Method for forming field oxide film in semiconductor device

A method for forming field oxide films in a semiconductor device, comprising the steps of:

• forming a first insulating film on a semiconductor substrate;
• forming a second insulating film on the first insulating film, said second insulating film being different from said first insulating film in material;
• etching the second insulating film, the first insulating film and the semiconductor substrate, in sequence, within element-isolating regions, to form a narrow trench and a wide trench;
• forming a third insulating film on the surfaces of the trenches through thermal oxidation;
• surface-treating the exposed surfaces of the third and the second insulating films in such a way that a subsequent insulating film might be deposited at rapid rate on the third insulating film but at slow rate on the second insulating film;
• depositing a blanket of a fourth insulating film over the resulting structure until the upper surface of the fourth becomes flat at a level higher than the height of the second insulating film;
• thermally treating the fourth insulating film to make it fine and dense;
• subjecting the fourth insulating film to etch back;
• and
• removing the second insulating film so as to create an element-isolating film structure in which the trenches are filled with the fourth insulating film.