Dear Fellows,

Thanks for your response and advice. I tried several parameters before,
time in air before soft bake (SB), temp of SB, time of SB, UV exposure
time, and mode of exposure contact. Yet none of them work.

Finally I realize the bubbling phenomenon is due to the nucleation or
trapping of gas on the substrate, so there is something with the
surface. Here I succeeded to avoid bubble formation after exposure by
“using prime grade wafer, heating up wafer at 150oC before or after
HMDS, then blow the surface with N2”. Prime grade wafer and HMDS are not
the main factors since I used them yet still see bubbling. I guess
“heating at 150oC” is the main solution to bubbling. Ironically
blowing
with surface N2 gas before spin coating does not foster bubbling.

Regards,
Zeta