Nanostructures on microstructured surfaces

Many applications require the combination of micro- and nanostructures. One interesting combination is to provide the master of a Fresnel lens (microstructure) with a moth-eye anti-reflective layer (nanostructure) (Wilson and Hutley in Opt Acta 29(7):993-1009 1982). Then in replication processes both the Fresnel lens and the anti-reflective layer are replicated in one process step. For example Fresnel lenses can be found in overhead projectors. It is state of the art to endow the planar backside of such a Fresnel lens with an anti-reflective layer. For the front side one would have to put the moth-eye nanostructure onto the Fresnel microstructure. But how to generate the combination of micro- and nanostructures, especially on such a large area? Interference lithography is suited very well for the generation of nanostructures on large areas. It provides a great depth of focus and can be used to generate surface relief nanostructures on already (pre-)microstructured surfaces. Unfortunately, by using conventional interference lithography processes for this purpose several negative effects during lithography prevent a successful exposure. In addition to that spin coating of photo resist on non-planar surfaces results in uneven resist thicknesses throughout the substrate. We have developed a novel, easy process that overcomes all the disadvantages mentioned above and furthermore the process works with almost arbitrary and varying resist thicknesses throughout the substrate. By using this novel process we were able to combine a moth-eye nanostructure with a Fresnel microstructure.