Hello,
     Everybody!Thank you for reading my problem!
     And I really appreciate Lydia, Thomas and huy vo's advices!

     Maybe I didn't express my problem clearly.
     My wafer is a 2-side oxidation Si wafer with abou 2um SiO2 on both sides.
     The PDMS layer covering the electroplated structures is one part of my
final device. The PDMS layer does not  only serve as protection of the
electroplated structures.

   And ,when I etch the Si wafer from the back side ,I use a chuck to protect
the front side.

   The problem occurs when the 500um Si is etched completely, the KOH solution
will overcome the bonding force between the PDMS layer and Si wafer.
   I wonder why the seed layer was stripped off and how KOH goes through to the
other side of the wafer. (The seed layer was sputtered on the 2um SiO2.)

  Helping your advice!
  Thank you!
                Yufeng Su