A processing method is provided for plasma etching features in a silicon nitride (SiN) film covered by a mask pattern. The method includes preparing a film stack on a substrate, the film stack containing a SiN film on the substrate and a mask pattern on the SiN film, forming a plasma from a process gas containing HBr gas, O2 gas, and a carbon-fluorine-containing gas, applying pulsed RF bias power to the substrate, and transferring the mask pattern to the SiN film by exposing the film stack to the plasma.