Dear Yang fu,

I have faced these problem during my work.  I would like to suggest you to
reduce the Hard baking timing. for first hard bake. your first Pr layer is
undergoing many thermal cycles, which is causing the bubbles.
As you mentioned that the hard bake temp is 130c. you can reduce it to 110C.
During the second PR process   the reduce the timing to half of regular one.

with these i got good result without disturbing the structure.
Best of Luck

-Abhay Joshi
University of Pune.