Block copolymers have been extensively studied for their ability to self-assemble into microdomain morphologies such as spheres, cylinders, and lamellae, with typical periodicities of 20-100 nm. Similar structures form when block copolymers are deposited as thin films on substrates; for example, very asymmetric block copolymers form spheres of the minority component in a matrix of the majority block, while less-asymmetric materials typically form cylinders in plane. These films can serve as excellent templates for nanofabrication, where the block copolymer’s nanodomain structure is faithfully reproduced in an inorganic material—but the final array of inorganic objects is, at best, only as good as the structure of the film from which it was derived. Consequently, we have worked intensively to develop methods to and manipulate the structure of the films. For example, the polygrain structure which these nanodomains normally form can be transformed to a single-crystal texture, over macroscopic areas, by a simple shearing process. Shear can also realign the domain orientation locally in films with an otherwise macroscopic orientation; to create complex orientation patterns on the millimeter scale; and even to transform spheres into cylinders. For spheres, we can exploit the selective wetting behavior of the two blocks to adjust the areal density of spheres (micelles) by at least an order of magnitude, all while keeping the micelle size fixed, simply through control of the film thickness.
We have employed these thin, substrate-supported block copolymer films to fabricate dense arrays of 20-40 nm metal or semiconductor particles: dots (from sphere-forming block copolymers) or lines (wires, from cylinder-formers), all with a size and spacing set through block copolymer molecular weight. As a particular example, we have used this approach on cylinder-forming films to fabricate centimeter-scale arrays of parallel metallic nanowires, with 33 nm pitch; due to their fine pitch, such wire grids can polarize an exceptionally broad range of wavelengths extending down into the deep ultraviolet (for 193 nm photolithography). We have also begun studying the shear-alignment of perpendicular ("standing") lamellae, which offer the possibility of higher aspect ratios.
See more of this Group/Topical: Materials Engineering and Sciences Division