The present application discloses a method for fabricating the semiconductor device including providing a substrate in a reaction chamber, forming an untreated silicon nitride film on the substrate, and forming a treated silicon nitride film on the untreated silicon nitride film. Forming the untreated silicon nitride film includes the steps of: (a) supplying a first silicon precursor into the reaction chamber, thereby allowing chemical species from the first silicon precursor to be adsorbed on the substrate, and (b) supplying a first nitrogen precursor into the reaction chamber, thereby nitriding the chemical species to deposit resultant silicon nitride. The step (a) and the step (b) are sequentially and repeatedly performed to form the untreated silicon nitride film. Forming the treated silicon nitride film includes the steps of: (c) supplying a second silicon precursor into the reaction chamber, thereby allowing chemical species from the second silicon precursor to be adsorbed on the untreated silicon nitride film, (d) performing a first hydrogen radical purging by supplying hydrogen radicals into the reaction chamber to reduce impurities in the chemical species from the second silicon precursor, and (e) supplying a second nitrogen precursor into the reaction chamber, thereby nitriding the chemical species from the second silicon precursor to deposit resultant silicon nitride. The step (c), the step (d), and the step (e) are sequentially and repeatedly performed to form the treated silicon nitride film. The untreated silicon nitride film and the treated silicon nitride film together form a silicon nitride layer.