Hi Peng Li,
first of all without step 3 how long are you waiting to put the two PDMS slab in
contact?
As far I know the time is a big issue due to the hydrophobic recovery of the
PDMS after a O2 plasma treatment. It has been reported in different papers that
you have to put in contact the two PDMS slab within 1 min after exposure  to O2
plasma. So you should try just to put a drop of methanol between the layers and
do your alignement.

Hope that will help you!

Cheers,

Abdou