A method for forming a low dielectric layer is provided to improve a gap-fill characteristic by performing a gap-fill process with a reactant having a liquefied property and hardening the low dielectric layer. A substrate loading process is performed to load a substrate including a plurality of patterns into an inside of a reaction chamber. The patterns are arranged apart from each other. A gap-fill process is performed to fill a gap between the patterns with a reactant by implanting a compound and H2O2 of a gas state into the inside of the reaction chamber. The reactant is formed by positioning -OH instead of -H disposed at an end of the compound. A hardening process is performed to harden the reactant between the patterns.
