I suspect you will have a problem with this since many people have tried it and
it hasn't worked. PECVD nitride has many pinholes and is not an effective KOH
etch mask. Only LPCVD nitride has been shown to be effective against a 80C KOH
etch etch for an extended period of time (a few hours or more). And
unfortunately it gets deposited at 800C -- beyond the meting point of most
metals used in silicon devices. People are currently trying TMAH (tetra methyl
ammonium hydroxide) as an etch since it attacks metals less.

I got around this in my thesis using a one-sided etching apparatus to do
backside etching in KOH while protecting the frontside. It's the only effective
way of protecting it against such a rigorous etch. There's always EDP and oxide
as a mask, but it's carcinogenic and dangerous to use.

The problem you face has been faced many times before by others, and I haven't
seen a suitable masking material, or solution yet. Hopefully others may be able
to direct you to different methods I'm not aware of. The best thing I've seen
is a one-sided etching apparatus. If you have access to the web, check out
http://cecelia.media.mit.edu:8080/htdocs/paper.html for a paper on it. Good
luck.

- Joe