Methods for isotropic etching at least a portion of a silicon-containing layer on a sidewall of high-aspect-ratio (HAR) apertures formed on a substrate in a reaction chamber are disclosed. The HAR aperture formed by plasma etching a stack of alternating layers of a first silicon-containing layer and a second silicon-containing layer, the second silicon-containing layer is different from the first silicon-containing layer. The method comprising the steps of: a) introducing a fluorine containing etching gas selected from the group consisting of nitrosyl fluoride (FNO), trifluoroamine oxide (F3NO), nitryl fluoride (FNO2) and combinations thereof into the reaction chamber; and b) removing at least a portion of the second silicon-containing layers by selectively etching the second silicon-containing layers versus the first silicon-containing layers with the fluorine containing etching gas to produce recesses between the first silicon-containing layers on the sidewall of the HAR aperture. Alternatively, the disclosed etching processes are cyclic etching processes.