Disclosed is a method of fabricating a semiconductor device. The method may include forming a mold layer on a substrate, the mold layer having a hole exposing a portion of the substrate, forming a phase transition layer with a void, in the hole, and thermally treating the phase transition layer to remove the void from the phase transition layer. The thermal treating of the phase transition layer may include heating the substrate to a first temperature to form a diffusion layer in the phase transition layer, and the first temperature may be lower than or equal to 55% of a melting point of the phase transition layer.