Semiconductor devices and methods of fabricating semiconductor devices that may include forming an insulation structure including insulation patterns that are sequentially stacked and vertically separated from each other to provide gap regions between the insulation patterns, forming a first conductive layer filling the gap regions and covering two opposite sidewalls of the insulation structure, and forming a second conductive layer covering the first conductive layer. A thickness of the second conductive layer covering an upper sidewall of the insulation structure is greater than a thickness of the second conductive layer covering a lower sidewall of the insulation structure.