US Patent 7332789 Isolation trenches for memory devices

Methods and apparatus are provided. A first dielectric plug is formed in a portion of a trench that extends into a substrate of a memory device so that an upper surface of the first dielectric plug is recessed below an upper surface of the substrate. The first dielectric plug has a layer of a first dielectric material and a layer of a second dielectric material formed on the layer of the first dielectric material. A second dielectric plug of a third dielectric material is formed on the upper surface of the first dielectric plug.