You might want to try hot phosphoric acid (180 degrees C) in a reflux type jar
to maintain evaporation rate of H2O. This process is very selective to oxide but
will remove silicon nitride from both sides of the wafer. If you use a thin
oxide underneath the silicon nitride you can safely etch the nitride with rie
etch using fluorine then wash away the oxide with BOE causing no damage to the
silicon. Bob Henderson