Silicon dioxide is an example of a suitable material for layer 28.The following are examples of the thicknesses of layers 20-28 which have been found to be useful for the fabrication of devices; layer 20 50 to 80 nanometers, layer 22 370 to 400 nanometers, layer 24 90 to 110 nanometers, layer 26 160 to 200nanometers, and layer 28 20 to 50 nanometers, with about 70, 385, 100, 180, and 40 nanometers