Abstract: Fabrication of a bottle-shaped trench is disclosed. A semiconductor substrate with a trench therein is provided. An ion-doped barrier layer is formed in the trench, exposing the upper portion surfaces of the sidewall of the trench. An ion implantation is performed on the upper portion surfaces of the sidewall of the trench to reduce the oxidation rate in the substrate near the upper portion of the trench. The ion-doped barrier layer is removed, exposing the lower portion and bottom surfaces of the sidewall of the trench. A thermal oxidation treatment is performed, forming an oxide layer on the surface of the trench. The thickness of the oxide layer on the upper portion of the sidewall surface is much thinner than that of the oxide layer on the lower portion of the sidewall surface or that of the bottom surface. A bottle-shaped trench is formed by removing the oxide layer.