Flash memory devices and fabrication method thereof

A method for fabricating a flash memory device, comprising:

• providing a semiconductor substrate;
• forming a first polysilicon layer on a surface of the semiconductor substrate;
• forming a hard mask layer on the first polysilicon layer;
• forming a plurality of first openings to expose the first polysilicon layer in the hard mask layer and first polysilicon silicon layer by etching the hard mask layer and the first polysilicon,
• forming a plurality of grooves in the semiconductor substrate by etching the semiconductor substrate along the first openings;
• forming liner oxide layers by oxidizing the first polysilicon layer exposed by the first openings;
• forming shallow trench isolation structures covering the liner oxide layers by filling the first openings and the grooves with an isolation material;
• forming second openings by removing the hard mask layer and the non-oxidized first polysilicon layer;
• forming a tunnel oxide layer on the surface of the semiconductor substrate at a bottom of each of the second openings;
• and
• forming a floating gate on each of the tunnel oxide layers.