Method of forming a gate layer with multiple ecthing steps

A method of manufacturing a semiconductor device comprising:

• forming a first insulating film over a semiconductor substrate;
• forming a first silicon film over the first insulating film;
• forming a second silicon film over the first silicon film;
• forming a mask layer in a predetermined pattern over the second silicon film;
• first etching the second silicon film in a depth such that the first silicon film is not exposed, in a first condition while using the mask layer as an etching mask;
• second etching a remaining portion of the second silicon film and the first silicon film in a depth such that the first insulating film is not exposed, in a second condition which gives a higher vertical etching component ratio than the first condition;
• and
• third etching a remaining portion of the first silicon film in a third condition of which a ratio of an etching rate for the first silicon film to an etching rate for the first insulating film is larger than that of the second condition,
• wherein an impurity concentration of a first conductivity type in the first silicon film is higher than an impurity concentration of the first conductivity type in the second silicon film.