Published on September 5, 2008
Slide 1: Fabrication of large area periodic nanostructures using Nanosphere Photolithography Wei Wu, Dibyendu Dey, Omer G. Memis, Alex Katsnelson and Hooman Mohseni Northwestern University July 2008 Slide 2: 2 Overview Introduction Simulations Experiments Results Conclusions Slide 3: 3 Introduction: nanofabrication methods Large areas of periodic nanostructures have many potential applications: from surface plasmons to ion pumps. Conventional top-down methods: either expensive or low-throughput Bottom-up methods: low cost, high throughput, but not well-developed Slide 4: 4 Novel methods: to bridge the gap between top-down and bottom-up methods Micro- and nano-spheres: uniform sizes and can be assembled into large areas of hexagonally close packed (HCP) monolayer Nanosphere lithography (NSL): use the vacancy between nano-spheres to generate nano-patterns Intro: Micro-/nano-spheres based lithography Slide 5: 5 λ=365 nm ~150 nm Intro: Micro-/Nanosphere photolithography Utilizes self-assembled ordered monolayer of silica micro-spheres to focus UV light and generate sub-wavelength periodic nanopatterns over a large area on photoresist Each single silica micro-sphere acts as a super-lens to focus the UV light Full width at Half Maximum (FWHM) of the focused light is much smaller and the intensity is much stronger. Slide 6: 6 Modelling & Simulations We used 3D-FDTD calculations to simulate the focus process of the micro-spheres. Normalized Intensity distribution with different sizes of spheres The FWHM (full width at half-maximum) of the focused light are almost independent of the sphere sizes. Sphere sizes deviation tolerant Highly uniform nanopatterns generated Slide 7: 7 Normalized intensity distribution with different wavelengths FWHM of the focused light becomes smaller as the wavelength gets smaller. (~203 nm down to ~121 nm) Even smaller nano-features can be obtained. 3D-FDTD calculations to simulate the focus process of the micro-spheres with different wavelengths of the light. Modelling & Simulations Slide 8: 8 Experiments Processing steps for NSP Slide 9: 9 Results: micro-sphere monolayer 10 μm A large area of hexagonally close packed (HCP) monolayer of silica spheres on the photoresist by the convective self-assembly method. HCP monolayer of spheres on top of photoresist Enlarged tilted view of micro-spheres on photoresist Slide 10: 10 Results: Nanoholes in positive photoresist 1 µm 250 nm Highly uniform nanoholes with high aspect ratio Larger nanoholes with the same periods Very uniform nanoholes with a high aspect ratio formed; the size of the holes can be changed with different exposure and development time. 1 µm Slide 11: 11 Results: controllable process Lattice period of the nanohole arrays is controlled by microsphere sizes Size of the nanoholes is controlled by exposure and development. Wu et al, Nanotechnology, 2007, 18, 485302 Slide 12: 12 Results: lift-off for gold nano-posts 5.0 μm 2.0 μm Using lift-off process with the positive photoresist holes, highly uniform gold nano-post arrays Gold post array by lift-off process Enlarged view of gold nanopillars Slide 13: 13 Results: nanopillars of negative photoresist 5.0 μm ~200 nm Applies into negative photoresist, highly uniform nanopillar arrays of photoresist was produced. An array of photoresist nano-pillars after development Enlarged view of the pillars Slide 14: 14 Results: lift-off for gold nanoholes 5.0 μm ~180 nm By use of the negative photoresist posts, highly uniform arrays of gold nano-holes were fabricated. Hexagonally packed gold array perforated with holes Enlarged view of the holes in gold film Slide 15: 15 Conclusions A mask-less deep sub-wavelength photolithography technique Convergence between the top-down and bottom-up approaches Fast, economical, high throughput and compatible with current photolithography technique Feature size as small as 180 nm, even smaller nano-features with a shorter wavelength Slide 16: 16 Questions? Thank you for your attention!