Block Copolymer Self-assembly on Ethylene Glycol (EG) Self-assembled Monolayer (SAM) for Nanofabrication
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Block Copolymer Self-assembly on Ethylene Glycol (EG) Self-assembled Monolayer (SAM) for Nanofabrication Dipu Borah1,2,3, Sozaraj Rasappa1,3, Barbara Kosmala1,2, Justin D. Holmes1,2,3, and Michael A. Morris1,2,3* 1 Department of Chemistry, University College Cork, College Road, Cork, Ireland 2 Tyndall National Institute, Lee Maltings, Dyke Parade, Cork, Ireland 3 Centre for Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin, Ireland ABSTRACT Nanostructure templates fabrication from P(S-b-MMA) thin films requires precise control of interfacial energies to achieve perpendicular orientation of microdomains to the substrate surface and can be obtained by modifying the oxide layer on silicon with a covalently anchored hydroxyl-terminated random copolymer P(S-r-MMA) termed a “neutral brush”. This commonly employed method enables precise fine-tuning of interfacial energies, but involves a lengthy process, requires starting materials that are commercially available but expensive, and results in a relatively thick under layer that can interfere with subsequent surface processing. We report here the microphase separation behaviour of an asymmetric P(S-b-MMA) diblock copolymer on electronic substrates modified with ethylene glycol (EG) self-assembled monolayer (SAM) as alternative to standard random copolymer brush. The diblock copolymer films deposited on EG SAMs upon thermal annealing spontaneously generates features with sub-lithographic resolution and pitch with perpendicular orientation. Selective etching provides a rapid route for the generation of PS template structures as the PMMA domains are etched at a faster rate. These templates can subsequently be used as etch masks to generate nanoscale features. We use state of the art lithography to generate sub-μm features and within these generate nm sized copolymer templates. Graphoepitaxy method proved a successful approach for the alignment of the microphase separated structures. This method of EG SAM driven self-0assembly provides a simple, rapid, yet tuneable approach for surface neutralization and nanofabrication technique for creating high density nanoscale features for the nanoelectronic industry. INTRODUCTION Diblock copolymer self-assembly at interfaces enables the generation of nanoscale structures in a parallel, scalable, bottom-up fashion [1]. Thin films of poly(styrene-b-methyl methacrylate), P(S-b-MMA) diblock copolymer of approximate volume ratio 70:30 on silicon substrate can be used to generate nanoporous templates [2, 3]. The cylindrical PMMA microdomains within these films are induced to align perpendicular to the interface. The PMMA domain can subsequently be removed either by dry etching or by wet chemical etching. The remaining cross-linked PS forms a stable thin film with pores that are tens of nanometers in center-to-center spacing, depending on the diblock copolymer molecular weight. These nanoporous templates have many more applications in the fabrication of semiconductor capacitors [4], metallic [5], and magnetic [6, 7]
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