US Patent 8119531 Mask and etch process for pattern assembly

A method of forming a trench is provided that includes providing a stack having a semiconductor layer or dielectric layer, a metal nitride layer, a leveling layer, and a first mask layer. First trenches are etched through the first mask layer and the leveling layer. The first mask layer is removed. A second mask layer is formed on the leveling layer. Second trenches are formed through the second mask layer, wherein the base of the second trenches do not extend through the metal nitride layer. The second mask layer is removed. Exposed portions of the metal nitride layer are etched selectively to the semiconductor layer and remaining portions of the leveling layer to extend the first trenches and the second trenches into contact with an upper surface of the semiconductor layer.