A method of filling a trench in a substrate ensures that a void or seam is not left in the material occupying the trench. First, a preliminary insulating layer is formed so as to extend contiguously along the bottom and sides of the trench and along an upper surface of the substrate. Impurities are then implanted into a portion of the preliminary insulating layer adjacent the top of the first trench to form a first insulating layer having a doped region and an undoped region. The doped region is removed to form a first insulating layer pattern at the bottom and sides of the first trench, and which first insulating layer pattern defines a second trench. The second trench is then filled with insulating material.