Enhanced order of block copolymer cylinders in single-layer films using a sweeping solidification front

The use of block copolymer (BCP) thin films as self-assembled 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 lithography 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 domains (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 orientation, 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 photoalignable side groups; ([14,15]) and shear can align BCP cylinders ([16]) and spheres ([17,18]) over square-centimeter areas, with an orientation 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 concentration, zone-casting from solution can produce macroscopically-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.