An attractive "unconventional" lithographic technique to pattern periodic, sub-100 nm features uses self-assembled block copolymer thin films as etch masks. Unfortunately, as-cast films lack the orientational and positional order of the microphase-separated domains that are necessary for many desired applications. Reviewed herein are techniques developed to guide the self-assembly process in thin films, which permit varying degrees of control over the patterns formed by the microdomains. Techniques that can control the out-of-plane order of the microdomains are first summarized. Then, techniques that control the lateral ordering are reviewed, beginning with those that generate large defect-free grains, then those that impart orientational order to the microdomains, and finally those that can control both the orientation and position of individual microdomains. Each technique is summarized with experimental examples and discussions regarding the mechanism of the guided self-assembly process.

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In common with many other structured fluids, block copolymers can be effectively oriented by shear. This susceptibility to shear alignment has previously been shown to hold even in thin films, containing as few as two layers of spherical microdomains, or even a single layer of cylindrical microdomains. A phenomenological model has been proposed [M. W. Wu, R. A. Register, and P. M. Chaikin, Phys. Rev. E 74, 040801(R) (2006)] to describe the alignment of such block-copolymer films, yielding the microdomain lattice order parameter as a function of shearing temperature, stress, and time. Here we directly test the central idea of that model, that the grains which are most misaligned with the shear direction are selectively destroyed, to reform in a direction more closely aligned with the shear. Films are first shear aligned from a polygrain state into a monodomain orientation and are then subjected to a second shear, at a variable stress (sigma) and misorientation angle (deltatheta) relative to the monodomain director, allowing the effects of sigma and deltatheta to be independently and systematically probed. For both cylinder-forming and sphere-forming block copolymers, these experiments confirm the basic premise of the model, as the stress required for realignment increases monotonically as deltatheta becomes smaller. For a cylinder-forming block copolymer, we find that the characteristic stress sigma(c) required to realign cylinders from one monodomain orientation to another is indistinguishable from that required to generate a monodomain orientation from the polygrain state. By contrast, the hexagonal lattice of spheres requires a value of sigma(c) more than 3 times as high for reorientation than for generation of the initial monodomain orientation.

Shear can impart a high degree of orientational order to block copolymer thin films containing two or more layers of spherical domains, but the orientational order appears to plateau at a limited level with increasing shear stress. At high stresses, the only defects which remain in the film are isolated dislocations, and the orientational order in the film is thus uniquely and inversely correlated with the dislocation density. These dislocations preferentially orient normal to the shear direction, which facilitates the sliding of layers of spheres relative to each other during shearing. A simple elastic continuum model provides a good quantitative description of the impact of isolated dislocations on the films' orientational order. At low dislocation densities, the apparent orientational order is limited by uncertainties in locating the positions of the spheres by atomic force microscopy, an effect which is quantitatively captured in this work.

Shear can impart a high degree of orientational order to supported block copolymer thin films containing one or more layers of cylindrical microdomains, leading to a striped pattern with a period of tens of nanometers extending over macroscopic (centimeter-squared) areas. Though the as-deposited films have a polygrain structure, after shearing at sufficiently high stresses the only defects which remain are isolated dislocations, and the orientational order can be quite high (nematic or twofold orientational order parameter >0.99, as measured by tapping-mode atomic force microscopy). The effect of isolated dislocations on orientational order is adequately captured by an isotropic elastic continuum model of the structure surrounding the dislocation, producing a linear decrease of order parameter with dislocation density. Even at zero dislocation density, however, the order parameter does not quite reach unity, due to small-amplitude undulations of the cylinders about their axes which persist in the transverse direction over several cylinder periods.