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Bit patterned media, including media fabricated with a gradient in composition, is being developed as a potential path to higher information storage density. The noise level in such media is significantly impacted by the precision of the ordering of the individual bits and by the narrowness of their size distribution. Block copolymers that phase separate on the appropriate length scale are one method of pattern generation that is receiving considerable attention. For cylinder forming block copolymer phases the ordering and degree of perpendicular alignment is largely determined by the matching of the substrate surface to the block copolymer. If the chemical properties of the substrate surface match the average for the block copolymer, then thin films of the block copolymer align perpendicularly on annealing. Although there are a number of examples where the substrate surface fortuitously matches the block copolymer, in general an orienting layer is necessary to provide the appropriate match. The most popular approach has been to synthesize a random copolymer with the same average composition as the block copolymer. In order to produce suitably thin orienting layers it has been necessary to chemically tether the random copolymer to the substrate. Previously used chemistry has not been suitable for noble metal substrates such as platinum. We have been developing an alternate approach using thiol functional groups which we anticipate will be more suitable for Pt capped substrates.
Bit patterned media, including media fabricated with a gradient in composition, is being developed as a potential path to higher information storage density. The formation of metal nanopillars with 20-30 nm repeat spacing and precisely controlled magnetic properties presents a significant challenge to current fabrication methods. We have been developing cylinder forming block copolymer phases as a method to generate the desired patterns coupled with the processing steps necessary to transfer the pattern into magnetic material. This involves spin coating of the polymer on an appropriate orienting layer, annealing to allow the pattern to form by self-organization of the block copolymer, solvent processing to remove the minority domain, electrodeposition to form a hard mask, followed by ion-milling to transfer the pattern to the magnetic material. We have demonstrated each step in this process and report on the quality of the pattern achieved.
Block copolymers that self-organize are of interest as templates for patterned media, as they potentially provide a low cost fabrication route. Poly(styrene)-Poly(methylmethacrylate) block co-polymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio self-assemble into a 2-D hexagonal phase in which the PS majority phase is continuous and surrounds cylinders of the minority, PMMA phase. For application of this phase to patterned media it is necessary that the cylinders of the minority phase be oriented perpendicular to the substrate surface. This can be achieved by a number of methods, including appropriate choice of substrate and use of a random co-polymer underlayer. Appropriate substrates include H-terminated silicon, some carbon coatings and some ITO glasses. Use of an acetic acid wash causes the minority PMMA component can be induced to be rearranged, giving rise to pores perpendicular to the substrate. Electrodeposition of a metal into the pores produces a hardmask which can be used with ion-milling to transfer the block co-polymer pattern onto a magnetic thin film.
Poly(styrene)-Poly(methylmethacrylate) block co-polymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio self-assemble into a 2-D hexagonal phase in which the PS majority phase is continuous and surrounds cylinders of the minority, PMMA phase. By UV irradiation and washing with acetic acid it is possible to remove the minority phase to leave empty channels. It is also possible to rearrange the PMMA phase with acetic acid to leave somewhat smaller pores. For most substrates the interactions between the polymer and the substrate surface are such that one block is preferentially adsorbed to the substrate resulting in alignment of the PMMA domains parallel to the substrate surface. It is possible to orient the polymer perpendicular to the surface by first adding a thin film of a random PS-PMMA co-polymer before applying the PS-b-PMMA block co-polymer. However thin films of the random PS-PMMA do not give good surface coatings, and thicker films are generally too thick for the pores in the PS-b-PMMA block co-polymer to be propagated to the substrate surface. For a few substrates, thin PS-b-PMMA films naturally adopt a perpendicular orientation after annealing, washing with acetic acid produces arrays of pores of diameter as small as 3 nm. For a number of other substrates the interaction between the polymer blocks and the surface is such that upon annealing the polymer rearranges to form micron sized domains which are not polymer coated, surrounded a areas which have a thicker polymer coating. We have observed this behavior with both carbon coated substrates and with ITO glass substrates. In both cases the areas of polymer are perpendicularly oriented, and upon washing with acetic acid give rise to pores that extend completely through the polymer film. In some cases films on ITO glass are continuous even after annealing. After washing with acetic acid it was possible to electrodeposit nickel into the pores to give nickel nano-pillars of 18 nm diameter.
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