Dear Yufeng,

I'm not sure whether I understood your problem correctly nor do I have
experience with PDMS; but I can tell you that we have often the problem
to etch only one side of a wafer in KOH, by protecting the other side of
getting in contact with the KOH. We employ a wafer chuck from idonus
(www.idonus.com) which seals one side of the wafer. This means that you
can have virtually anything on one side of the wafer being protected
from KOH. Hopefully this helps you.

Regards,

Thomas Overstolz, PhD student
University of Neuchatel
Institute of Microtechnology
Sensors, Actuators and Microsystems Laboratory
Optical MEMS division