In a method of fabricating a semiconductor device, isolation structures are formed in a substrate to define active regions. Conductive structures are formed on the substrate to cross over at least two of the active regions and the isolation structures, the conductive structures extending in a first direction. An interfacial layer is conformally formed on the substrate in contact with the conductive structures. A first insulation layer is provided on the interfacial layer, wherein the first insulation layer is formed using a flowable chemical vapor deposition (CVD) process, and wherein the interfacial layer reduces a tensile stress generated at an interface between the conductive structures and the first insulation layer while the first insulation layer is formed.