> Is there any etchant that etches oxide but not nitride?

Any HF-based solution, such as 10:1 HF, 5:1 BHF, or 10:1 BHF
will etch thermal silicon dioxide, low-temperature LPCVD oxide, PSG, etc.
much faster than LPCVD silicon nitride (both stoichiometric and low-stress).

PECVD nitride is a much "lower quality" nitride than LPCVD, however,
any can etch almost as fast as oxide, depending on the deposition
conditions.
If you're using PECVD nitride and want a low HF etch rate,
deposit it at higher temperature to reduce the amount of hydrogen in the
material.
I've also found that silicon-rich PECVD nitride (with higher refractive
index)
can etch 100X more slowly than a low-index nitride.

For plasma etches, I don't know of anything that is very selective.

        --Kirt Williams Agilent Technologies