Sorry, I left out an important point about Si membranes and fracture
strength.

At Redwood Microsystems, if our target was 50 um, we would etch to about
70 um thick using KOH.  Then, we would etch the last 20 um (10 um per
side on a two-sided etch) in HF:nitric:acetic (HNA).  This latter etch
is isotropic, and left rounded fillets at the corners/edges of the
membrane.  Such rounded fillets were essential to realizing the
strongest membranes possible.  Without the fillets, the fracture
strength was quite low, as you would expect for any sharp edge or corner
in crystalline silicon.

I should add, it is more difficult to control HNA than KOH.  Best to buy
a pre-mix from a commercial source (don't mix your own), and again use a
temperature-controlled bath.

Al

---
Albert K. Henning, PhD
Director of MEMS Technology
NanoInk, Inc.
215 E. Hacienda Avenue
Campbell, CA  95008
408-379-9069  ext 101
ahenning@nanoink.net