The fastest solution that I know is mixture of 25:14:10 of Nitric Acid (wt 70%),
Acetic Acid (wt 100%) and Hydrofluoric Acid (wt 48%). Etching is in room
temperature.
The etch rate of n-type Si is about 13um/min, etch rate of SiO2 is 90 times
lower than Si and etch rate of  Silicon Nitride is about 11A/min.

Regards,

Andrzej Prochaska