Directed Self-assembly Of Block Copolymers Research Articles

Boundary-Directed Epitaxy of Block Copolymers Toward Sub-10 nm Fabrication

The directed self-assembly (DSA) of block copolymers (BCPs) can be used to produce nanometer-scale patterns without the cost and process complexity of state-of-the-art optical nanolithography. Thus, DSA is of potential use in a wide variety of semiconductor technologies, such as finFETs and biosensors. To control the position and orientation of BCP domains and create technologically useful patterns, conventional DSA mechanisms often rely on topographically complex structures or high-resolution chemical patterns that are difficult to fabricate. In comparison, a newly discovered mechanism for the DSA of BCPs – termed “boundary-directed epitaxy” – utilizes the chemical contrast at the boundaries between a substrate and relatively wide chemical stripe.[1] As a result, aligned and high density-multiplied BCP nanopatterns with half-pitch of 6.4 nm have been created using stripes as wide as 100 nm. By obviating the need for high-resolution chemical patterns, boundary-directed epitaxy offers exciting opportunities for the fabrication of sub-10 nm nanostructures.[1] Boundary-directed epitaxy of block copolymers. R.M. Jacobberger, V. Thapar, G.-P. Wu, T.-H. Chang, V. Saraswat, A.J. Way, K.R. Jinkins, Z. Ma, P.F. Nealey, S.-M. Hur, S. Xiong, M.S. Arnold, Nature Communications 11, 4151 (2020), 10.1038/s41467-020-17938-3. Figure 1

Orbital Angular Momentum from Self-Assembled Concentric Nanoparticle Rings.

Ring-shaped nanostructures can focus, filter, and manipulate electromagnetic waves, but are challenging to incorporate into devices using standard nanofabrication techniques. Directed self-assembly (DSA) of block copolymers (BCPs) on lithographically patterned templates has successfully been used to fabricate concentric rings and spirals as etching masks. However, this method is limited by BCP phase behavior and material selection. Here, a straightforward approach to generate ring-shaped nanoparticle assemblies in thin films of supramolecular nanocomposites is demonstrated. DSA is used to guide the formation of concentric rings with radii spanning 150-1150nm and ring widths spanning 30-60nm. When plasmonic nanoparticles are used, ring nanodevice arrays can be fabricated in one step, and the completed devices produce high-quality orbital angular momentum (OAM). Nanocomposite DSA simplifies and streamlines nanofabrication by producing metal structures without etching or deposition steps; it also introduces interparticle coupling as a new design axis. Detailed analysis of the nanoparticle ring assemblies confirms that the supramolecular system self-regulates the spatial distribution of its components, and thus exhibits a degree of flexibility absent in DSA of BCPs alone, where structures are determined by polymer-pattern incommensurability. The present studies also provide guidelines for developing self-regulating DSA as an alternative to incommensurability-driven methods.

Mesoporous Titanium Oxynitride Monoliths from Block Copolymer-Directed Self-Assembly of Metal-Urea Additives.

This report describes a simple one-pot soft-templating and ammonolysis-free approach to synthesize mesoporous crystalline titanium oxynitride by combining block copolymer-directed self-assembly with metal sol and urea precursors. The Pluronic F127 triblock copolymer was employed to structure-direct titanium-oxo-acetate sol nanoparticles and urea-formaldehyde into ordered hybrid mesostructured monoliths. The hybrid composites were directly converted into mesoporous crystalline titanium oxynitride and retained macroscale monolithic integrity up to 800 °C under nitrogen. Notably, the urea-formaldehyde additive provided nitrogen and rigid support to the inorganic mesostructure during crystallization. The resultant mesoporous titanium oxynitride exhibited good electrochemical catalytic activity toward hydrogen evolution reaction in 1 M KOH aqueous medium under applied bias. Our results suggest an inexpensive and safe pathway to generate ordered mesoporous crystalline metal oxynitride structures suitable for catalyst and energy-storage applications.

Lamellar Orientation of a Block Copolymer via an Electron-Beam Induced Polarity Switch in a Nitrophenyl Self-Assembled Monolayer or Si Etching Treatments

Directed self-assembly (DSA) was investigated on self-assembled monolayers (SAMs) chemically modified by electron beam (EB) irradiation, which is composed of 6-(4-nitrophenoxy) hexane-1-thiol (NPHT). Irradiating a NPHT by EB could successfully induce the orientation and selective patterning of block copolymer domains. We clarified that spatially-selective lamellar orientations of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) could be achieved by a change of an underlying SAM. The change of an underlying SAM is composed of the transition of an NO2 group to an NH2 group, which is induced by EB. The modification in the polarity of different regions of the SAM with EB lithography controlled the lamellar orientation of PS-b-PMMA. The reduction of the NPHT SAM plays an important role in the orientation of block copolymer. This method might significantly simplify block copolymer DSA processes when it is compared to the conventional DSA process. By investigating the lamellae orientation with EB, it is clarified that only suitable annealing temperatures and irradiation doses lead to the vertical orientation. We also fabricated pre-patterned Si substrates by EB lithographic patterning and reactive ion etching (RIE). DSA onto such pre-patterned Si substrates was proven to be successful for subdivision of the lithographic patterns into line and space patterns.

Emergent symmetries in block copolymer epitaxy

The directed self-assembly (DSA) of block copolymers (BCPs) has shown promise in fabricating customized two-dimensional (2D) geometries at the nano- and meso-scale. Here, we discover spontaneous symmetry breaking and superlattice formation in DSA of BCP. We observe the emergence of low symmetry phases in high symmetry templates for BCPs that would otherwise not exhibit these phases in the bulk or thin films. The emergence phenomena are found to be a general behavior of BCP in various template layouts with square local geometry, such as 44 and 32434 Archimedean tilings and octagonal quasicrystals. To elucidate the origin of this phenomenon and confirm the stability of the emergent phases, we implement self-consistent field theory (SCFT) simulations and a strong-stretching theory (SST)-based analytical model. Our work demonstrates an emergent behavior of soft matter and draws an intriguing connection between 2-dimensional soft matter self-assembly at the mesoscale and inorganic epitaxy at the atomic scale.

Block Copolymer Self-Assembly Directed Hierarchically Structured Materials from Nonequilibrium Transient Laser Heating

We highlight two recent approaches operating far from equilibrium for the synthesis of hierarchical porous thin film materials by coupling block copolymer-directed self-assembly with transient laser heating. In first block copolymer-induced writing by transient heating experiments, or B-WRITE, an all-organic block copolymer–resols hybrid film is heated by submillisecond carbon dioxide laser irradiation, directly generating 3D mesoporous continuous resin structures and shapes. Harnessing the highly unique resin materials properties under laser heating conditions, in the second approach block copolymer-directed resin templating is coupled with nanosecond pulsed excimer laser annealing to generate complementary crystalline silicon nanostructures. The underlying structure formation mechanisms for such laser-induced organic and inorganic nanostructured materials are discussed, emphasizing that the nonequilibrium nature of these transient laser annealing approaches opens up vast and new processing windows beyon...

Polymer-based multiferroic nanocomposites via directed block copolymer self-assembly.

The existence of ferroelectricity and ferromagnetism in multiferroic materials and their coupling enables the manipulation of the electric polarization with applied magnetic field and vice versa, opening many doors for the practical applications. However, the preparation of polymeric multiferroic nanocomposites is often accompanied with aggregation of magnetic particles inside the ferroelectric polymeric matrix. To overcome this issue, we developed a simple and straightforward method to obtain multiferroic nanocomposites with an exceptional and selective dispersion of magnetic nanoparticles, using self-assembly of poly(vinylidene fluoride) (PVDF)-based block copolymers. Magnetic cobalt ferrite nanoparticles modified with gallic acid are selectively incorporated within poly(2-vinylpyridine) (P2VP) domains of the lamellar block copolymer due to strong hydrogen bond formation between the ligand and the P2VP block. Using this approach, phase separation between the blocks is improved, which leads to an increase in the degree of crystallinity, whereas the selective dispersion of nanoparticles inside amorphous domains prevents changes in the crystalline phase of the ferroelectric block. The obtained nanocomposites demonstrate both ferroelectric and magnetic properties without large conductive losses at high electric field, making them good candidates for improved multiferroic devices.

Directing Block Copolymer Self-Assembly on Patterned Substrates.

Self-assembling block copolymer films provide access to a variety of different nanostructured patterns in one, two, and three dimensions. However, in the absence of any templating, these nanostructures suffer from defects, often limiting utility. Directed block copolymer self-assembly uses patterned substrates that effectively suppress defect formation and allow the creation of desired patterns. The two main directed self-assembly techniques, chemoepitaxy and graphoepitaxy, employ chemically and topographically patterned substrates, respectively, to direct the block copolymer assembly in thin films. Their successful application in generating defect-free patterns in films of block copolymers exhibiting particular morphologies is summarized in this concept article. The possible role of directed self-assembly in extending nanostructured patterning from two to three dimensions is also discussed.

Polymersomes with high loading capacity prepared by direct self-assembly of block copolymers in drugs

We develop a new method to prepare block copolymer vesicles with high loading capacity of drugs which can then be released in a controlled manner. The block copolymers, including PS-b-PAA, PS-b-PEO, and biocompatible PCL-b-PEO, can directly self-assemble into vesicles in aspirin and encapsulate aspirin by solvent annealing with ethanol which imparts mobility to the originally solid block copolymers and aspirin molecules. Aspirin associates with the hydrophilic blocks after premixing, firstly leading to the formation of bilayer structures. During solvent annealing, the bilayers are wrapped into vesicles to enclose aspirin that fills the cores of the vesicles. The interactions between block copolymers and aspirin were probed by FT-IR, and the formation of aspirin-loaded vesicles were confirmed by transmission electron microscopy and dynamic light scattering. The loading content of aspirin in the extracted vesicles is 59 ± 5%, higher than that of conventional vesicles formed in liquid systems with dilute drugs. The release rate and final release amount of aspirin from vesicles in aqueous solutions can be controlled by addition of different amount of n-dioxane or by changing pH values.

Imprint strategy for directed self-assembly of block copolymers

The directed self-assembly (DSA) of block copolymers (BCP) has attracted high interest for the definition of nanostructures in an almost self-forming way when adequate boundary conditions are given. At present, grapho- and chemo-epitaxy are the workhorses but they require precisely patterned substrates to serve as the guiding pattern. Nanoimprint may replace this laborious pre-patterning of each substrate by employing an adequate stamp that can be used multiple times, inducing the guided DSA from the top of the film. Here, the DSA of BCPs is revisited in view of the specific nanoimprint situation. As a consequence, the BCP layer is imprinted in a partial cavity-filling mode, using a stamp of sufficient height provided with a conventional anti-sticking layer; substrate pre-treatment is minimized or rather avoided. Even with a highly preferential substrate it is possible to obtain vertical lamellae that are largely oriented in parallel to the stamp edges in PS-b-PMMA (polystyrene-block-polymethyl methacrylate) already after 3min of imprint. The vertical lamellae are at least 70nm high, freestanding on the substrate. Though optimization is required the results indicate the high potential of nanoimprint to simplify the DSA of BCPs for technical applications, also beyond Si technology.

Integration of a templated directed self-assembly-based hole shrink in a short loop via chain

The use of directed self-assembly (DSA) of cylinder forming block copolymers (BCP) for contact hole shrink applications has gained increased attention due to the dimensions that can be achieved with this materials. Recent work has focused on engineering the dimensions and surface energy of the templates to obtain straight profiles of the cylinders assembled in them. However, the impact of process optimization on defect formation is measured using scanning electron microscopy before and after transferring the BCP features to a hardmask, which provides limited information about the presence of defects or three-dimensional morphologies in the polymer structures. To identify the presence of single defects in arrays of various densities and sizes, we use Kelvin and chain structures available in the IMEC 28-nm node via chain electrical test vehicle, Everest, in combination with templated DSA. We tuned the surface energy and dimensions of the templates with the use of random copolymers and through the exposure conditions, respectively. Finally, the contact holes obtained with templated DSA of BCP were subsequently transferred into a relevant stack to apply advanced metallization processes and, ultimately, validated electrically.

Post-directed-self-assembly membrane fabrication for in situ analysis of block copolymer structures

Full characterization of the three-dimensional structures resulting from the directed self-assembly (DSA) of block copolymers (BCP) remains a difficult challenge. Transmission electron microscope (TEM) tomography and resonant soft x-ray scattering have emerged as powerful and complementary methods for through-film characterization; both techniques require samples to be prepared on specialized membrane substrates. Here we report a generalizable process to implement BCP DSA with density multiplication on silicon nitride membranes. A key feature of the process developed here is that it does not introduce any artefacts or damage to the polymer assemblies as DSA is performed prior to back-etched membrane formation. Because most research and applications of BCP lithography are based on silicon substrates, process variations introduced by implementing DSA on a silicon nitride/silicon stack versus silicon were identified and mitigated. Using full-wafers, membranes were fabricated with different sizes and layouts to enable both TEM and x-ray characterization. Finally, both techniques were used to characterize structures resulting from the DSA of lamella-forming BCP with density multiplication.

Formation of Periodically-Ordered Calcium Phosphate Nanostructures by Block Copolymer-Directed Self-Assembly

Structuring ionic solids at the nanoscale with block copolymers (BCPs) is notoriously difficult due to solvent incompatibilities and strong driving forces for crystallization of the inorganic material. Here, we demonstrate that elucidating pathway complexity in the BCP-directed self-assembly of an ionic solid, amorphous calcium phosphate (ACP), is a key component in obtaining nanostructured, bulk composite materials in which the nanostructure is the result of thermodynamically controlled BCP self-assembly, i.e., exhibiting sequences of bulk morphologies as known from typical equilibrium BCP phase diagrams. Specifically, we identify three critical pathway “decision points” for the evaporation-induced self-assembly of composites from ultrasmall, organosilicate-modified amorphous calcium phosphate nanoparticles (osm-ACP-NPs) and poly(isoprene)-block-poly(2-(dimethylamino)ethyl methacrylate) (PI-b-PDMAEMA) block copolymers. Using this strategy enabled us to obtain composites with hexagonal, cubic network, and...

Mesoporous monoliths of inverse bicontinuous cubic phases of block copolymer bilayers.

Solution self-assembly of block copolymers into inverse bicontinuous cubic mesophases is a promising new approach for creating porous polymer films and monoliths with highly organized bicontinuous mesoporous networks. Here we report the direct self-assembly of block copolymers with branched hydrophilic blocks into large monoliths consisting of the inverse bicontinuous cubic structures of the block copolymer bilayer. We suggest a facile and scalable method of solution self-assembly by diffusion of water to the block copolymer solution, which results in the unperturbed formation of mesoporous monoliths with large-pore (>25 nm diameter) networks weaved in crystalline lattices. The surface functional groups of the internal large-pore networks are freely accessible for large guest molecules such as protein complexes of which the molecular weight exceeded 100 kDa. The internal double-diamond (Pn3m) networks of large pores within the mesoporous monoliths could be replicated to self-supporting three-dimensional skeletal structures of crystalline titania and mesoporous silica.

Computational study of directed self-assembly for contact-hole shrink and multiplication

We use three-dimensional self-consistent field theory (SCFT) to study the directed self-assembly (DSA) of cylinder-forming block copolymers in a peanut-shaped (also called egg-box) prepattern. The design of the prepattern shape will target the pitch reduction of the contact holes. The idea is that the DSA of block copolymers will not only lead to reduced critical dimensions relative to the template but will also repair defects in the guiding prepatterns and produce defect-free contact holes. We also study blends of block copolymers and homopolymers with various lengths and volume fractions. Using SCFT simulations, we establish the effects of the added homopolymer on defectivity, the process window, and the properties of the formed cylinders. In an attempt to quantify the effect of thermal fluctuations on the placement of the cylinders, we resort to complex Langevin simulations and perform a stochastic sampling of the assembled morphologies.

New route toward integrating large nickel nanocrystals onto mesoporous carbons

Integrating large and magnetic transition metal nanocrystals onto mesoporous carbons is of great importance owing to their low cost, easy separation and great potential in various applications. Herein we firstly report large nickel nanocrystals (Ni NCs, 30–75nm) in situ integrated onto mesoporous carbons (MC), synthesized via block copolymer-directed self-assembly of nickel acetate and 8-quinolinol modified chitosan under neutral conditions assisted by tetraethyl orthosilicate before pyrolysis and silica removal. This neutral one-pot synthesis not only renders evenly dispersed Ni NCs firmly attached on the mesoporous carbonaceous framework without using any stabilizer, but also creates larger Ni NCs as compared to those prepared by acidic and basic synthetic routes. It is found that all nickel catalysts can be magnetically separated easily; and the Ni NC size, microstructure and the carbonaceous support morphology are well adjusted by variation of the pyrolysis temperature, which in turn affects the catalytic properties of Ni NCs towards 4-nitrophenol reduction. The Ni NC functionalized mesoporous carbon pyrolyzed at 750°C (Ni–MC–750) simultaneously possessing twinned microstructure and carbon nanotube morphology, exhibits the highest catalytic efficiency. High-energy X-ray diffraction, N2 adsorption/desorption, transmission electron microscopy, infrared spectroscopy, X-ray absorption fine structure and Raman spectroscopy studies, as well as the comparative catalytic tests have demonstrated these points.

Colloidal inverse bicontinuous cubic membranes of block copolymers with tunable surface functional groups.

Analogous to the complex membranes found in cellular organelles, such as the endoplasmic reticulum, the inverse cubic mesophases of lipids and their colloidal forms (cubosomes) possess internal networks of water channels arranged in crystalline order, which provide a unique nanospace for membrane-protein crystallization and guest encapsulation. Polymeric analogues of cubosomes formed by the direct self-assembly of block copolymers in solution could provide new polymeric mesoporous materials with a three-dimensionally organized internal maze of large water channels. Here we report the self-assembly of amphiphilic dendritic-linear block copolymers into polymer cubosomes in aqueous solution. The presence of precisely defined bulky dendritic blocks drives the block copolymers to form spontaneously highly curved bilayers in aqueous solution. This results in the formation of colloidal inverse bicontinuous cubic mesophases. The internal networks of water channels provide a high surface area with tunable surface functional groups that can serve as anchoring points for large guests such as proteins and enzymes.

Rapid-flux-solvent-atmosphere method for tailoring the morphology of titania substrates over a large area via direct self-assembly of block copolymers

A fast method for the preparation of block-copolymer-based hybrid composite nanostructures and titania substrates well oriented over a large area, is illustrated.

Device-oriented graphene nanopatterning by mussel-inspired directed block copolymer self-assembly

Directed self-assembly of a block copolymer is successfully employed to fabricate device-oriented graphene nanostructures from CVD grown graphene. We implemented mussel-inspired polydopamine adhesive in conjunction with the graphoepitaxy principle to tailor graphene nanoribbon arrays and a graphene nanomesh located between metal electrodes. Polydopamine adhesive was utilized for facile and damage-free surface treatment to complement the low surface energy of pristine graphene. Our process minimizes the damage to the ideal graphitic structures and electrical properties of graphene during the nanopatterning process. Multi-channel graphene nanoribbon arrays and a graphene nanomesh were successfully fabricated between metal electrodes.

Pattern transfer using block copolymers

To meet the increasing demand for patterning smaller feature sizes, a lithography technique is required with the ability to pattern sub-20 nm features. While top-down photolithography is approaching its limit in the continued drive to meet Moore's law, the use of directed self-assembly (DSA) of block copolymers (BCPs) offers a promising route to meet this challenge in achieving nanometre feature sizes. Recent developments in BCP lithography and in the DSA of BCPs are reviewed. While tremendous advances have been made in this field, there are still hurdles that need to be overcome to realize the full potential of BCPs and their actual use.