A fabrication method of an integrated circuit semiconductor device includes: forming a plurality of low dielectric pattern apart from each other on a substrate, the plurality of low dielectric pattern having a lower dielectric constant than the substrate; after forming the low dielectric pattern, forming a flow layer to bury the low dielectric pattern on the substrate; forming an epitaxial layer on the flow layer; and forming a transistor in the substrate comprising the low dielectric pattern buried by the flow layer and in the epitaxial layer.