ELEMENT ISOLATION FILM FORMING METHOD AND METHOD FOR MANUFACTURING SEMICONDUCTOR DEVICE USING THE METHOD

PROBLEM TO BE SOLVED: To provide an element isolation film forming method and to provide a method for manufacturing a semiconductor device using the film forming method. SOLUTION: This element isolation film forming method includes a step in which a semiconductor substrate having an active region and a field region is prepared, a step in which a first insulation layer and a mask layer are successively formed on the semiconductor substrate, a step in which the first insulation layer and the mask layer are patterned to form a trench on the semiconductor substrate, a step in which at least one step is formed on the semiconductor substrate in the upper part of the trench, a step in which a second insulation layer covering the inner surface of the semiconductor substrate defining the trench is formed, a step in which a liner layer is successively formed over the whole surface of the semiconductor substrate, a step in which a third insulation layer, thick enough to fill the trench, is formed over the whole surface of the semiconductor substrate, a step in which the third insulation film, the liner layer, and the mask layer are polished flat on the same level until the mask layer is partially exposed, and a step in which the first insulation layer is removed by etching.