Method of quasi-atomic layer etching of silicon nitride

A method of etching, comprising:

• providing a substrate in a plasma processing apparatus, wherein the plasma processing apparatus includes a first power source and a second power source, and the second power source operates at a higher frequency than the first power source, and wherein the substrate includes a first material containing silicon nitride and a second material that is different from the first material;
• forming a first chemical mixture in the plasma processing apparatus by plasma-excitation of a first process gas consisting essentially of hydrogen and optionally a noble gas;
• exposing the first material on the substrate to the first chemical mixture in the plasma processing apparatus;
• after the exposing the first material to the first chemical mixture, forming a second chemical mixture in the plasma processing apparatus by plasma-excitation of a second process gas containing N, F, O, and optionally a noble element;
• exposing the first material on the substrate to the second plasma-excited process gas in the plasma processing apparatus to selectively etch the first material relative to the second material;
• and
• wherein during the forming of the first chemical mixture and exposing the first material to the first chemical mixture, the second power source is off.