Thanks for all your discussions and advices. I have solved the stick
prolem by extending the softbake time from 5mins to 30 mins at 95C.
and before the bake, I put my irregularly shaped sample on the wipe,
and use microscope glass silde to cut off the bead edge. This really
works, and also very easy to operate. I hope this could be some info
for later someone encounters the same problem.

But, after exposure, I measured the depth of my SU-8 2050 channel, the
profiler reads a depth of only 10um, while my spin speed is 4000rpm
for 30s which should be around 40um suggested by microchem
instruction. Who can explain this ?

Thank you very much

Steven