Dear Harish Hi!!! How thick is your Diaphram? Depending on that you would
have to etch it in 2 or 3 steps.
You can also use TMAH to etch your Silicon. I have used TMAH to etch Si to
get a membrane of 20 microns. KOH will give you a rough surface compared to
TMAH and so if that is not a problem then you could use KOH. Unpolished
wafer is no problem for KOH etching. Unpolished wafer can be a problem for
lithography. YOu would have to probably expose your wafer 2 times and this
means extra work because you have to align 2 times and develop two times.
Oxide is a good mask as long as it is thick. I use 15 min dry, 120 min wet
and then 30 min dry (1100C) for oxidation. This gives around 321nm thick
oxide. If you have to etch for a long time then best is to do etching for 5
to 6 hours then remove the oxide and oxidize it again and then again etch it
in KOH or TMAH.

Hope this helps.

Arti Tibrewala
[email protected]