The present invention relates to a nonvolatile semiconductor memory apparatus capable of writing and erasing data and a method of manufacturing it and, more particularly, to a nonvolatile semiconductor memory apparatus in which a tunnel current flows through an insulating film comprising a relatively thick (400 to 2,000 angstroms) oxide film and charges are injected into a floating gate to write or erase data utilizing a phenomenon wherein an electric field tends to be concentrated on a roughened portion formed on the surface of a polycrystalline silicon (polysilicon) film, and a method of manufacturing it.
In the manufacture of a nonvolatile memory apparatus, when a polysilicon film to be used as a floating gate or a writing gate is formed, a roughened portion is formed on the surface of the polysilicon film. This is done to prepare a nonvolatile memory, i.e., an EEPROM. In such a memory, a predetermined voltage is applied to the polysilicon film portion to cause an electric field concentration phenomenon on the roughened portion, and even when a relatively thick (e.g., 800 angstroms) oxide film is formed as an electron discharge source, a tunnel current is flowed through the oxide film to electrically control erase and write operations.
However, in order to control the shape of the roughened portion on the surface of the polysilicon film, deposition conditions (e.g., a deposition temperature, a deposition rate, and the like) must be optimally set during a polysilicon deposition step. In addition, when oxidation conditions (e.g., an oxidation temperature, an oxidation atmosphere, and the like) are set during an oxidation step following the deposition step, a strict control operation is necessary. Therefore, when the roughened portion is formed on the surface of the polysilicon film, it is difficult to control its shape, and the resultant roughened shape often does not correspond to the target shape, thus impairing the reliability of the semiconductor memory apparatus.