Hi Li Wang,

at our lab at ETH Zurich, we use two protection schemes in parallel:

1) We use a mechanical wafer holder (from AMMT, www.ammt.de) that
protects the backside and the edge of the wafer by a special triple
O-ring clamping scheme. The advantage of their design is the way that
the two holder pieces fit together. The clamping stress is minimized
(we can work with wafers down to 200 µm thick) and there is no risk
when tightening the screws (no torque wrench or special pattern
necessary). Also, a backside venting hole avoids pressure built-up
during heating of the assembly in the KOH bath.

2) For the cases where we overetch and membranes are etched through,
we apply a protective PMMA coating on the wafer front side as a safety
measure. The material is called X AR-PC 5000/4 and can be obtained
from Allresist (www.allresist.de) in Germany. However, it withstands
KOH only for a brief time and only serves as a backup. For longer
etching processes I'd always recommend a chuck solution as above.

I hope that helps,

Jan

*******************************

Dr. Jan Lichtenberg
Team Leader Biosensor and Microfluidics Group

Physical Electronics Laboratory
ETH Zurich
Hoenggerberg, HPT H4.2
CH-8093 Zurich, Switzerland

Phone:  +41-1-633 2187
Fax:    +41-1-633 1054
E-mail: lichtenb@iqe.phys.ethz.ch

*******************************

Saturday, June 5, 2004, 11:21:11 PM, you wrote:

LW> Hello,

LW> I want to protect my front side during KOH etching because there is
LW> polysilicon on it. I designed a jig which seal the wafer with O-ring and
LW> sandwiched the wafer in between two plastic pieces, fastened with six
LW> screws. But my problem is that the forces on the screw is kind of difficult
LW> to control and the wafers were broken several times in testing when the
LW> wafers were etched thinner. Another way I tried is PDMS protection. I put
LW> PDMS on the front side. The problem I met is that the PDMS will be detached
LW> from the wafer after I put the wafer into hot KOH for 2 hours. Anyone has
LW> any idea of KOH protection, please email me to help me out. Thanks a lot.

LW> Li Wang