You can simply use a hot plate for heating so long as you re-circulate
the solution. You can do this by using a small pump with the input tube
at the bottom of your container and the return at the top of the
container. Use the shortest tubing possible. Also use a hotplate with a
spinner so you can put a magnetic stir bar in the bottom for added
agitation, and maybe a tube with n2 bubbling for extra agitation. This
will all improve the temp and mixture stabilization. Once final note,
use a lid with a hole for a probe for constant feed back to the
hotplate, the lid will help to keep the TMAH from evaporating away,
which changes etch rate, and surface roughness as the solution depletes.

Many of our researchers have performed 8" etches in a large container
with the wafer horizontal, using a Teflon holder to keep the wafer off
the bottom. In this instance recirculation is not necessary, just a stir
bar and the lid setup. Horizontal immersion keeps the wafer close to the
heat source thus making a smaller region to control.

Good luck.

Joe Lonjin
Penn State Nanofabrication Facility

-----Original Message-----
From: [email protected] [mailto:[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?