guys for your suggestions.

It seems the SU-8 was not softbaked for a long enough time. I tried a longer
time than recommended and this time it is not soft and sticky. However, I do get
some other problems in softbake. I saw a lot of divots forming in softbake.

The normal cleaning process using acetone IPA and DI water works well for my Cr
mask with SU-8 residue on it.

Peeling SU-8 off from 4'' silicon wafer with Polystyrene and toluene still make
me headache. The SU-8 is very crispy, which breaks into pieces when stripping it
off. I think it may be caused by the uneven surface with a lot of divots. It may
also be contaminated by Polystyrene during softbake. My peeled off SU-8 is
translucent rather than transparent. Any other thoughts?