Guys,

I have a project to be completed by next weekend. Here are the details,

I have to make micron length scale cantilever beams of silicon. So I =
have a 400 um thick silicon wafer with a micron thick layer of oxide on =
both sides. I have patterned the back side oxide into windows that will =
be etched to go all the way through the wafer. The front side is =
patterned into beams. I would like to etch the silicon from the front =
side to go as deep as 30 um and the back side to etch all the way =
through =96 about 400 microns. Our cleanroom has a table top technics =
RIE machine with SF6 and CHF3 gases and we have KOH for wet etching.=20

Could you please suggest an etch recipe in which the oxide stays atleast =
till the end of the etch step? I find that all the oxide is etched away =
in 50% KOH @ 90 degrees C in less than 2 hours. I would appreciate any =
kind of help or advice.

Nagappa