Embodiments relate to a flash memory device and a method of manufacturing a flash memory device that may improve a reliability of process by obtaining a Depth of Focus (DOF) in an exposure process. In embodiments, a method may include sequentially stacking an oxide film, a floating gate poly film, an ONO film, a control gate poly film, and a BARC (Bottom AntiReflect Coating) on a semiconductor substrate, forming a photoresist pattern for a stack gate on the BARC, and etching the BARC, the control gate poly film, the ONO film and the floating gate poly film at once by using the photoresist pattern until the oxide film is exposed.