Guiding Polymers to Perfection: Macroscopic Alignment of Nanoscale Domains

Nanoscale diblock copolymer domains are aligned via top-down/bottom-up hierarchical assembly. Grating substrates template cylinder alignment with demonstrated 5000:1 aspect ratio for 100 μm domains extendable to arbitrary length scales. Depending on trough depth and amount of deposited polymer, aligned domains are (1) confined to the channels or (2) expanded across the grating frequently with (3) a complete absence of defects. This methodology can be exploited in hybrid hard/soft matter systems for electronics, catalysis, and sensors. Fabrication of macroscopic domains of periodic nanoscale structures using self-organizing systems has garnered significant attention because of the simplicity and low cost of the method.1 Ultrathin diblock copolymer films, in particular, are promising candidates for bottom-up nanotemplates in hybrid organic-inorganic electronic,2 optical, and magnetic3-5 devices. These systems self-assemble into microphase separated domains with a length scale tunable through the heart of the nanoscale (10-100 nm); however, without further constraint, the domains have no preferred orientation and form a disordered “fingerprint” structure.6,7 Localized alignment of cylindrical domains has been reported using solvent prewetting,8 electric fields,9 directional crystallization,10 and other schemes. These approaches, though, tend to produce either only short-range order or random domain orientation. In this paper, we present a new general method for defining an orientation and eliminating defects via the introduction of geometric substrate anisotropy.11 Graphoepitaxy12-14 is applied to the cylindrical diblock copolymer phase to overcome the stubborn disorder intrinsic to this structure. Three novel results will be presented: (1) alignment of cylindrical polymer domains in confined volumes, (2) extension of this substrate-induced alignment above and beyond the confined volumes, and (3) virtually defect-free domains in contrast to results obtained using other cylinder alignment techniques. Alignment is achieved using lithographically assisted self-assemblysa combined top-down/ bottom-up methodology. Diblock copolymer composed of polystyrene and polyisoprene, denoted PS-b-PI, with a molecular weight of 22 000 g/mol and a polydispersity of 1.08, was modified to polystyrene-block-poly(ethylene-alt-propylene) (PS-b-PEP) by selective hydrogenation of the polyisoprene block.15 PSb-PEP has 27 wt % PS so that, in the bulk, the copolymer forms hexagonally packed PS cylinders in the PEP matrix. The spacing between the cylinders is 26.6 nm and the natural thickness of one layer of cylinders, L, equals 23 nm as determined by a calibrated AFM measurement. Thin films of this diblock copolymer were spin-cast (at 5000 rpm) from 1.55% toluene solutions onto flat and topographically patterned silicon nitride substrates. These substrates, containing varied square wave grating patterns, were prepared in a silicon nitride layer by electron beam lithography using a Hitachi S-2700 scanning electron microscope (SEM) and reactive ion etching (Figure 1b). These films consist of cylindrical microdomains of PS embedded in a PEP matrix when annealed (115-135 °C) above the glass transition temperature under an argon atmosphere. Each grating contained troughs of different widths ranging from 200 nm to 1.5 μm in increments of 100 nm; the trough length was always 100 μm. Crest widths were a constant ∼750 nm. Gratings of two different depths, 35 and 95 nm, were used for these experiments. These depths were selected to comfortably accommodate one layer and three layers of cylinders, respectively. Before discussing the behavior of PS-b-PEP thin films on patterned substrates, it is useful to review their behavior on a typical flat silicon nitride substrate. PS prefers to wet the silicon nitride/polymer interface whereas PEP exhibits an * Corresponding author. E-mail: [email protected]. † The University of Chicago ‡ Argonne National Laboratory. NANO LETTERS 2004 Vol. 4, No. 2 273-276 10.1021/nl035005j CCC: $27.50 © 2004 American Chemical Society Published on Web 12/25/2003 Report Documentation Page Form Approved