Molecular pathways for defect annihilation in directed self-assembly

Directed Self-Assembly by Sparsely Prepatterned Substrates with Self-Responsive Polymer Brushes

The guiding pattern in the chemoepitaxially directed self-assembly (DSA) of block copolymers is often fabricated by periodically functionalizing homogeneously random copolymer brushes tethered on a substrate. The prepatterned copolymer brushes constitute a soft penetrable surface, and their two components can in principle locally segregate in response to the overlying self-assembly process of block copolymers. To reveal how the self-responsive behavior of the copolymer brushes affects the directing effect, we develop a dissipative particle dynamics model to explicitly include the prepatterned polymer brushes and implement it to simulate the DSA of a cylinder-forming diblock copolymer melt on the sparse pattern of polymer brushes. Through large-scale dynamic simulations, we identify the windows of the content of the random copolymer, the film thickness, and the diameter of the patterned spot, for the formation of perfectly ordered hexagonal patterns composed of perpendicular cylinders. Our dynamic simulations reveal that the random copolymer brushes grafted on the unpatterned area exhibit a remarkable self-responsive ability with respect to the self-assembly of the diblock copolymers overlying them, which may widen the effective window of the content of the random copolymer. Within the processing windows of these key parameters, defect-free patterns are successfully achieved both in simulations and in experiments with sizes as large as a few micrometers for 4-fold density multiplications. This work demonstrates that highly efficient computer simulations based on an effective model can provide helpful guidance for experiments to optimize the critical parameters and even may promote the application of DSA.

Density Multiplication of Highly Periodic Sub-5 nm Supramolecular Dendrimer Cylinders on Block Copolymer Lamellar-Assisted High-Resolution Patterns

Density multiplication in nanopatterning is one of the most efficient techniques for increasing the resolution of the inherent patterns. Thus far, most of the density multiplication techniques integrate bottom-up (or top-down) patterning onto guide patterns prepared by the top-down approach. Although the bottom-up approach exhibits several advantages of cost-effectiveness and high resolution, very few studies have reported bottom-up patterning within a bottom-up template. In this study, the density multiplication of supramolecular cylinders into a block copolymer (BCP)-based guide lamellar pattern is demonstrated by the directed self-assembly (DSA) of a dendrimer and BCPs for the first time. Supramolecular cylinders of sub-5 nm scale are confined into trenches based on 50 and 100 nm scales of a lamellar polystyrene (PS)-poly(methyl methacrylate) (PMMA) BCP, which led to 10×-level to 20×-level density multiplication. Moreover, the orientation of the dendrimer is dependent on the dendrimer film thickness, and the corresponding mechanism is revealed. Notably, the strong guiding effect from the high-resolution guide patterns improved the ordering behavior in the highly curved pattern. Graphoepitaxy via the confinement of an ultrahigh-resolution dendrimer into the guide pattern based on BCP demonstrates promise as a density multiplication method for generating highly ordered nanostructures and complex structures.

Complex-tensor theory of simple smectics

Matter self-assembling into layers generates unique properties, including structures of stacked surfaces, directed transport, and compact area maximization that can be highly functionalized in biology and technology. Smectics represent the paradigm of such lamellar materials - they are a state between fluids and solids, characterized by both orientational and partial positional ordering in one layering direction, making them notoriously difficult to model, particularly in confining geometries. We propose a complex tensor order parameter to describe the local degree of lamellar ordering, layer displacement and orientation of the layers for simple, lamellar smectics. The theory accounts for both dislocations and disclinations, by regularizing singularities within defect cores and so remaining continuous everywhere. The ability to describe disclinations and dislocation allows this theory to simulate arrested configurations and inclusion-induced local ordering. This tensorial theory for simple smectics considerably simplifies numerics, facilitating studies on the mesoscopic structure of topologically complex systems.

Determining Structure and Thermodynamics of A-b-(B-r-C) Copolymers

The self-assembly of block copolymers (BCPs) is dictated by their segregation strength, χN, and while there are well-developed methods for determining χ in the weak and strong segregation regimes, it is challenging to accurately measure χ of copolymers with intermediate segregation strengths, especially when copolymers have inaccessible order-disorder transition temperatures. χ_eff is often approximated by using strong segregation theory (SST), but utilizing these values to estimate the interface width (w_m) of BCPs in the intermediate segregation regime often results in predictions that deviate significantly from measured values. Therefore, we propose using the extent of mixing, quantified as the normalized interface width w_m/L₀, where L₀ is the block copolymer pitch, as a thermodynamic parameter. We experimentally measure w_m and L₀ for a series of lamellar A-b-(B-r-C) copolymers via resonant soft X-ray reflectivity and extract values of χ_effN based on previous data collected for A-b-B copolymers. The composition profiles measured via reflectivity match the extracted χ_effN values, while those calculated with SST predict much more mixed composition profiles. The extracted χ_eff values agreed quantitatively between copolymers of different molecular weights. We believe that this methodology will be well-suited for block copolymers used in lithographic applications due to their inaccessible order-disorder transition temperatures, intermediate values of χN, and the importance of w_m for line edge roughness metrics.

Sequential Brush Grafting for Chemically and Dimensionally Tolerant Directed Self-Assembly of Block Copolymers

We report a method for the directed self-assembly (DSA) of block copolymers (BCPs) in which a first BCP film deploys homopolymer brushes, or "inks", that sequentially graft onto the substrate's surface via the interpenetration of polymer molecules during the thermal annealing of the polymer film on top of existing polymer brushes. By selecting polymer "inks" with the desired chemistry and appropriate relative molecular weights, it is possible to use brush interpenetration as a powerful technique to generate self-registered chemical contrast patterns at the same frequency as that of the domains of the BCP. The result is a process with a higher tolerance to dimensional and chemical imperfections in the guiding patterns, which we showcase by implementing DSA using homopolymer brushes for the guiding features as opposed to more robust cross-linkable mats. We find that the use of "inks" does not compromise the line width roughness, and the quality of the DSA as a lithographic mask is verified by implementing a robust "dry lift-off" pattern transfer.

Optimized design of block copolymers with covarying properties for nanolithography

The ability to impart multiple covarying properties into a single material represents a grand challenge in manufacturing. In the design of block copolymers (BCPs) for directed self-assembly and nanolithography, materials often balance orthogonal properties to meet constraints related to processing, structure and defectivity. Although iterative synthesis strategies deliver BCPs with attractive properties, identifying materials with all the required attributes has been difficult. Here we report a high-throughput synthesis and characterization platform for the discovery and optimization of BCPs with A-block-(B-random-C) architectures for lithographic patterning in semiconductor manufacturing. Starting from a parent BCP and using thiol-epoxy 'click' chemistry, we synthesize a library of BCPs that cover a large and complex parameter space. This allows us to readily identify feature-size-dependent BCP chemistries for 8-20-nm-pitch patterns. These blocks have similar surface energies for directed self-assembly, and control over the segregation strength to optimize the structure (favoured at higher segregation strengths) and defectivity (favoured at lower segregation strengths).

Suppressing high-dimensional crystallographic defects for ultra-scaled DNA arrays

While DNA-directed nano-fabrication enables the high-resolution patterning for conventional electronic materials and devices, the intrinsic self-assembly defects of DNA structures present challenges for further scaling into sub-1 nm technology nodes. The high-dimensional crystallographic defects, including line dislocations and grain boundaries, typically lead to the pattern defects of the DNA lattices. Using periodic line arrays as model systems, we discover that the sequence periodicity mainly determines the formation of line defects, and the defect rate reaches 74% at 8.2-nm line pitch. To suppress high-dimensional defects rate, we develop an effective approach by assigning the orthogonal sequence sets into neighboring unit cells, reducing line defect rate by two orders of magnitude at 7.5-nm line pitch. We further demonstrate densely aligned metal nano-line arrays by depositing metal layers onto the assembled DNA templates. The ultra-scaled critical pitches in the defect-free DNA arrays may further promote the dimension-dependent properties of DNA-templated materials.

Droplet-like Defect Annihilation Mechanisms in Hexagonal Cylinder-Forming Block Copolymers

The annihilation of typical individual defects in hexagonal cylinder-forming block copolymers is investigated using the self-consistent field theory (SCFT) in conjunction with the string method. Usually, defect removal in two-dimensional hexagonal patterns involves reorganizing the cylindrical domains. Unlike atoms in solid crystals, the self-assembled cylindrical domains of block copolymers are "soft". Thus, the kinetic motions of the cylindrical domains resemble liquid droplets. Dislocations in hexagonal patterns are eliminated via creating and removing cylindrical domains. Our results show that new cylindrical domains are created via either a nucleation-like process or a fission-like process, whereas excessive domains are eliminated via a fusion-like or evaporation-like process. For weakly segregated block copolymers, the nucleation-like and evaporation-like processes are preferred.

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films

Transitional composition between two thin-film morphologies of the block copolymer, polystyrene-block-poly(tert-butyl acrylate) (PS-b-PtBuA), was investigated using near-field infrared spectroscopy and atomic force microscopy mechanical measurements. These techniques allowed block identification with nanoscale spatial resolution and elucidated the material's sub-surface composition. PS was found to form coronae around the PtBuA block in spherical valleys on flat areas of the film, and coronae of PtBuA surrounding the PS lamellae were observed at the edge of the polymer film, where parallel lamellae are formed. Furthermore, we found that the peak position and width varied by location, which may be a result of block composition, chain tension, or substrate interaction.

Self-Aligned Assembly of a Poly(2-vinylpyridine)-b-Polystyrene-b-Poly(2-vinylpyridine) Triblock Copolymer on Graphene Nanoribbons

Directed self-assembly (DSA) of block copolymers is one of the most promising patterning techniques for patterning sub-10 nm features. However, at such small feature sizes, it is becoming increasingly difficult to fabricate the guiding pattern for the DSA process, and it is necessary to explore alternative guiding methods for DSA to achieve long-range ordered alignment. Here, we report the self-aligned assembly of a triblock copolymer, poly(2-vinylpyridine)-b-polystyrene-b-poly(2-vinylpyridine) (P2VP-b-PS-b-P2VP) on neutral graphene nanoribbons with the gap consisting of a P2VP-preferential silicon oxide (SiO₂) substrate via solvent vapor annealing. The assembled P2VP-b-PS-b-P2VP demonstrated long-range, one-dimensional alignment on the graphene substrate in a direction perpendicular to the boundary of the graphene and substrate with a half-pitch size of 8 nm, which greatly alleviates the lithography resolution required for traditional chemoepitaxy DSA. A wide processing window is demonstrated with the gap between graphene stripes varying from 10 to 100 nm, overcoming the restriction on widths of guiding patterns to have commensurate domain spacing. When the gap was reduced to 10 nm, P2VP-b-PS-b-P2VP formed a straight-line pattern on both the graphene and the substrate. Monte Carlo simulations showed that the self-aligned assembly of the triblock copolymer on the graphene nanoribbons is guided at the boundary of parallel and perpendicular lamellae on graphene and SiO₂, respectively. Simulations also indicate that the swelling of a system allows for rapid rearrangement of chains and quickly anneal any misaligned grains and defects. The effect of the interaction strength between SiO₂ and P2VP on the self-assembly is systematically investigated in simulations.

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Understanding Kinetics of Defect Annihilation in Chemoepitaxy-Directed Self-Assembly

Directed self-assembly (DSA) of block copolymers (BCP) has attracted considerable interest from the semiconductor industry because it can achieve semiconductor-relevant structures with a relatively simple process and low cost. However, the self-assembling structures can become kinetically trapped into defective states, which greatly impedes the implementation of DSA in high-volume manufacturing. Understanding the kinetics of defect annihilation is crucial to optimizing the process and eventually eliminating defects in DSA. Such kinetic experiments, however, are not commonly available in academic laboratories. To address this challenge, we perform a kinetic study of chemoepitaxy DSA in a 300 mm wafer fab, where the complete defectivity information at various annealing conditions can be readily captured. Through extensive statistical analysis, we reveal the statistical model of defect annihilation in DSA for the first time. The annihilation kinetics can be well described by a power law model, indicating that all dislocations can be removed by sufficiently long annealing time. We further develop image analysis algorithms to analyze the distribution of dislocation size and configurations and discover that the distribution stays relatively constant over time. The defect distribution is determined by the role of the guiding stripe, which is found to stabilize the defects. Although this study is based on polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA), we anticipate that these findings can be readily applied to other BCP platforms as well.

Large Area Patterning of Nanoparticles and Nanostructures: Current Status and Future Prospects

Nanoparticles possess exceptional optical, magnetic, electrical, and chemical properties. Several applications, ranging from surfaces for optical displays and electronic devices, to energy conversion, require large-area patterns of nanoparticles. Often, it is crucial to maintain a defined arrangement and spacing between nanoparticles to obtain a consistent and uniform surface response. In the majority of the established patterning methods, the pattern is written and formed, which is slow and not scalable. Some parallel techniques, forming all points of the pattern simultaneously, have therefore emerged. These methods can be used to quickly assemble nanoparticles and nanostructures on large-area substrates into well-ordered patterns. Here, we review these parallel methods, the materials that have been processed by them, and the types of particles that can be used with each method. We also emphasize the maximal substrate areas that each method can pattern and the distances between particles. Finally, we point out the advantages and disadvantages of each method, as well as the challenges that still need to be addressed to enable facile, on-demand large-area nanopatterning.

Self-assembled mesostructured Co0.5Fe2.5O4 nanoparticle superstructures for highly efficient oxygen evolution

Self-assembly of colloidal nanoparticles (NPs) into well-defined superstructures has been recognized as one of the most promising ways to fabricate rationally-designed functional materials for a variety of applications. Introducing hierarchical mesoporosity into NP superstructures will facilitate mass transport while simultaneously enhancing the accessibility of constituent NPs, which is of critical importance for widening their applications in catalysis and energy-related fields. Herein, we develop a colloidal co-assembly strategy to construct mesostructured, carbon-coated Co_0.5Fe_2.5O₄ NP superstructures (M-C@CFOSs), which show great promise as highly efficient electrocatalysts for the oxygen evolution reaction (OER). Specifically, organically-stabilized SiO₂ NPs are employed as both building blocks and sacrificial template, which co-assemble with Co_0.5Fe_2.5O₄ NPs to afford binary NP superstructures through a solvent drying process. M-C@CFOSs are obtainable after in situ ligand carbonization followed by the selective removal of SiO₂ NPs. The hierarchical mesoporous structure of M-C@CFOSs, combined with the conformal graphitic carbon coating derived from the native organic ligands, significantly improves their electrocatalytic performance as OER electrocatalysts when compared with nonporous Co_0.5Fe_2.5O₄ NP superstructures. This work establishes a new and facile approach for designing NP superstructures with hierarchical mesoporosity, which may find wide applications in energy storage and conversion.

Nanoscratch-Directed Self-Assembly of Block Copolymer Thin Films

Directed self-assembly (DSA) of block copolymer (BCP) thin films is of particular interest in nanoscience and nanotechnology due to its superior ability to form various well-aligned nanopatterns. Herein, nanoscratch-DSA is introduced as a simple and scalable DSA strategy allowing highly aligned BCP nanopatterns over a large area. A gentle scratching on the target substrate with a commercial diamond lapping film can form uniaxially aligned nanoscratches. As applied in BCP thin films, the nanoscratch effectively guides the self-assembly of overlying BCPs and provides highly aligned nanopatterns along the direction of the nanoscratch. The nanoscratch-DSA is not material-specific, allowing more versatile nanofabrication for various functional nanomaterials. In addition, we demonstrate that the nanoscratch-DSA can be utilized as a direction-controllable and area-selective nanofabrication method.

Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order-disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

Kinetic Pathways of Block Copolymer Directed Self-Assembly: Insights from Efficient Continuum Modeling

We introduce a computationally efficient continuum technique to simulate the complex kinetic pathways of block copolymer self-assembly. Subdiffusive chain dynamics is taken into account via nonlocal Onsager coefficients. An application to directed self-assembly of thin films of diblock copolymers on patterned substrates reveals the conditions under which experimentally observed metastable structures intervene in the desired thin-film morphology. The approach generalizes easily to multiblock copolymers and more complex guiding patterns on the substrate, and its efficiency allows for the systematic optimization of guiding patterns and process conditions.

Thermodynamics and ordering kinetics in asymmetric PS-b-PMMA block copolymer thin films

The ordering kinetics of standing cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymers (molecular weight: 39 kg mol^-1) close to the order-disorder transition is experimentally investigated following the temporal evolution of the correlation length at different annealing temperatures. The growth exponent of the grain-coarsening process is determined to be 1/2, signature of a curvature-driven ordering mechanism. The measured activation enthalpy and the resulting Meyer-Neldel temperature for this specific copolymer along with the data already known for PS-b-PMMA block copolymers in strong segregation limit allow investigation of the interplay between the ordering kinetics and the thermodynamic driving force during the grain coarsening. These findings unveil various phenomena concomitantly occurring during the thermally activated ordering kinetics at segmental, single chain, and collective levels.

Kinetically-arrested single-polymer nanostructures from amphiphilic mikto-grafted bottlebrushes in solution: a simulation study

Solution self-assembly of molecular bottlebrushes offers a rich platform to create complex functional organic nanostructures. Recently, it has become evident that kinetics, not just thermodynamics, plays an important role in defining the self-assembled structures that can be formed. In this work, we present results from extensive molecular dynamics simulations that explore the self-assembly behavior of mikto-grafted bottlebrushes when the solvent quality for one of the side blocks is changed by a rapid quench. We have performed a systematic study of the effect of different structural parameters and the degree of incompatibility between side chains on the final self-assembled nanostructures in the low concentration limit. We found that kinetically-trapped complex nanostructures are prevalent as the number of macromonomers increases. We performed a quantitative analysis of the self-assembled morphologies by computing the radius of gyration tensor and relative shape anisotropy as the different relevant parameters were varied. Our results are summarized in terms of non-equilibrium morphology diagrams.

Role of Defects in Achieving Highly Asymmetric Lamellar Self-Assembly in Block Copolymer/Homopolymer Blends

Lamellar structure is a prominent state in soft condensed matter. Swelling lamellar layers to highly asymmetric structures by a second component is a facile, cost-effective strategy to impart materials with adaptive size and tunable properties. One key question that remains unsolved is how defects form and affect the asymmetric lamellar order. This study unravels the role of defects by swelling a miktoarm block copolymer with a homopolymer. Ordered lamellae first lose translational order by a significant increase in the number of dislocations and then lose orientational order by the generation of disclinations. The homopolymers are not uniformly distributed in defective lamellae and primarily segregate in the vicinity of disclination cores. The free energy of defects is mainly contributed by molecular splay and significantly alleviated by an increased radius of local curvature. This study provides direct evidence to reveal the role of defects and lamellar order in block copolymer/homopolymer blends and also sheds light on understanding analogous structural transitions in other soft systems, including lyotropic liquid crystals, phospholipid membranes, and polymer nanocomposites.

Three-dimensional line edge roughness in pre- and post-dry etch line and space patterns of block copolymer lithography

In this work, we employ large-scale coarse-grained molecular dynamics (CGMD) simulations to study the three-dimensional line edge roughness associated with line and space patterns of chemo-epitaxially directed symmetric block copolymers (BCPs) on a flat substrate. The di-block copolymer chain length and interaction parameters are validated with the experimental BCP period, L0 and corresponding molecular weight. Defect-free lamellae are formed, after which the system is quenched below the glass transition temperature before selectively dry-etching off one of the BCP phases. The effect of varying etch-selectivity on post-etch resist domain morphology was studied. The roughness of the polymer domain was evaluated over three process stages: annealing, pre-etching, and post-etching. Power spectral density plots were then generated to elucidate the contributions of low and high frequency roughness for the three process stages. The roughness results obtained from simulations are shown to be in close agreement with the roughness result obtained from analyzing experimental SEM images. Parameters like the Hurtz roughness exponent and correlation length inherent to the process and the BCP were also revealed from the experimental study.

Strategies for Increasing the Rate of Defect Annihilation in the Directed Self-Assembly of High-χ Block Copolymers

Directed self-assembly (DSA) of high-χ block copolymer thin films is a promising approach for nanofabrication of features with length scale below 10 nm. Recent work has highlighted that kinetics are of crucial importance in determining whether a block copolymer film can self-assemble into a defect-free ordered state. In this work, different strategies for improving the rate of defect annihilation in the DSA of a silicon-containing, high-χ block copolymer film were explored. Chemo-epitaxial DSA of poly(4-methoxystyrene-block-4-trimethylsilylstyrene) with 5× density multiplication was implemented on 300 mm wafers by using production level nanofabrication tools, and the influence of different processes and material parameters on dislocation defect density was studied. It was observed that only at sufficiently low χN can the block copolymer assemble into well-aligned patterns within a practical time frame. In addition, there is a clear correlation between the rate of the lamellar grain coarsening in unguided self-assembly and the rate of dislocation annihilation in DSA. For a fixed chemical pattern, the density of kinetically trapped dislocation defects can be predicted by measuring the correlation length of the unguided self-assembly under the same process conditions. This learning enables more efficient screening of block copolymers and annealing conditions by rapid analysis of block copolymer films that were allowed to self-assemble into unguided (commonly termed fingerprint) patterns.

Molecular Modeling of EUV Photoresist Revealing the Effect of Chain Conformation on Line-Edge Roughness Formation

Extreme ultraviolet lithography (EUVL) is a leading-edge technology for pattern miniaturization and the production of advanced electronic devices. One of the current critical challenges for further scaling down the technology is reducing the line-edge roughness (LER) of the final patterns while simultaneously maintaining high resolution and sensitivity. As the target sizes of features and LER become closer to the polymer size, polymer chain conformations and their distribution should be considered to understand the primary sources of LER. Here, we proposed a new approach of EUV photoresist modeling with an explicit description of polymer chains using a coarse-grained model. Our new simulation model demonstrated that interface variation represented by width and fluctuation at the edge of the pattern could be caused by characteristic changes of the resist material during the lithography processes. We determined the effect of polymer chain conformation on LER formation and how it finally contributed to LER formation with various resist material parameters (e.g., Flory-Huggins parameter, molecular weight, protected site ratio, and Tg).

Direct Imaging of Interfacial Fluctuations in Confined Block Copolymer with in Situ Slow-Scan-Disabled Atomic Force Microscopy

Using environmentally controlled, high-speed atomic force microscopy (AFM), we examine dynamic fluctuations of topographically confined poly(styrene-block-methyl methacrylate) (PS-b-PMMA) cylinders. During thermal annealing, fluctuations drive perturbations of the block copolymer (BCP) interface between polymer domains, leading to pattern roughness. Whereas previous investigations have examined roughness in room-temperature and kinetically quenched samples, we directly visualize the dynamics of PS/PMMA interfaces in real space and time at in situ temperatures above the glass transition temperature, T_g. Imaging under these experimentally challenging thermal annealing conditions is critical to understanding the inherent connection between thermal fluctuations and BCP pattern assembly. Through the use of slow-scan-disabled AFM, we dramatically improve the imaging time resolution for tracking polymer dynamics. Fluctuations increase in intensity with temperature and, at high temperatures, become spatially coherent across their confining potential. Additionally, we observe that topographic confinement suppresses fluctuations and correlations in the proximity of the guiding field. In situ imaging at annealing temperatures represents a significant step in capturing the dynamics of chain mobility at BCP interfaces.

Shear-solvo defect annihilation of diblock copolymer thin films over a large area

Achieving defect-free block copolymer (BCP) nanopatterns with a long-ranged orientation over a large area remains a persistent challenge, impeding the successful and widespread application of BCP self-assembly. Here, we demonstrate a new experimental strategy for defect annihilation while conserving structural order and enhancing uniformity of nanopatterns. Sequential shear alignment and solvent vapor annealing generate perfectly aligned nanopatterns with a low defect density over centimeter-scale areas, outperforming previous single or sequential combinations of annealing. The enhanced order quality and pattern uniformity were characterized in unprecedented detail via scattering analysis and incorporating new mathematical indices using elaborate image processing algorithms. In addition, using an advanced sampling method combined with a coarse-grained molecular simulation, we found that domain swelling is the driving force for enhanced defect annihilation. The superior quality of large-scale nanopatterns was further confirmed with diffraction and optical properties after metallized patterns, suggesting strong potential for application in optoelectrical devices.

Influence of Homopolymer Addition in Templated Assembly of Cylindrical Block Copolymers

Templated assembly of cylindrical block copolymers provides a promising strategy for patterning holes at the nanoscale. However, remaining challenges include the ability to achieve defect-free patterns and to generate architectures useful for device patterning. The aim of this work is to gain insight into the influence of homopolymer addition on the assembly of a cylindrical block copolymer in confined space. To do so, a concerted examination that relies on experiments and simulations is carried out for different block copolymer/homopolymer blends. It is shown that by adding a majority block homopolymer with low molecular weight (compared to the blocks that make up the block copolymer), the pattern quality is significantly improved and a larger defect-free window is obtained in terms of template dimensions for two-hole features in elliptical confinements. The redistribution of the homopolymer chains effectively enables the assembly of two cylinders, despite the geometrical mismatch between the elliptical shape of the confinement and the natural hexagonal ordering of the unguided block copolymer. Monte Carlo simulations show that the homopolymer segregates to the spaces in the template that are entropically unfavorable for the block copolymer. This work serves to highlight the importance of optimizing block copolymer formulation.

Controlling Self-Assembly in Gyroid Terpolymer Films By Solvent Vapor Annealing

The efficacy with which solvent vapor annealing (SVA) can control block copolymer self-assembly has so far been demonstrated primarily for the simplest class of copolymer, the linear diblock copolymer. Adding a third distinct block-thereby creating a triblock terpolymer-not only provides convenient access to complex continuous network morphologies, particularly the gyroid phases, but also opens up a route toward the fabrication of novel nanoscale devices such as optical metamaterials. Such applications, however, require the generation of well-ordered 3D continuous networks, which in turn requires a detailed understanding of the SVA process in terpolymer network morphologies. Here, in situ grazing-incidence small-angle X-ray scattering (GISAXS) is employed to study the self-assembly of a gyroid-forming triblock terpolymer during SVA, revealing the effects of several key SVA parameters on the morphology, lateral order, and, in particular, its preservation in the dried film. The robustness of the terpolymer gyroid morphology is a key requirement for successful SVA, allowing the exploration of annealing parameters which may enable the generation of films with long-range order, e.g., for optical metamaterial applications.

Defect Annihilation Pathways in Directed Assembly of Lamellar Block Copolymer Thin Films

Defects in highly ordered self-assembled block copolymers represent an important roadblock toward the adoption of these materials in a wide range of applications. This work examines the pathways for annihilation of defects in symmetric diblock copolymers in the context of directed assembly using patterned substrates. Past theoretical and computational studies of such systems have predicted minimum free energy pathways that are characteristic of an activated process. However, they have been limited to adjacent dislocations with opposite Burgers vectors. By relying on a combination of advanced sampling techniques and particle-based simulations, this work considers the long-range interaction between dislocation pairs, both on homogeneous and nanopatterned substrates. As illustrated here, these interactions are central to understanding the defect structures that are most commonly found in applications and in experimental studies of directed self-assembly. More specifically, it is shown that, for dislocation dipoles separated by several lamellae, multiple consecutive free energy barriers lead to effective kinetic barriers that are an order of magnitude larger than those originally reported in the literature for tightly bound dislocation pairs. It is also shown that annihilation pathways depend strongly on both the separation between dislocations and their relative position with respect to the substrate guiding stripes used to direct the assembly.

Nano-confinement of block copolymers in high accuracy topographical guiding patterns: modelling the emergence of defectivity due to incommensurability

Extreme ultraviolet interference lithography (EUV-IL) is used to manufacture topographical guiding patterns to direct the self-assembly of block copolymers. High-accuracy silicon oxide-like patterns with trenches ranging from 68 nm to 117 nm width are fabricated by exposing a hydrogen silsesquioxane (HSQ) resist layer using EUV-IL. We investigate how the accuracy, the low line width roughness and the low line edge roughness of the resulting patterns allow achieving DSA line/space patterns of a PS-b-PMMA (polystyrene-block-poly methyl methacrylate) block copolymer of 11 nm half-pitch with low defectivity. We conduct an in-depth study of the dependence of the DSA pattern morphology on the trench width and on how the neutral brush covers the guiding pattern. We identify the relation between trench width and the emergence of defects with nanometer precision. Based on these studies, we develop a model that extends available free energy models, which allows us to predict the patterning process window.

Curvature as a Guiding Field for Patterns in Thin Block Copolymer Films

Experimental data on thin films of cylinder-forming block copolymers (BC)-free-standing BC membranes as well as supported BC films-strongly suggest that the local orientation of the BC patterns is coupled to the geometry in which the patterns are embedded. We analyze this phenomenon using general symmetry considerations and numerical self-consistent field studies of curved BC films in cylindrical geometry. The stability of the films against curvature-induced dewetting is also analyzed. In good agreement with experiments, we find that the BC cylinders tend to align along the direction of curvature at high curvatures. At low curvatures, we identify a transition from perpendicular to parallel alignment in supported films, which is absent in free-standing membranes. Hence both experiments and theory show that curvature can be used to manipulate and align BC patterns.

High aspect ratio nanotubes assembled from macrocyclic iminium salts

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>10³), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

Influence of the long-range ordering of gold-coated Si nanowires on SERS

Controlling the location and the distribution of hot spots is a crucial aspect in the fabrication of surface-enhanced Raman spectroscopy (SERS) substrates for bio-analytical applications. The choice of a suitable method to tailor the dimensions and the position of plasmonic nanostructures becomes fundamental to provide SERS substrates with significant signal enhancement, homogeneity and reproducibility. In the present work, we studied the influence of the long-range ordering of different flexible gold-coated Si nanowires arrays on the SERS activity. The substrates are made by nanosphere lithography and metal-assisted chemical etching. The degree of order is quantitatively evaluated through the correlation length (ξ) as a function of the nanosphere spin-coating speed. Our findings showed a linear increase of the SERS signal for increasing values of ξ, coherently with a more ordered and dense distribution of hot spots on the surface. The substrate with the largest ξ of 1100 nm showed an enhancement factor of 2.6 · 10³ and remarkable homogeneity over square-millimetres area. The variability of the signal across the substrate was also investigated by means of a 2D chemical imaging approach and a standard methodology for its practical calculation is proposed for a coherent comparison among the data reported in literature.

Engineering the Kinetics of Directed Self-Assembly of Block Copolymers toward Fast and Defect-Free Assembly

Directed self-assembly (DSA) of block copolymers (BCPs) can achieve perfectly aligned structures at thermodynamic equilibrium, but the self-assembling morphology can become kinetically trapped in defective states. Understanding and optimizing the kinetic pathway toward domain alignment is crucial for enhancing process throughput and lowering defectivity to levels required for semiconductor manufacturing, but there is a dearth of experimental, three-dimensional studies of the kinetic pathways in DSA. Here, we combined arrested annealing and TEM tomography to probe the kinetics and structural evolution in the chemoepitaxy DSA of PS- b-PMMA with density multiplication. During the initial stages of annealing, BCP domains developed independently at first, with aligned structures at the template interface and randomly oriented domains at the top surface. As the grains coarsened, the assembly became cooperative throughout the film thickness, and a metastable stitch morphology was formed, representing a kinetic barrier. The stitch morphology had a three-dimensional structure consisting of both perpendicular and parallel lamellae. On the basis of the mechanistic information, we studied the effect of key design parameters on the kinetics and evolution of structures in DSA. Three types of structural evolutions were observed at different film thicknesses: (1) immediate alignment and fast assembly when thickness < L₀ ( L₀ = BCP natural periodicity); (2) formation of stitch morphology for 1.25-1.45 L₀; (3) fingerprint formation when thickness >1.64 L₀. We found that the DSA kinetics can be significantly improved by avoiding the formation of the metastable stitch morphology. Increasing template topography also enhanced the kinetics by increasing the PMMA guiding surface area. A combination of 0.75 L₀ BCP thickness and 0.50 L₀ template topography achieved perfect alignment over 100 times faster than the baseline process. This research demonstrates that an improved understanding of the evolution of structures during DSA can significantly improve the DSA process.