An apparatus for processing a substate includes: a chamber having a gas inlet and a gas outlet; a substrate support disposed in the chamber; a plasma generator; and a controller programmed to: (a) place a substrate on the substrate support, the substrate having a pattern, (b) supply a first reactive species into the chamber to adsorb the first reactive species onto the pattern of the substrate, (c) partially purge the first reactive species from the chamber to adjust an amount of a residual first reactive species in the chamber, (d) supply a second reactive species into the chamber, and (e) expose the substrate to a plasma generated from the residual first reactive species and the second reactive species by the plasma generator to form a film on the pattern of the substrate.