I am afraid you will find that using those gases to etch silicon will also
etch your silcon nitride mask. You might be better off using photoresist as
your masking material. What shape are you looking for in the silicon etch.
Anisotropic or isotropic. How much undercut can you stand between interface
of the masking material and the silicon interface? Drop me a line and I will
discuss further with you. Bob Henderson