Methods of selectively etching silicon nitride

A method of selectively etching silicon nitride relative to silicon oxide, the method comprising:

• flowing a fluorine-containing precursor and a first gas mixture into a semiconductor processing chamber containing a substrate;
• forming a plasma of the fluorine-containing precursor and the first gas mixture;
• flowing a sulfur-containing precursor and a second gas mixture into the semiconductor processing chamber, the sulfur-containing precursor and the second gas mixture present in a ratio of sulfur-containing precursor to second gas mixture in a range of from 0.01 to 5;
• forming a plasma of the sulfur-containing precursor and the second gas mixture;
• and
• etching the substrate, the substrate having a plurality of alternating layers of silicon oxide and silicon nitride thereon and a trench formed through the plurality of alternating layers, and the silicon nitride layers are selectively etched relative to the silicon oxide layers at an etch selectivity of greater than or equal to 500:1, wherein the semiconductor processing chamber is maintained at a pressure in a range of from 5 millitorr to 100 Torr and a temperature of less than or equal to 500° C.