A method of forming a film on a substrate that includes an etching layer and a mask formed on the etching layer. The method comprises (a) exposing the substrate, in a reaction chamber, to a precursor to dispose precursor particles on at least a sidewall of a recess in the etching layer; (b) supplying an inhibitor gas and a modification gas to the reaction chamber to generate a plasma; and (c) modifying the precursor particles on the sidewall into a protective film while the inhibitor gas and the modification gas are supplied in the reaction chamber.