An apparatus and method process a substrate in a first session and a second session. In the first session, a hybrid gas application cycle is performed in a chamber that holds the substrate. A first gas is introduced for a first time period so components of the first gas adsorb onto the substrate. Subsequently, a second gas is introduced for a second time period so the second gas reacts with the components of the first gas to provide a protective layer on sidewalls of a pattern of the substrate, and the second gas etches a bottom portion of the pattern, a ratio of the first time period to the second time period being a use-ratio. Then, in a second session, the hybrid gas application cycle is repeated with a different use-ratio that corresponds with a vertical dimension of the pattern.