US Patent 7419870 Method of manufacturing a flash memory device

Provided is a method of manufacturing a flash memory device. In the method, after forming a cell string and source/drain selection transistors, it forms a first oxide film in which a sidewall oxide film and a buffering oxide film are stacked, a nitride film, and a second oxide film for spacer on the overall structure. Then, source/drain contact holes are formed. Thus, the source/drain selection transistors are prevented from being exposed while etching the source/drain contact holes, which enhances the reliability of the flash memory device.