Hello Craig,

Is it very important for your aplication to make the membrane out of Silicon? I
think you could play with the number of cycles to achieve a fairly thin membrane
(I am not very sure that the membrane would be the same thickness across the
entire area...it tends to be etched more in the center and lesser as you go
towards the edge)

One of the alternatives (if you can afford to have a different material ) is to
use to LPCVD silicon nitride on the other side. I have been etching through 300
micron silicon wafer to reach the nitride layer on the other side which behaves
as a very good etch stop (so even if you over do the DRIE cycles to get rid of
all the silicon acrros the area of the hole), the membranes are still very flat
and uniform.

I thought of sharing it would you since I am doing something similar ...working
with 3 microns nitride membranes...though it might be completely irrelevant to
your application as the nitride is non-conductive...well...but discussing idea
never hurts:)

Thanks

Ashish