Micro-195-W wax works well as an etch barrier/stop for KOH at temperatures up
to at least 80 C. It can be applied by heating the part or wafer on a hot plate
to 100-125 C and then melting the wax directly from a stick onto the part. The
wax has a sharp melting temperature (195 F = 90 C) and low viscosity once it
melts, so it is easy to work with. It is most reliable to wax the piece to be
protected face-to-face with a wafer that is coated with silicon nitride or
another KOH-impervious layer. We have done this many times successfully with Si
wafers waxed to SiN-coated Si wafer. For consistent success, it is very
important to be make sure that both wafers are clean and have no KOH-etchable
layers extending to the edges that contact the KOH.

The part can then be removed by heating and melting the wax, and the wax
stripped off in a TCA reflux system, where it comes off in a couple of minutes.
In general, it seems to require a hot, chlorinated solvent to remove.

Micro-195-W is Walnut Hill's part number for a white, microcrystalline wax with
a 195 F melting temp. It is a purified form of the better-known "black wax,"
which may work just as well for this application (We haven't tried it). Walnut
Hill can be reached at 1-800-NEED-WAX! (no joke)

Stuart Wenzel
Berkeley MicroInstruments
Phone: (510) 231-5710
Fax: (510) 231-5711