Low Cost Method for Generating Periodic Nanostructures by Interference Lithography Without the Use of an Anti-Reflection
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Low Cost Method for Generating Periodic Nanostructures by Interference Lithography Without the Use of an Anti-Reflection Coating Omree Kapon1,2, Merav Muallem1,2, Alex Palatnik1,2, Hagit Aviv1,2 ,and Yaakov. R. Tischler1,2 1 Department of Chemistry, Bar-Ilan University, Ramat-Gan 5920002, Israel 2 Bar-Ilan University Institute for Nanotechnology and Advanced Materials, Ramat-Gan 5920002, Israel ABSTRACT Interference lithography has proven to be a useful technique for generating periodic subdiffraction limited nanostructures. Interference lithography can be implemented by exposing a photoresist polymer to laser light using a two-beam arrangement or a one-beam configuration based on a Lloyd’s Mirror Interferometer. For typical photoresist layers, an anti-reflection coating must be deposited on the substrate to prevent adverse reflections from cancelling the holographic pattern of the interfering beams. For silicon substrates, such coatings are typically multilayered and complex in composition. By thinning the photoresist layer to a thickness well below the quarter wavelength of the exposing beam, we demonstrate that interference gratings can be generated without an anti-reflection coating on the substrate. We used ammonium dichromate doped polyvinyl alcohol as the positive photoresist because it provides excellent pinhole free layers down to thicknesses of 40 nm, and can be cross-linked by a lowcost single mode 457 nm laser and etched in water. Gratings with a period of 320 nm and depth of 4 nm were realized, as well as a variety of morphologies depending on the photoresist thickness. This simplified interference lithography technique promises to be useful for generating periodic nanostructures with high fidelity and minimal substrate treatments. INTRODUCTION Today, interference lithography (IL) is at the heart of FinFET1 semiconductor device fabrication, in which it is used to fabricate sub-100nm device features over large area, which is critical for increasing transistor performance and device density, while reducing cost. IL is useful in fabricating photonic integrated circuits2-3, holographic gratings,4 and threedimensional polymer photonic crystal structures.5-6 IL has also been implemented as an attractive low-cost method for fabricating periodic nanostructures,7 due to the simplicity of not needing a photomask,8-9 the ability to pattern a complete substrate with a single exposure, and the periodicity of the obtained structures. IL is based on the principle of periodic constructive and destructive interference of two light waves. In 1967, IL was first demonstrated in photoresist for making diffraction gratings.10,11 In 1997, Berger et al. showed that a two dimensional pattern could be recorded using a set of three plane waves.12 In the last two decades, IL has been used in many applications such as micro-sieves,13 nanowires14 for future optoelectronic devices, and electrical circuit printing.15 A useful technique for generating diffraction gratings is a Lloyd’s Mirror Interferometer configuration4 with a single
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