Morten is right - heavily boron-doped silicon etches much slower than
undoped or lightly-doped silicon in anisotropic wet etchants like KOH and
TMAH, and makes a nice etch-stop.  There is a significant amount of
literature out there on this subject.  Keep in mind that because boron is a
substitutional impurity and has a smaller covalent radius than silicon,
boron doping creates a residual tensile stress in the silicon.

The more significant issue is creating an etch-stop at your target thickness
of 29µm.  I would say that thermally diffusing boron from a solid or gaseous
source would be prohibitively time consuming (email me if you'd like a
detailed explanation).  I can't speak to the feasibility of ion implantation
because I don't know what kind of penetration depths can be achieved with
ion beams.

I fabricated ~5um thick membranes as part of my MS thesis research.  I doped
one side of the wafer with boron to create an etch stop, and I measured the
etch progress by periodically removing the wafers from the etchant
(refluxing 25wt% TMAH @ 90°C) and measuring the etch depth with a stylus
profilometer.  This method, or something similar (without the etch-stop),
could work for your application provided that you know the initial thickness
of your wafer with an accuracy of +/- 1µm or so.  Please contact me if you'd
like to discuss this further.

Good luck,

--
Brian C. Stahl
Graduate Student Researcher
UCSB Materials Research Laboratory
brian.stahl@gmail.com / bstahl@mrl.ucsb.edu
Cell: (805) 748-5839
Office: MRL 3117A


On Mon, Dec 21, 2009 at 12:10 AM, Morten Aaroe  wrote:

> 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