Hi all,

as first thanks for your useful suggestions.

I need membranes of thickness 29+/-0.5um, lateral size of the membranes is
5x5 mm^2

Here is how i decided to go on: i deposited LPCVD silicon nitride (low
stress).

I patterned the nitride.

I've started to etch the silicon with KOH 40% at 80 deg. After about 1
hour etching i measured the membrane thicness.

I've found that all the membranes are 8um thinner in proximity of the
membrane frame! I'm very surprised of this! Central region of the membrane
is thicker than the region close to the border!

At this point i continued etching at room temperature (25 deg). After 24
hours etching i measured again the membrane thickness, and found again
that all the membranes are still 8um thinner in proximity of the borders.

Why this strange behaviour ??

Thanks!
Andrea