Dry-etch selectivity

A method of etching a patterned substrate in a substrate processing region of a substrate processing chamber, wherein the patterned substrate has an exposed silicon nitride region and an exposed second material region, the method comprising:

• flowing a fluorine-containing precursor into a remote plasma region fluidly coupled to the substrate processing region while forming a plasma in the plasma region to produce plasma effluents;
• pulsing a power of the plasma at a plasma frequency with a plasma duty cycle;
• flowing the plasma effluents through a showerhead separating the remote plasma region from the substrate processing region;
• and
• etching the exposed silicon nitride region, wherein the substrate processing region is maintained substantially plasma free during the etching operation, and wherein the patterned substrate is maintained substantially unbiased during the etching operation.