Various embodiments herein relate to methods, apparatus, and systems for etching a feature in a substrate. Typically the feature is etched in a dielectric-containing stack. The etching process involves cyclically etching the feature and depositing a protective film on sidewalls of the partially etched feature. These stages are repeated until the feature reaches its final depth. The protective film may have a particular composition, for example including at least one of a tungsten carbonitride, a tungsten sulfide, tin, a tin-containing compound, molybdenum, a molybdenum-containing compound, a ruthenium carbonitride, a ruthenium sulfide, an aluminum carbonitride, an aluminum sulfide, zirconium, and a zirconium-containing compound. A number of optional steps may be taken including, for example, doping the mask layer, pre-treating the substrate prior to deposition, removing the protective film from the sidewalls, and oxidizing any remaining protective film.