In a thin film stack deposited on a substrate, a first layer can have a non-uniform thickness. A second layer, disposed so that the first layer is between the substrate and the second layer, can have a non-uniform thickness. A first pattern can be formed on the second layer to define first areas. The second layer can be etched in the first areas to form first holes having varying depths. A second pattern can be formed on the first layer in the first holes to define second areas. Each second area can be smaller than a corresponding first area. The first layer can be etched in the second areas to form second holes having varying depths. An imprint of the etched thin film stack can be formed such that the first holes and the second holes form a plurality of diffractive elements having varying sizes on the imprint.