Method of manufacturing a semiconductor device

A method of manufacturing a semiconductor device, the method comprising:

• forming a mask pattern having a first opening partially exposing a surface of a substrate;
• forming a trench in the substrate using the mask pattern;
• forming an insulating pattern in the first opening and the trench;
• removing the mask pattern to form a second opening exposing an inchoate active region of the substrate;
• forming a first silicon layer, on the exposed active region and the insulating pattern, having an opened seam overlying the inchoate active region;
• forming a silicon oxide layer on the first silicon layer;
• heat treating the first silicon layer at a temperature sufficient to induce silicon migration so as to cause the opened seam to be closed via the silicon migration, wherein the first silicon layer is heat-treated under an atmosphere including hydrogen, and the silicon migration is promoted by a reduction reaction between hydrogen and the silicon oxide layer;
• forming a second silicon layer on the first silicon layer after having closed the opened seam;
• and
• performing a planarization process until the insulating pattern is exposed such that a remainder of the first silicon layer represents a floating gate pattern in the second opening.