Elena,
You will have to mix the solution - you can try a magnetic stirrer  - the
best is a recirculation pump. The reaction is exothermic so the solution
heats up near the wafer. Also, it is warmest at the top of the vessel.

Shay

-----Original Message-----
From: [email protected]
Subject: [mems-talk] back-side etch: uniform temperature

Does any one know of heaters (say a ceramic plate that is submerged
vertically in solution) that provide uniform temperature throughout the
etching solution? I am doing a back side etch on an 8'' wafer, using a very
large container filled with TMAH, but the wafer etches differently at the
bottom and at the top. Has any one run into this problem and figured out a
solution?