Sacrificial-post templating method for block copolymer self-assembly

Directed Self-Assembly of Polystyrene-Block-Polyhedral Oligomeric Silsesquioxane Monolayer by Nano-Trench for Nanopatterning

This work pioneers to combine fast self-assembly of polyhedral oligomeric silsesquioxanes (POSS) nanocage-based giant surfactants with high etching contrast and directed self-assembly for reliable long-range lateral order to create well-aligned sub-10 nm line nanopatterns via reactive ion etching (RIE). Polystyrene-block-oligo(dimethylsiloxane) substituted POSS (PS-b-oDMS7POSS) with seven oligo(dimethylsiloxane) at the corners of the POSS nanocage and one polystyrene (PS) tail is designed and synthesized as a giant surfactant with self-assembly behaviors like block copolymer (BCP). In contrast to BCP, oDMS7POSS gives a volume-persistent "nanoatom" particle with higher mobility for fast self-assembly and higher segregation strength with PS for smaller feature size. By taking advantage of directed self-assembly using nano-trench fabricated by electron beam lithography, well-ordered nanostructured monolayer with well-aligned parallel oDMS7POSS cylinders can be formed by confined self-assembly within the nano-trench. With the optimization of the RIE treatment using O2 as an etchant, the high etching contrast from the oDMS7POSS and PS gives the formation of well-defined line nanopatterns with sub-10 nm critical dimension that can serve as a mask for pattern transfer in lithography. These results demonstrate a cost-effective approach for nanopatterning by utilizing a creatively designed giant surfactant with sub-10 nm feature size and excellent etching contrast for modern lithographic applications.

Responsive blends of block copolymers stabilize the hexagonally perforated lamellae morphology

Blends of block copolymers can form phases and exhibit features distinct from the constituent materials. We study thin film blends of cylinder-forming and lamellar-forming block copolymers across a range of substrate surface energies. Blend materials are responsive to interfacial energy, allowing selection of pure or coexisting phases based on surface chemistry. Blending stabilizes certain motifs that are typically metastable, and can be used to generate pure hexagonally perforated lamellar thin films across a range of film thicknesses and surface energies. This tolerant behavior is ascribed to the ability of blend materials to redistribute chains to stabilize otherwise high-energy defect structures. The blend responsiveness allows the morphology to be spatially defined through multi-tone chemical surface patterns.

Non-Bulk Morphologies of Extremely Thin Block Copolymer Films Cast on Topographically Defined Substrates Featuring Deep Trenches: The Importance of Lateral Confinement

Directed self-assembly of block copolymers is evolving toward applications that are more defect-tolerant but still require high morphological control and could benefit from simple, inexpensive fabrication processes. Previously, we demonstrated that simply casting ultra-thin block copolymer films on topographically defined substrates leads to hierarchical structures with dual patterns in a controlled manner and unraveled the dependence of the local morphology on the topographic feature dimensions. In this article, we discuss the extreme of the ultraconfined thickness regime at the border of film dewetting. Additional non-bulk morphologies are observed at this extreme, which further elaborate the arsenal of dual patterns that could be obtained in coexistence with full placement control. It is shown that as the thickness confinement approaches its limit, lateral confinement imposed by the width of the plateaus becomes a critical factor influencing the local morphology.

Autonomous discovery of emergent morphologies in directed self-assembly of block copolymer blends

The directed self-assembly (DSA) of block copolymers (BCPs) is a powerful approach to fabricate complex nanostructure arrays, but finding morphologies that emerge with changes in polymer architecture, composition, or assembly constraints remains daunting because of the increased dimensionality of the DSA design space. Here, we demonstrate machine-guided discovery of emergent morphologies from a cylinder/lamellae BCP blend directed by a chemical grating template, conducted without direct human intervention on a synchrotron x-ray scattering beamline. This approach maps the morphology-template phase space in a fraction of the time required by manual characterization and highlights regions deserving more detailed investigation. These studies reveal localized, template-directed partitioning of coexisting lamella- and cylinder-like subdomains at the template period length scale, manifesting as previously unknown morphologies such as aligned alternating subdomains, bilayers, or a "ladder" morphology. This work underscores the pivotal role that autonomous characterization can play in advancing the paradigm of DSA.

Dual Block Copolymer Morphologies in Ultrathin Films on Topographic Substrates: The Effect of Film Curvature

The ability to create mixed morphologies using easily controlled parameters is crucial for the integration of block copolymers in advanced technologies. We have previously shown that casting an ultrathin block copolymer film on a topographically patterned substrate results in different deposited thicknesses on the plateaus and in the trenches, which leads to the co-existence of two patterns. In this work, we highlight the dependence of the dual patterns on the film profile. We suggest that the steepness of the film profile formed across the plateau edge affects the nucleation of microphase-separated domains near the plateau edges, which influences the morphology that develops on the plateau regions. An analysis of the local film thicknesses in multiple samples exhibiting various combinations of plateau and trench widths for different trench depths enabled the construction of phase diagrams, which unraveled the intricate dependence of the formed patterns not only on the curvature of the film profile but also on the fraction of the film that resides in the trenches. Our analysis facilitates the prediction of the patterns that would develop in the trenches and on the plateaus for a given block copolymer film of known thickness from the dimensions of the topographic features.

Hierarchical, Self-Assembled Metasurfaces via Exposure-Controlled Reflow of Block Copolymer-Derived Nanopatterns

Nanopatterning for the fabrication of optical metasurfaces entails a need for high-resolution approaches like electron beam lithography that cannot be readily scaled beyond prototyping demonstrations. Block copolymer thin film self-assembly offers an attractive alternative for producing periodic nanopatterns across large areas, yet the pattern feature sizes are fixed by the polymer molecular weight and composition. Here, a general strategy is reported which overcomes the limitation of the fixed feature size by treating the copolymer thin film as a hierarchical resist, in which the nanoscale pattern motif is defined by self-assembly. Feature sizes can then be tuned by thermal reflow controlled locally by irradiative cross-linking or chemical alteration using lithographic ultraviolet light or electron beam exposure. Using blends of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) with PS and PMMA homopolymers, we demonstrate both self-assembled PS grating and hexagonal hole patterns; exposure-controlled reflow is then used to reduce the hole diameter by as much as 50% or increase the PS grating linewidth by more than 180%. Transferring these nanopatterns, or their inverse obtained by a lift-off approach, into silicon yields structural colors that may be prescriptively controlled based on the nanoscale feature size. Furthermore, patterned exposure enables area-selective feature size control, yielding uniform structural color patterns across centimeter square areas. Electron beam lithography is also used to show that the lithographic resolution of this selective-area control can be extended to the nanoscale dimensions of the self-assembled features. The exposure-controlled reflow approach demonstrated here takes a pivotal step toward fabricating complex, hierarchical optical metasurfaces using scalable self-assembly methods.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Metallic Nanomeshes Fabricated by Multimechanism Directed Self-Assembly

The directed self-assembly of block copolymers (BCPs) is a powerful motif for the continued scaling of feature sizes for nanoscale devices. A multimechanism directed self-assembly (MMDSA) method is described that generates orthogonal meshes from a polystyrene-b-poly-2-vinylpyridine BCP that is subsequently metallized with Pt. The MMDSA process takes advantage of three different mechanisms, trench wall guidance, edge nucleation, and underlayer guidance, to align the mesh with respect to substrate features. The mechanisms and their interactions are investigated via both experiments and dissipative particle dynamics simulations. MMDSA is applied to produce well-aligned conductive nanomeshes and then is extended to fabricate multicomponent metallic structures with 2D/3D hybrid morphologies.

Controlled Spacing between Nanopatterned Regions in Block Copolymer Films Obtained by Utilizing Substrate Topography for Local Film Thickness Differentiation

Various types of devices require hierarchically nanopatterned substrates, where the spacing between patterned domains is controlled. Ultraconfined films exhibit extreme morphological sensitivity to slight variations in film thickness when the substrate is highly selective toward one of the blocks. Here, it is shown that using the substrate's topography as a thickness differentiating tool enables the creation of domains with different surface patterns in a fully controlled fashion from a single, unblended block copolymer. This approach is applicable to block copolymers of different compositions and to different topographical patterns and thus opens numerous possibilities for the hierarchical construction of multifunctional devices.

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.

Microphase Segregation of Diblock Copolymers Studied by the Self-Consistent Field Theory of Scheutjens and Fleer

We used the self-consistent field (SCF) formalism of Scheutjens and Fleer (SF-SCF) to complement existing theoretical investigations on the phase behavior of block copolymer melts. This method employs the freely jointed chain (FJC) model for finite chain length and systematic differences exist compared to the classical SCF predictions. We focus on the critical and hexagonal (HEX) to lamellar (LAM) phase transition region at intermediate and strong segregations. Chain length (N) dependence of the critical point (χcr) was found to be χcrN=10.495(1+4/N). The characteristic spacing (D) of LAM was found as D=4/3N at the critical conditions. We present SF-SCF predictions for the phases single gyroid (SG), double gyroid (DG) and hexagonally perforated lamellar (HPL), in the region where HEX and LAM compete. At χN=30, N=300; we found SG and HPL were metastable with respect to LAM or HEX, DG was stable in a narrow region of the asymmetry ratio. In contrast to the latest predictions, at strong segregation χN=120, DG was found to be metastable. From the structural evolution of HPL, we speculate that this may be an intermediate phase that allows the system to go through various connectivity regimes between minority and majority blocks.

Non-native three-dimensional block copolymer morphologies

Self-assembly is a powerful paradigm, wherein molecules spontaneously form ordered phases exhibiting well-defined nanoscale periodicity and shapes. However, the inherent energy-minimization aspect of self-assembly yields a very limited set of morphologies, such as lamellae or hexagonally packed cylinders. Here, we show how soft self-assembling materials-block copolymer thin films-can be manipulated to form a diverse library of previously unreported morphologies. In this iterative assembly process, each polymer layer acts as both a structural component of the final morphology and a template for directing the order of subsequent layers. Specifically, block copolymer films are immobilized on surfaces, and template successive layers through subtle surface topography. This strategy generates an enormous variety of three-dimensional morphologies that are absent in the native block copolymer phase diagram.

Rapid ordering of block copolymer thin films

Block-copolymers self-assemble into diverse morphologies, where nanoscale order can be finely tuned via block architecture and processing conditions. However, the ultimate usage of these materials in real-world applications may be hampered by the extremely long thermal annealing times-hours or days-required to achieve good order. Here, we provide an overview of the fundamentals of block-copolymer self-assembly kinetics, and review the techniques that have been demonstrated to influence, and enhance, these ordering kinetics. We discuss the inherent tradeoffs between oven annealing, solvent annealing, microwave annealing, zone annealing, and other directed self-assembly methods; including an assessment of spatial and temporal characteristics. We also review both real-space and reciprocal-space analysis techniques for quantifying order in these systems.

Selective directed self-assembly of coexisting morphologies using block copolymer blends

Directed self-assembly (DSA) of block copolymers is an emergent technique for nano-lithography, but is limited in the range of structures possible in a single fabrication step. Here we expand on traditional DSA chemical patterning. A blend of lamellar- and cylinder-forming block copolymers assembles on specially designed surface chemical line gratings, leading to the simultaneous formation of coexisting ordered morphologies in separate areas of the substrate. The competing energetics of polymer chain distortions and chemical mismatch with the substrate grating bias the system towards either line/space or dot array patterns, depending on the pitch and linewidth of the prepattern. This is in contrast to the typical DSA, wherein assembly of a single-component block copolymer on chemical templates generates patterns of either lines/spaces (lamellar) or hexagonal dot arrays (cylinders). In our approach, the chemical template encodes desired local spatial arrangements of coexisting design motifs, self-assembled from a single, sophisticated resist.

There is a limited range of structures available in nanolithography using directed self-assembled block copolymers. Here, Black and co-workers expand directed self-assembly chemical patterning by using a blend of lamellar and cylinder forming block copolymers on surface chemical line gratings.

Multilayer block copolymer meshes by orthogonal self-assembly

Continued scaling-down of lithographic-pattern feature sizes has brought templated self-assembly of block copolymers (BCPs) into the forefront of nanofabrication research. Technologies now exist that facilitate significant control over otherwise unorganized assembly of BCP microdomains to form both long-range and locally complex monolayer patterns. In contrast, the extension of this control into multilayers or 3D structures of BCP microdomains remains limited, despite the possible technological applications in next-generation devices. Here, we develop and analyse an orthogonal self-assembly method in which multiple layers of distinct-molecular-weight BCPs naturally produce nanomesh structures of cylindrical microdomains without requiring layer-by-layer alignment or high-resolution lithographic templating. The mechanisms for orthogonal self-assembly are investigated with both experiment and simulation, and we determine that the control over height and chemical preference of templates are critical process parameters. The method is employed to produce nanomeshes with the shapes of circles and Y-intersections, and is extended to produce three layers of orthogonally oriented cylinders.

Universal pattern transfer methods for metal nanostructures by block copolymer lithography

A universal block copolymer pattern transfer method was demonstrated to produce Co nanostructures consisting of arrays of lines or dots from a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) diblock copolymer. Three processes were used: liftoff, a damascene process, and ion beam etching using a hard mask of tungsten, including a sacrificial poly(methyl methacrylate) layer under the PS-b-PDMS for the etch and liftoff processes. The ion beam etch process produced the most uniform magnetic arrays. A structural and magnetic comparison in terms of uniformity, edge roughness and switching field distribution has been reported.

Simulation methods for solvent vapor annealing of block copolymer thin films

Recent progress in modelling the solvent vapor annealing of thin film block copolymers is examined in the context of a self-consistent field theory framework. Key control variables in determining the final microdomain morphologies include swelling ratio or swollen film solvent volume fraction, swollen film thickness, substrate and vapor atmosphere surface energies, effective volume fraction, and effective Flory-Huggins interaction parameter. The regime of solvent vapor annealing studied is where the block copolymer has a high enough Flory-Huggins parameter that ordered structures form during swelling and are then trapped in the system through quenching. Both implicit and explicit consideration of the solvent vapor is considered to distinguish the cases in which solvent vapor leads to a non-bulk morphology. Block-selective solvents are considered based on the experimental systems of polystyrene-b-polydimethylsiloxane annealed with toluene and heptane. The results of these simulations are compared with these experiments.

Photopatterning of cross-linkable epoxide-functionalized block copolymers and dual-tone nanostructure development for fabrication across the nano- and microscales

The self-assembly of block copolymers in thin films provides an attractive approach to patterning 5-100 nm structures. Cross-linking and photopatterning of the self-assembled block copolymer morphologies provide further opportunities to structure such materials for lithographic applications, and to also enhance the thermal, chemical, or mechanical stability of such nanostructures to achieve robust templates for subsequent fabrication processes. Here, model lamellar-forming diblock copolymers of polystyrene and poly(methyl methacrylate) with an epoxide functionality are synthesized by atom transfer radical polymerization. We demonstrate that self-assembly and cross-linking of the reactive block copolymer materials in thin films can be decoupled into distinct, controlled process steps using solvent annealing and thermal treatment/ultraviolet exposure, respectively. Conventional optical lithography approaches can also be applied to the cross-linkable block copolymer materials in thin films and enable simultaneous structure formation across scales-micrometer scale patterns achieved by photolithography and nanostructures via self-assembly of the block copolymer. Such materials and processes are thus shown to be capable of self-assembling distinct block copolymers (e.g., lamellae of significantly different periodicity) in adjacent regions of a continuous thin film.

Large-area nanosquare arrays from shear-aligned block copolymer thin films

While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.