Shear‐Induced Alignment in Thin Films of Spherical Nanodomains

Abstract

Nanofabrication is witnessing a rapid trend towards using self-assembled templates as a cost-effective method of generating densely patterned surfaces. In particular, templating with block copolymer (BCP) thin films, until recently an area of essentially academic interest, has become increasingly popular in the semiconductor industry. BCPs are macromolecules composed of two (a ªdiblock copolymerº, diBCP) or more chemically distinct, covalently connected, polymer chains, which are typically immiscible in bulk. Molecular connectivity prohibits macroscopic phase separation; instead, BCPs ªmi-crophase separateº, forming nanoscale domains. In diBCPs where one block is much shorter than the other, the minority blocks self-assemble into spheres within a matrix of the majority block; if the length disparity is less pronounced, the nanodomains are cylindrical or lamellar.[1] The self-assembled polymeric patterns obtained in this fashion can be used as templates for lithography. [2] This economical and versatile patterning technique, capable of creating arrays of dielectric, [3] metallic, [4] quantum, [5] or magnetic [6,7] dots spaced only a few tens of nanometers apart, is fully compatible with silicon semiconductor processing [8] and is being applied to the fabrication of devices including magnetic hard drives [9] and, very recently, nanocrystal flash memories.[10] The Achilles' heel of this fabrication technique is lack of addressability, which limits data storage density to well below the theoretical maximum of one bit per dot. While the arrays typically display excellent short-range order, only limited long-range order can be achieved by traditional self-assembly because of the existence of topological defects. Recent research efforts have been directed at guiding the self-assembly process to induce long-range in-plane order in the array of BCP nanodomains that define the templated structures. The high degree of isotropy imposed by the hexagonal packing of the spherical nanodomains complicates the alignment problem: electric fields, although highly successful for in-plane alignment of cylinder-forming BCPs, [11] which define the templates for stripe arrays, have not been profitably applied to arrays of BCP spheres. Good alignment, however, can be achieved by using straight, microfabricated substrate features (mesas [12] or troughs [13,14] ) to impose a preferential lattice orientation (graphoepitaxy). While the alignment persists everywhere along such a straight feature, it extends for at most a few micrometers in the direction normal to the feature. The degree of alignment obtained by graphoepitaxy can be significantly improved by increasing the mobility of the polymer chains via thermal or solvent [15] annealing, yet these methods are (by themselves) ineffective in imposing a predefined lattice direction. We demonstrate below that a previously developed shearing technique, [16] when applied to thin films of sphereforming BCPs, results in BCP dot-array templates with excellent long-range order. The process involves s...

Pattern formation mechanisms in sphere-forming diblock copolymer thin films

“…Order was induced through vacuum annealing above the glass transition temperature of both blocks and below the T ODT of the block copolymer. In this system, the PEP block wets the air interface, while PS wets the silicon oxide substrate [25,26], i.e., there is an asymmetric wetting condition (see schematic in Fig. 1).…”

Dynamics of dislocations in a two-dimensional block copolymer system with hexagonal symmetry

“…Since block copolymer domains nucleate at random positions with a random orientational phase, the appearance of topological defects at the interfaces of different domains is completely unavoidable, and several strategies, such as graphoepitaxy or external ordering fields, have been employed to obtain ordered hexagonal and lamellar masks with a low density of topological defects [11,12]. For example, by employing a shearing technique, Angelescu et al [13] showed that thin films of sphere-forming block copolymers can be well aligned over square-centimetre regions. Compared with thermal treatments, where the density of topological defects can hardly be reduced below 200 dislocations per square micrometre, in shear-aligned hexagonal patterns the orientational order was perfect over a 2.5 × 2.5 mm 2 area while the translational order was limited only by the presence of a reduced number of dislocations (on average six dislocations per square micrometre).…”

Nanopatterns Produced by Directed Self-Assembly in Block Copolymer Thin Films

“…Shear alignment of thin films was shown to be very efficient on spherical and cylindrical block copolymer phases, if it is performed at a temperature between the highest of the two glass transition temperature and the order-disorder transition temperature of the diblock [80]: T g < T < T ODT . In other cases, however, a good alignment is obtained with a high defect density [81] (Fig.…”