Fabrication of Large Area Periodic Nanostructures Using Nanosphere Photolithography
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- Wu, W., Dey, D., Memis, O.G. et al. Nanoscale Res Lett (2008) 3: 351. doi:10.1007/s11671-008-9164-y
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Large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications, such as photonic band-gap materials, high dense data storage, and photonic devices. We have developed a maskless photolithography method—Nanosphere Photolithography (NSP)—to produce a large area of uniform nanopatterns in the photoresist utilizing the silica micro-spheres to focus UV light. Here, we will extend the idea to fabricate metallic nanostructures using the NSP method. We produced large areas of periodic uniform nanohole array perforated in different metallic films, such as gold and aluminum. The diameters of these nanoholes are much smaller than the wavelength of UV light used and they are very uniformly distributed. The method introduced here inherently has both the advantages of photolithography and self-assembled methods. Besides, it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes.
Large area periodic nanostructures exhibit unique optical and electronic properties and have been applied into many areas, such as photonic band-gap materials , high dense data storage , and photonic devices . To fabricate these periodic nanostructures, standard photolithography methods cannot easily reach the resolution required. High-resolution methods such as e-beam lithography and focal ion beam milling are too slow to reach a large area because of their inherent serial property. Nano-imprint methods are fast to be applied, but it needs to use the mold, which requires the same resolutions as the patterns. So, it also benefits from the development of fast, economic, and high throughput fabrication methods with a high resolution.
We have developed a novel photolithography technique, the Nanosphere Photolithography (NSP) technique [4, 5], which utilizes a self-assembled ordered monolayer of hexagonally close packed (HCP) micro-spheres as nano-jets  to generate sub-wavelength regular patterns over a large area on standard photoresist. Here, we will fabricate periodic metallic nanoholes perforated in gold and aluminum films using NSP technique. We used NSP to produce a large area of hexagonally packed nanopillars of negative photoresist with a strong undercut. Using these nanopillars, we produced large area uniform nanoholes perforated in different metal layers with controlled thickness by lift-off process. The diameter of the nanoholes is about 180 nm and the period of the hexagonal array is 1 μm, which is depending on the size of the microspheres used. The exposure wavelength we used for NSP is a broadband source centered about 400 nm, which is far greater than the pattern size.
We have presented a large area of uniform metallic nanoholes of about 180 nm produced by Nanosphere Photolithography technique with a broadband wavelength centered at 400 nm. Our simulation results show that even smaller nanoholes with tunable periods can be generated with a shorter wavelength. The techniquedemonstrated here supplies an alternative routine for manufacturing large areas of periodic nanostructures.