Selective Deposition of Passivating Layer During Spacer Etching

A method for processing a substrate includes: forming a mandrel over the substrate including an underlying layer, the mandrel having a top surface and sidewalls, the substrate including an exposed surface including a portion of the underlying layer; conformally depositing a spacer material over the substrate, the spacer material covering the top surface and the sidewalls of the mandrel and the portion of the underlying layer; in a plasma processing chamber, exposing the substrate to a plasma generated in the plasma processing chamber from a first halogen-containing process gas, a second halogen-containing process gas, and a carbon-containing passivating agent, the exposing anisotropically etching the spacer material; and removing the mandrel to form free-standing spacers from sidewall portions of the spacer material covering the sidewalls of the mandrel.