Narrow etched gaps or features in multi-period thin-film structures

A method, comprising:

• forming a first multi-layer film on a substrate;
• etching the first multi-layer film to form first structures;
• planarizing the first structures with a planarizing material;
• removing a portion of the planarizing material to expose a top surface of each of the first structures;
• forming a second multi-layer film on the remaining planarizing material and the top surfaces of the first structures;
• and
• etching the second multi-layer film to form second structures, each of the second structures being aligned with and in contact with the top surface of a corresponding one of the first structures and forming a multi-period structure therewith, formation of the second structures thereby resulting in a plurality of multi-period structures, a first multi-period structure of the multi-period structures being separated from an adjacent multi-period structure by a spacing across all layers of the first multi-period structure.