Method for forming trench isolation structure

A method for forming a trench isolation structure is provided. A patterned mask layer with an opening is formed on a substrate. The substrate is etched to form a trench in the substrate under the opening. A first pullback process is performed to the patterned mask layer to broaden the caliber of the opening. The trench is partially filled with a first dielectric material, in which a recess is formed on the first dielectric material. A second pullback process is performed in the patterned mask layer to broaden the caliber of the opening again. An etching back process is performed in the first dielectric material to broaden the caliber and depth of the recess. The trench is fully filled with a second dielectric material.