Development of Block Co-Polymers as Self-Assembling Templates for Magnetic Media and Spin-Valves
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0941-Q06-10
Development of Block Co-Polymers as Self-Assembling Templates for Magnetic Media and Spin-Valves Vishal Vitthal Warke1, Chris Redden1, Martin Bakker1, David Nikles1, Kunlun Hong2, Jimmy Mays2, and Phil Britt2 1 Department of Chemistry and MINT Center, University of Alabama, Tuscaloosa, AL, 35487 2 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, TN, 37831
ABSTRACT Poly (Styrene)-Poly (Methylmethacrylate) block co-polymer (PS-b-PMMA) of 36000 g/mol molecular weight, with a composition of 67% PS and 33% PMMA, Polydispersity index of 1.09 were synthesized. Thin films of ca. 30 nm thickness were prepared by spin-coating from toluene solution onto a range of substrates including are carbon overcoated hard drive substrate and Indium Tin Oxide (ITO) glass. After annealing, AFM showed that the polymer had nanophase separated into cylinders of PMMA in a continuous film of PS, as expected from the composition of the polymer. On the carbon coated substrate annealing produced significant dewetting of the substrate to give micron scale holes where the substrate was not covered by the polymer. In the covered areas the orientation of the nanophase was PMMA cylinders perpendicular to the substrate. Films on ITO were predominantly oriented perpendicular to the ITO surface. Washing with glacial acetic acid resulted in rearrangement of the PMMA component to open pores in the film. Electrodeposition of nickel onto this substrate lead to nickel pillars of estimated 18 nm diameter. INTRODUCTION Block copolymers (BCP) are macromolecules consisting of two or more chemically different polymer segments of a single type of monomer unit, covalently bonded together. Thin films of block copolymers have been studied extensively because of their potential applications as templates to fabricate nanostructured materials. Some block copolymers can self-assemble into well-defined structures. The shape and size of the structures formed by using block copolymers can be easily manipulated by changing the properties of the block copolymer e.g. molecular weight, weight fraction of each block, and segmental interaction parameter (the FloryHuggins parameter χ)1-3. The interaction parameter decides the lower size limit of the microdomain, and the mobility of the segments limits the largest size that can be achieved. Block copolymer lithography can be used to fabricate patterned dot arrays of magnetic and semiconducting materials1, 4-6. Orientation of the polymer domains is important to their use as templates. In general it is the parallel orientation of the domains that is generally thermodynamically more stable. However, it is diblock copolymers with cylindrical microdomains oriented perpendicular to the surface they are attracting most attention from researchers. The spatial orientation of block copolymers can be controlled by external fields such as electrical fields 2, 7-9, 13, 19 or by
controlling the interfacial interactions, e.g. using a random copolymer brush10. Selective solvent vapor annealing11 can a
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