Single-Grain Lamellar Microdomain from a Diblock Copolymer

Fast assembly of ordered block copolymer nanostructures through microwave annealing

Block copolymer self-assembly is an innovative technology capable of patterning technologically relevant substrates with nanoscale precision for a range of applications from integrated circuit fabrication to tissue interfacing, for example. In this article, we demonstrate a microwave-based method of rapidly inducing order in block copolymer structures. The technique involves the usage of a commercial microwave reactor to anneal block copolymer films in the presence of appropriate solvents, and we explore the effect of various parameters over the polymer assembly speed and defect density. The approach is applied to the commonly used poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) and poly(styrene)-b-poly(2-vinylpyridine) (PS-b-P2VP) families of block copolymers, and it is found that the substrate resistivity, solvent environment, and anneal temperature all critically influence the self-assembly process. For selected systems, highly ordered patterns were achieved in less than 3 min. In addition, we establish the compatibility of the technique with directed assembly by graphoepitaxy.

Block copolymer nanolithography: translation of molecular level control to nanoscale patterns

The self-asembly of block copolymers is a promising platform for the "bottom-up" fabrication of nanostructured materials and devices. This review covers some of the advances made in this field from the laboratory setting to applications where block copolymers are in use.

Fabrication of stable nanocylinder arrays in highly birefringent films of an amphiphilic liquid-crystalline diblock copolymer

An amphiphilic liquid-crystalline diblock copolymer (LCDC) with a highly birefringent cyanobiphenyl (CB) group as a mesogen was prepared by atom-transfer radical polymerization. The obtained LCDC showed a well-defined structure and a narrow molecular-weight distribution. In its spin-coated films, both liquid-crystalline (LC) alignment and microphase-separated nanostructures were systematically studied. Random LC arrangement and ambiguous microphase separation were observed in as-prepared films because of the high viscosity at room temperature. Upon annealing of the films in an isotropic phase of the LCDC, the CB mesogens self-organized into a smectic texture of a conic fan and the microphase separation proceeded completely. It is the supramolecular cooperative motion that enables the LCDC to hierarchically assemble into a regular patterning of normally aligned nanocylinders to the substrate, dispersed in the out-of-plane-oriented mesogens. With the help of homogeneous alignment of the CB groups induced by a rubbing technique, uniform patterning of highly ordered nanocylinders parallel to the rubbing direction was successfully fabricated in the plane. The fabricated perpendicular and parallel patterning of nanocylinders dispersed in the highly birefringent films with the CB block as the majority phase show good stability under room light, indicating their potential applications as nanotemplates for preparing advanced nanoscaled materials.

Target patterns induced by fixed nanoparticles in block copolymer films

It is well-known that thin films of cylinder-forming block copolymers (BCP) can exhibit a transition from a perpendicular to a parallel cylinder orientation with respect to the supporting solid substrate upon varying film thickness. We show that wave-like oscillations between these morphologies can be induced through the introduction of nanoparticles (NP) into flow-coated and annealed BCP films where the particles span the film thickness and are fixed by irreversible adsorption to the supporting substrate. We hypothesize that these novel "target" patterns arise from residual stresses that build up in the film while undergoing thermal treatment and film formation, and we support this hypothesis by showing the suppression of this type of pattern formation in films that are first thermally annealed near their glass transition T(g) to relax residual stress. Similar undulating height patterns are also observed in heated homopolymer films with nanoparticles, consistent with our thermally induced stress hypothesis of the target pattern formation in BCP films and pointing to the general nature of wave-like thermally induced height variations in heated heterogeneous polymer films. Similar wave patterns should be induced by lithographically etched substrate patterns arising in device fabrication using BCP materials, which makes the phenomena of technological interest. These target patterns also potentially provide valuable information about the presence of residual stresses in cast films that arise during their processing.

Characterization of the dynamics of block copolymer microdomains with local morphological measures

We investigate the structure formation in thin films of cylinder forming block copolymers. With in situ scanning probe microscopy image sequences can be recorded with high temporal (2 min per frame) and spatial (10 nm) resolution. We compare different image processing methods for quantitative analysis of the large amount of data. Computing local Minkowski functionals yields local geometrical and morphological information about the observed structures and enables us to track their evolution with time. An alternative characterization method is to reduce the gray scale images to their skeleton and to classify and count the branching points of the skeletonized structure. We tracked the temporal evolution of these measures and computed correlation functions.

A simple route to highly oriented and ordered nanoporous block copolymer templates

Controlling the orientation and lateral ordering of the block copolymer microdomains is essential to their use as templates and scaffolds for the fabrication of nanostructured materials. In addition, a process must be robust, simple to implement, and rapid, and should not introduce disruptive processing steps that would impede their use. Here, we describe thin films of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymers, spin-coated from mixed solvents that show highly oriented, cylindrical microdomains with a high degree of order on a wide range of substrates, including silicon oxide, polystyrene, germanium, polyimide, and poly(butylene terephthalate). In addition, the preferential solvation of the P4VP block with an alcohol caused a surface reconstruction that resulted in the formation of a nanoporous film upon drying. The evaporation of gold onto the reconstructed films produced thermally stable and reactive ion etching resistant films.

Facile preparation of macroscopic soft colloidal crystals with fiber symmetry

A facile, efficient way to fabricate macroscopic soft colloidal crystals with fiber symmetry by drying a latex dispersion in a tube is presented. A transparent, stable colloidal crystal was obtained from a 25 wt % latex dispersion by complete water evaporation for 4 days. The centimeter-long sample was investigated by means of synchrotron small-angle X-ray diffraction (SAXD). Analysis of a large number of distinct Bragg peaks reveals that uniaxially oriented colloidal crystals with face-centered cubic lattice structure were formed. The measurement of evaporation rates under different conditions indicates that the water evaporates primarily through the optically clear regions (i.e., via the solid material) even when the region is more than 2 mm thick.

Directed copolymer assembly on chemical substrate patterns: a phenomenological and single-chain-in-mean-field simulations study of the influence of roughness in the substrate pattern

The directed assembly of lamella-forming copolymer systems on substrates chemically patterned with rough stripes has been studied using a Helfrich-type, phenomenological theory and Single-Chain-in-Mean-Field (SCMF) simulations. The stripe period matches that of the lamellar spacing in the bulk. The effect of the line edge roughness (LER) of the substrate pattern on the microphase-separated morphology was investigated considering two generic types of substrate LER with a single characteristic wavelength imposed on the edges of the stripes: undulation and peristaltic LER. In both cases, the domain interfaces are pinned to the rough stripe boundary at the substrate and, thus, are deformed. We study how this deformation decays as a function of the distance from the substrate. The simple theory and the SCMF simulations demonstrate that one of the basic factors determining the decay of the roughness transferred into the self-assembled morphology is the characteristic LER wavelength of the substrate pattern; i.e., the distance over which the roughness propagates away from the substrate increases with wavelength. However, both approaches reveal that, for a quantitative understanding of the consequences of substrate LER, it is important to consider the interplay of the pattern wavelength with the other characteristic length scales of the system, such as the film thickness and the bulk lamellar spacing. For instance, in thin films, the induced deformation of the lamellar interface decays slower with distance from the patterned surface than in thicker films. It is shown that the phenomenological theory can capture many of the same qualitative results as the SCMF simulations for copolymer assembly on substrate patterns with LER, but, at the same time, is limited by an incomplete description of the constraints on the polymer chain conformations imposed by the substrate.

Block-copolymer ordering with a spatiotemporally heterogeneous mobility

Motivated by recent zone annealing measurements on stripe-forming block-copolymer films [B. C. Berry, Nano Lett. 7, 2789 (2007)], we study block-copolymer ordering with a spatiotemporally heterogeneous mobility. Specifically, we implement a time- and space-dependent mobility field in the relaxation of a diblock copolymer self-consistent field theory. The model includes a gradient in the local mobility and intrinsic nanoscale mobility variations characteristic of glass phenomenology. The simulations demonstrate that a spatiotemporally heterogeneous mobility can have a significant influence on microdomain ordering in block-copolymer systems, and that nanoscale dynamic heterogeneities associated with glass formation can impact the structure of the ordered block-copolymer microphase.

Unconventional methods for forming nanopatterns

Nanostructured materials have become an increasingly important theme in research, in no small part due to the potential impacts this science holds for applications in technology, including such notable areas as sensors, medicine, and high-performance integrated circuits. Conventional methods, such as the top-down approaches of projection lithography and scanning beam lithography, have been the primary means for patterning materials at the nanoscale. This article provides an overview of unconventional methods - both top-down and bottom-up approaches - for generating nanoscale patterns in a variety of materials, including methods that can be applied to fragile molecular systems that are difficult to pattern using conventional lithographic techniques. The promise, recent progress, advantages, limitations, and challenges to future development associated with each of these unconventional lithographic techniques will be discussed with consideration given to their potential for use in large-scale manufacturing.

Orientational order in block copolymer films zone annealed below the order--disorder transition temperature

We report measurements of rapid ordering and preferential alignment in block copolymer films zone annealed below the order-disorder transition temperature. The orientational correlation lengths measured after approximately 5 h above the glass-transition temperature ( approximately 2 microm) were an order of magnitude greater than that obtained under equivalent static annealing. The ability to rapidly process polymers with inaccessible order-disorder transition temperatures suggests zone annealing as a route toward more robust nanomanufacturing methods based on block copolymer self-assembly.

Photoinduced alignment of nanocylinders by supramolecular cooperative motions

A linearly polarized laser beam was used to control nanocylinders self-assembled in an amphiphilic diblock liquid-crystalline copolymer consisting of flexible poly(ethylene oxide) as a hydrophilic block and poly(methacrylate) containing an azobenzene moiety in the side chain as a hydrophobic liquid-crystalline segment. The perfect array of poly(ethylene oxide) nanocylinders was achieved, aligned perpendicularly to the polarization direction of the actinic light by supramolecular cooperative motions between the ordered azobenzene and microphase separation. By the simple and convenient way of photocontrol, the macroscopic parallel patterning of nanocylinders can be easily obtained in an arbitrary area.

Ordering in Asymmetric Block Copolymer Films by a Compressible Fluid

We examine the morphological structures of asymmetric poly(ethylene oxide)-b-poly(1,1'-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films upon annealing in a compressible fluid, supercritical CO2 (Sc-CO2). The strong affinity between PFOMA and CO2 is found to induce phase segregation when annealing PEO-b-PFOMA films at the same temperature as compared with vacuum. In vacuum, PEO-b-PFOMA films remain disordered from 80 to 180 degrees C, whereas, in Sc-CO2 at 13.9 MPa, an upper order-disorder transition (UODT) between 116 and 145 degrees C is found. In Sc-CO2, the observed ordered structure is layers of PEO spheres embedded in the matrix of PFOMA, followed by a brush layer, in which PEO wets the substrate. The swelling isotherms of PFOMA and PEO in CO2 are correlated with the Sanchez-Lacombe equation of state (SLEOS) to estimate the interaction parameters, XPFOMA-CO2 and XPEO-CO2. The phase segregation (order) induced by CO2 relative to vacuum at a given temperature is explained in terms of two factors: (1) copolymer volume fraction upon dilution with CO2, phi, and (2) the relative interaction parameter, DeltaX= XPEO-CO2 - XPFOMA-CO2. The latter factor favors order and is dominant at low temperatures over the phi factor, which always favors disorder. At high temperatures (above the T(ODT)), the preferential swelling of PFOMA by CO2 is less pronounced ( DeltaX decreases), and the copolymer is disordered.

Flow-enhanced epitaxial ordering of brush-like macromolecules on graphite

Long-range orientational order in monolayers of brush-like macromolecules was achieved during spreading of a polymer melt on the surface of highly oriented pyrolytic graphite. The combination of wetting-induced flow and epitaxial adsorption of poly(n-butylacrylate) side chains on graphite led to the formation of large domains of uniaxially oriented rodlike molecules. The domain size varied from ca. 1 to 10 microm which is noticeably larger than the submicrometer-sized mosaic domains typically observed upon adsorption from solution. The increase in the degree of order is attributed to the flow-enhanced diffusion of the macromolecules within spreading monolayers which facilitates the epitaxial alignment of the large macromolecules. The diffusion coefficient was shown to increase linearly with the spreading rate. Even though the ordering occurred during flow, no correlation was observed between the molecular orientation and the flow direction. Thus, the role of the flow was not to induce the molecular orientation but to facilitate the intrinsic ordering process. This finding can inspire and lead to new strategies for constructing large scale ordered structures on surfaces.

Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates

Parallel processes for patterning densely packed nanometre-scale structures are critical for many diverse areas of nanotechnology. Thin films of diblock copolymers can self-assemble into ordered periodic structures at the molecular scale (approximately 5 to 50 nm), and have been used as templates to fabricate quantum dots, nanowires, magnetic storage media, nanopores and silicon capacitors. Unfortunately, perfect periodic domain ordering can only be achieved over micrometre-scale areas at best and defects exist at the edges of grain boundaries. These limitations preclude the use of block-copolymer lithography for many advanced applications. Graphoepitaxy, in-plane electric fields, temperature gradients, and directional solidification have also been demonstrated to induce orientation or long-range order with varying degrees of success. Here we demonstrate the integration of thin films of block copolymer with advanced lithographic techniques to induce epitaxial self-assembly of domains. The resulting patterns are defect-free, are oriented and registered with the underlying substrate and can be created over arbitrarily large areas. These structures are determined by the size and quality of the lithographically defined surface pattern rather than by the inherent limitations of the self-assembly process. Our results illustrate how hybrid strategies to nanofabrication allow for molecular level control in existing manufacturing processes.