Abstract: The method for forming an isolation region in the present invention mainly includes the following steps. First, a pad layer is formed on a semiconductor substrate and an oxidation masking layer is formed on the pad layer. The oxidation masking layer, the pad layer, and the substrate are then patterned to form trenches in the substrate. The pad layer is removed laterally to form undercut structures under the oxidation masking layer. A doped layer is conformably formed on the oxidation masking layer, the undercut structures of the pad layer, and the substrate in the trenches. Next, a thermally oxidizing step is carried out to oxidize the doped layer to form an oxidized layer conformably on the oxidation masking layer, the undercut structures of the pad layer, and the substrate in the trenchs. A dielectric layer is formed over the substrate to fill up the trenches and cover over the pad layer and the oxidation masking layer. The dielectric layer is planarized downward to portions of the oxidation masking layer.