Patent ID: 8809199

Claim:
A method for processing a substrate, comprising: preparing a film stack on a substrate, the film stack containing a silicon nitride (SiN) film on the substrate and a mask pattern on the SiN film; forming a plasma from a process gas containing HBr gas, O 2 gas, and a carbon-fluorine-containing gas; and transferring the mask pattern to the SiN film by exposing the film stack to the plasma, wherein the transferring comprises: etching through less than an entire thickness of the SiN film in a main etch (ME) step, where a first pulsed RF bias power is applied to the substrate during the ME step; and thereafter, etching through a remaining thickness of the SiN film and stopping on the substrate in an over etch (OE) step, wherein a second pulsed RF bias power is applied to the substrate during the OE step, the first pulsed RF bias power being greater than the second pulsed RF bias power, and wherein the transferring forms a protection layer on the mask pattern when RF bias power is not applied to the substrate, the protection layer protecting the mask pattern when the RF bias power is applied to the substrate and increasing the etch selectivity of the SiN film relative to the mask pattern.