DOI: 10.1002/adma.200602904 Enhanced Order of Block Copolymer Cylinders in Single-Layer Films Using a Sweeping Solidification Front** By Dan E. Angelescu,* Judith H. Waller, Douglas H. Adamson, Richard A. Register, and Paul M. Chaikin The use of block copolymer (BCP) thin films as self-as- sembled templates has become increasingly popular as an economical nanofabrication technique, with new applications and techniques constantly being developed. This bottom-up approach to nanofabrication is extremely versatile; BCP films have been used as sacrificial contact masks for thin-film li- thography to produce nanometer-scale periodic patterns in a wide variety of materials, [1–3] which have been investigated in the data storage sector for high-density magnetic [4,5] and nano-crystal FLASH memories. [6,7] While the nanoscale do- mains (spheres, cylinders, etc.) typically organize into grains of micron or smaller size, with no overall orientation, several methods have been developed to direct the domain orienta- tion, or simply increase the grain size, in BCP films. In the first category, epitaxy [8,9] can direct the alignment over large areas, but requires substrates pre-patterned at the nanometer scale; graphoepitaxy [10–12] creates needle-like grains very long in one dimension, but at most a few microns wide; electric fields [13] generate alignment parallel to the field direction over square- micron areas; polarized light can create arbitrary orientation patterns in liquid-crystalline block copolymers with photoa- lignable side groups; [14,15] and shear can align BCP cylinders [16] and spheres [17,18] over square-centimeter areas, with an orien- tation specified by the shear direction. In the second category, uniform solvent annealing [19] can increase BCP grain size to a few microns without imposing a preferential direction to the pattern; by creating a moving gradient in solvent concentra- tion, “zone-casting” from solution can produce macro- scopically-aligned specimens. [20] Similarly, uniform thermal annealing [21,22] increases the grain size without imposing a preferential direction, but the grain size grows only as the 1 / 4 power of annealing time; the effect of a moving temperature gradient on a block copolymer thin film has not been reported previously. Hashimoto et al. [23] showed that sweeping a strong tempera- ture gradient through a bulk specimen of a lamellar BCP, such that the material is heated above its order-disorder transition temperature, can produce strong alignment; as the material cools in the gradient, ordered lamellae grow from the disor- dered phase, with the lamellar planes aligned normal to the direction of the moving gradient. Though directional solidifi- cation has not been reported for neat BCP thin films, direc- tional crystallization of a small-molecule solvent from a BCP solution has been reported by Thomas and co-workers; [24,25] crystallization of the solvent concentrates the BCP and drives it to order, similar to what zone-casting [20] achieves through solvent evaporation. When the solvent is directionally crystal- lized on a cold substrate, an aligned BCP film is left on the surface of the frozen solvent, but with a defect density higher than obtained with the methods discussed in the preceding paragraph. Our present study demonstrates the dramatic reduction in defect density that can be achieved in single-layer films of a neat cylinder-forming BCP through the application of a sweeping thermal solidification front. Additionally, we dem- onstrate that ordering can be further enhanced when the so- lidification front is advanced in an oscillatory fashion, rather than simple unidirectional motion. To the best of our knowl- edge we are the first to apply such thermal treatments to thin films of “soft” matter bereft of atomic-scale crystalline order, despite routine application of similar techniques in metallurgy and semiconductor fabrication to purify, uniformly dope, or create single crystals of “hard” materials, [26] or in polymer science to create directionally crystallized polymer samples. [27] Figure 1 displays a schematic of the setup used to apply such strong, slowly-advancing thermal gradients to thin (30 nm thick) BCP films. The substrate is an oxide-covered silicon wafer, onto which heater and sensor elements are mi- crofabricated and the block copolymer thin film deposited. The inset in Figure 1 shows an optical micrograph of the cen- tral area of the wafer, which incorporates the heater and the resistive temperature detectors (RTDs). The resistive heater COMMUNICATION Adv. Mater. 2007, 19, 2687–2690 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2687 – [*] Dr. D. E. Angelescu [+] Department of Physics, Princeton University Princeton, NJ 08540 (USA) E-mail: angelscu@alumni.princeton.edu J. H. Waller Department of Materials Science, Oxford University Oxford OX1 3PH (UK) Dr. D. H. Adamson Princeton Materials Institute, Princeton University Princeton, NJ 08540 (USA) Prof. R. A. Register Department of Chemical Engineering, Princeton University Princeton, NJ 08540 (USA) Prof. P. M. Chaikin Department of Physics, Princeton University Princeton, NJ 08540 (USA) [+] Current address: Schlumberger-Doll Research, 1 Hampshire St., Cambridge, MA 02141, USA. [**] We gratefully acknowledge financial support from the National Science Foundation through the Princeton Center for Complex Materials (DMR-0213706).