Study on the combined effects of solvent evaporation and polymer flow upon block copolymer self-assembly and alignment on topographic patterns

Solvent-assisted self-assembly of block copolymer thin films

Solvent-assisted block copolymer self-assembly is a compelling method for processing and advancing practical applications of these materials due to the exceptional level of the control of BCP morphology and significant acceleration of ordering kinetics. Despite substantial experimental and theoretical efforts devoted to understanding of solvent-assisted BCP film ordering, the development of a universal BCP patterning protocol remains elusive; possibly due to a multitude of factors which dictate the self-assembly scenario. The aim of this review is to aggregate both seminal reports and the latest progress in solvent-assisted directed self-assembly and to provide the reader with theoretical background, including the outline of BCP ordering thermodynamics and kinetics phenomena. We also indicate significant BCP research areas and emerging high-tech applications where solvent-assisted processing might play a dominant role.

Directed Self-Assembly of Block Copolymer Thin Films Using Minimal Topographic Patterns

We demonstrate that a minimal topographic pattern with a confinement depth (D) much less than the domain spacing of block copolymers (L0) can be used to achieve highly ordered hexagonal arrays or unidirectionally aligned line patterns over large areas. Cylinder-forming poly(styrene-b-ethylene oxide) (PS-b-PEO) thin films were prepared on a series of minimal single trench patterns with different widths (W) and D. Upon thermal annealing, hexagonal arrays of cylindrical microdomains propagated away from the edges of a single trench, providing insight into the minimum pitch (P) of the trench necessary to fully order hexagonal arrays. The confinement trench D of 0.30L0, the W in the range of 1.26L0 to 2.16L0, and the P as long as 18.84L0 were found to be effective for the generation of laterally ordered hexagonal arrays with the density amplification up by a factor of 17, within the minimally patterned trench surfaces of 100 μm by 100 μm. Furthermore, we produced line patterns of cylindrical microdomains by using solvent vapor annealing on the minimally patterned trench surfaces. However, highly aligned line patterns could be achieved only on the patterned surface with P = 5.75L0, W = 1.26L0, and D = 0.30L0 because the influence of the minimally patterned trench surface on the lateral ordering decreased as the P and W increase at the fixed D, resulting in poor ordering. These findings suggest that the minimal topographic pattern is more effective in guiding hexagonal arrays than in guiding line patterns.

Self-Assembly of PS-b-PDMS on a Tunable PDMS Template with Nanoscale Channels and Enhanced Anisotropic Wetting

In this article, we systematically studied the self-assembly of poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) on a poly(dimethylsiloxane) (PDMS) substrate with nanoscale channels. The channeled PDMS substrate was achieved by a simple replica molding method. To decrease the effect that the subsequent solvent treatments had in distorting the soft PDMS substrate, a simple UV/O3 treatment was provided before the self-assembly, resulting in a relatively stable, harder and hydrophilic silicon oxide (SiO2) layer on the channeled PDMS surface. Ultimately, the isotropic SiO2 nanopatterns with spherical and long cylindrical morphologies were successfully fabricated by the self-assembly of two kinds of PS-b-PDMS on the PDMS substrate with nanoscale channels, respectively. In particular, we demonstrated that the introduction of isotropic SiO2 patterns is an effective approach to greatly enhance anisotropic wetting rather than that of the anisotropic structure with channels.

Solvent vapor annealing of block copolymers in confined topographies: commensurability considerations for nanolithography

The directed self-assembly of block copolymer (BCP) materials in topographically patterned substrates (i.e., graphoepitaxy) is a potential methodology for the continued scaling of nanoelectronic device technologies. In this Communication, an unusual feature size variation in BCP nanodomains under confinement with graphoepitaxially aligned cylinder-forming poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP is reported. Graphoepitaxy of PS-b-P4VP BCP line patterns (CII ) is accomplished via topo-graphy in hydrogen silsequioxane (HSQ) modified substrates and solvent vapor annealing (SVA). Interestingly, reduced domain sizes in features close to the HSQ guiding features are observed. The feature size reduction is evident after inclusion of alumina into the P4VP domains followed by pattern transfer to the silicon substrate. It is suggested that this nano-domain size perturbation is due to solvent swelling effects during SVA. It is proposed that using a commensurability value close to the solvent vapor annealed periodicity will alleviate this issue leading to uniform nanofins.

Highly Aligned Block Copolymer Thin Films by Synergistic Coupling of Static Graphoepitaxy and Dynamic Thermal Annealing Fields

Directed self-assembly of cylinder forming block copolymer (c-BCP) thin films via a dynamic thermal field on multidimensional symmetric graphoepitaxy channels is reported. A synergy of dynamic thermal and static boundary fields induces highly aligned c-BCP cylinders inside the channels with a power law dependence of orientational order parameter f, on trench width, f ∼ d^-0.3, analogous to dual-field alignment of semiconducting metals and liquid crystals on graphoepitaxy surfaces, f' ∼ d^-1. Static thermal annealing of identical films in a vacuum oven for several days fails to produce comparable results. Furthermore, we demonstrate global c-BCP cylinder alignment over mesas and trenches by tuning the synergy between the dynamic thermal field and asymmetry of the graphoepitaxy static field.

Sub-10 nm feature size PS-b-PDMS block copolymer structures fabricated by a microwave-assisted solvothermal process

Block copolymer (BCP) microphase separation at surfaces might enable the generation of substrate features in a scalable, manufacturable, bottom-up fashion provided that pattern structure, orientation, alignment can be strictly controlled. A further requirement is that self-assembly takes place within periods of the order of minutes so that continuous manufacturingprocesses do not require lengthy pretreatments and sample storageleading to contamination and large facility costs. We report here microwave-assisted solvothermal (in toluene environments) self-assembly and directed self-assembly of a very low molecular weight cylinder-forming polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP on planar and patterned silicon nitride (Si3N4) substrates. Good pattern ordering was achieved in the order of minutes. Factors affecting BCP self-assembly, notably anneal time and temperature were studied and seen to have significant effects. Graphoepitaxy to direct self-assembly in the BCP yielded promising results producing BCP patterns with long-range translational alignment commensurate with the pitch period of the topographic patterns. This rapid BCP ordering method is consistent with the standard thermal/solvent anneal processes.

Long-Range Ordered Crystallization Structure in the Micromolded Diblock Copolymer Thin Film

Confined crystallization of the micromolded poly(butadiene)-block-poly(ε-caprolactone) (PB-b-PCL) diblock copolymer thin film was studied in this work. The long-range regular ordering of the PCL crystal with crystallographic b-axis parallel to the long-axis of the channel was detected, as indicated by the electron diffraction and grazing-incidence X-ray diffraction experimental results. This preferential crystallographic orientation is mainly because that PCL block crystallization was readily influenced by the geometric effect, then, the fast-growth direction (crystallographic b-axis) was forced to extend along the long-axis of the channel to grow long. Moreover, the substrate induced ordering of the block copolymer restricted the "in-plane" molecular diffusion in the residual layer, and cross-channel crystallization was precluded. Hence, micromolding seems to be a promising method for tailoring the nanoscale crystallization of block copolymer in thin films.

Surface-directed dewetting of a block copolymer for fabricating highly uniform nanostructured microdroplets and concentric nanorings

Through a combination of nanoimprint lithography and block copolymer self-assembly, a highly regular dewetting process of a symmetric diblock copolymer occurs whereby the hierarchal formation of microdroplets and concentric nanorings emerges. The process is driven by the unique chemical properties and geometrical layout of the underlying patterned silsesquioxane micrometer-sized templates. Given the presence of nonpreferential substrate-polymer interactions, directed dewetting was utilized to produce uniform arrays of microsized droplets of microphase separated polystyrene-block-poly(methyl methylacrylate) (PS-b-PMMA), following thermal annealing at 180 °C. Microdroplets with diameters greater than 400 nm exhibited a hexagonal close-packed arrangement of nanodots on the surface with polydomain ordering. At the droplet periphery, the polydomain ordering was severely disrupted because of a higher in-plane radius of curvature. By reducing the droplet size, the in-plane radius of curvature of the microdroplet becomes significant and the PMMA cylinders adopt parallel structures in this confined geometry. Continuous scaling of the droplet results in the generation of isolated, freestanding, self-aligned, and self-supported oblique nanorings (long axis ∼250-350 nm), which form as interstitial droplets between the larger microdroplets. Optical and magnetic-based nanostructures may benefit from such hierarchal organization and self-supporting/aligned nanoring templates by combining more than one lithography technique with different resolution capabilities.

Chemical interactions and their role in the microphase separation of block copolymer thin films

The thermodynamics of self-assembling systems are discussed in terms of the chemical interactions and the intermolecular forces between species. It is clear that there are both theoretical and practical limitations on the dimensions and the structural regularity of these systems. These considerations are made with reference to the microphase separation that occurs in block copolymer (BCP) systems. BCP systems self-assemble via a thermodynamic driven process where chemical dis-affinity between the blocks driving them part is balanced by a restorative force deriving from the chemical bond between the blocks. These systems are attracting much interest because of their possible role in nanoelectronic fabrication. This form of self-assembly can obtain highly regular nanopatterns in certain circumstances where the orientation and alignment of chemically distinct blocks can be guided through molecular interactions between the polymer and the surrounding interfaces. However, for this to be possible, great care must be taken to properly engineer the interactions between the surfaces and the polymer blocks. The optimum methods of structure directing are chemical pre-patterning (defining regions on the substrate of different chemistry) and graphoepitaxy (topographical alignment) but both centre on generating alignment through favourable chemical interactions. As in all self-assembling systems, the problems of defect formation must be considered and the origin of defects in these systems is explored. It is argued that in these nanostructures equilibrium defects are relatively few and largely originate from kinetic effects arising during film growth. Many defects also arise from the confinement of the systems when they are 'directed' by topography. The potential applications of these materials in electronics are discussed.