A manufacturing method of a flash memory device is provided to improve an interference effect between a cell and a cell by lowering an effective field oxide height. A tunnel insulation film(102), a first conductive film, and a device isolation mask are formed on a top of a semiconductor substrate(100). The device isolation mask is formed by laminating a buffer insulation film, an etch stop film, and a hard mask film. A trench(114) is formed by etching the semiconductor substrate of a device isolation region. An insulation film is formed on a top of the device isolation mask including the trench. A first oxide film(118) is formed by oxidizing a surface of an exposed first conductive film. A high dielectric insulation film(120) and a second oxide film(122) are successively formed on a top of the first oxide film and a device isolation film(116a). A second conductive film is formed on a top of the second oxide film of the high dielectric film.
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