Heavy doping with Boron is an effective etch stop for KOH, but will of
course change the electrical properties of your membrane - I don't know if
this is a problem in your application. Additionally, I'm not sure what the
achievable penetration depth is for Boron doping - it might well be short
of 29um.

Another way of controlling membrane thickness is to simply measure it
while etching, although this requires some specialized equipment.
I know that some commercial pressure sensors are fabricated with this type
of setup. See for instance:

Adee and Steinbrüchel, Non-destructive, interferometric method for
measuring the thickness of a silicon membrane, Thin Solid Films
Volume 306, Issue 1, August 1997, Pages 171-173

Hope it helps.

// Morten

On Sun, December 20, 2009 8:24 pm, Andrea Mazzolari wrote:
> Hi all,
>
> i need to realize silicon memabranes of thickness 29um. I can not start
> from SOI wafers, cos the membrane will remain under stress state due to
> the oxide in the frame.
>
> I plan to realize the job by KOH etching, but how to precisely control the
> membrane thickness ? Is there any suggestion ?
>
> Thanks,
> Andrea