semiconductor materials typically the first substrate 10 comprises a semiconductor substrate that has a generally conventional thickness each of the first dielectric layer 12 and the second dielectric layer 16 may comprise any of several dielectric materials non limiting examples include oxides nitrides and oxynitrides particularly of silicon but oxides nitrides and oxynitrides of other elements are not excluded the first dielectric layer 12 and the second dielectric layer 16 may comprise the same or different dielectric materials and may be formed using any of several methods non limiting examples include ion implantation methods thermal or plasma oxidation or nitridation methods chemical vapor deposition methods and physical vapor deposition methods typically each of the first dielectric layer 12 and the second dielectric layer 16 comprises a silicon oxide dielectric material formed to a thickness from 100 to 5000 nanometers the stop layer 14 typically comprises a hard mask material in general hard mask materials may be selected from the same group of dielectric materials as the first dielectric layer 12 and the second dielectric layer 16 however the stop layer 14 comprises a different material than the first dielectric layer 12 and the second dielectric layer 16 and thus the stop layer has a different relative etch rate.