Defect Structure in Thin Films of a Lamellar Block Copolymer Self-Assembled on Neutral Homogeneous and Chemically Nanopatterned Surfaces

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.

Competitive Registration Fields for The Development of Complex Block Copolymer Structures by A Layer-by-Layer Approach

Block copolymer (BCP) self-assembly in thin films is an elegant method to generate nanometric features with tunable geometrical configurations. By combining directed assembly and hybridization methods, advances in nano-manufacturing have been attested over the past decades with flagship applications in lithography and optics. Nevertheless, the range of geometrical configurations is limited by the accessible morphologies inherent to the energy minimization process involved in BCP self-assembly. Layering of nanostructured BCP thin films has been recently proposed in order to enrich the span of nanostructures derived from BCP self-assembly with the formation of non-native heterostructures such as double-layered arrays of nanowires or dots-on-line and dots-in-hole hierarchical structures. In this work, the layer-by-layer method is further exploited for the generation of nano-mesh arrays using nanostructured BCP thin films. In particular, a subtle combination of chemical and topographical fields is leveraged in order to demonstrate design rules for the controlled registration of a BCP layer on top of an underneath immobilized one by the precise tuning of the interfacial chemical field between the two BCP layers.

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.

Dynamic Ordering in High-χ Block Copolymer Lamellae Based on Cross-Sectional Orientational Alignment

Further development of next-generation block copolymer (BCP) lithography processes is contingent on comprehensive studies of the ordering dynamics of high-χ BCPs that can form sub-10 nm features on thin films. However, quantitative analyses of the degree of ordering on the surface and cross sections of thin films have been difficult to execute. To tackle this challenge, we employ a perpendicular lamella-forming high-χ BCP, poly(polyhedral oligomeric silsesquixone-block-2,2,2-trifluoroethyl methacrylate) (PMAPOSS-b-PTFEMA), and reveal that the high-χ PMAPOSS-b-PTFEMA requires three times the activation energy (E_a) compared to that of poly(styrene-block-methyl methacrylate) (PS-b-PMMA) for defect annihilation, at E_a = 2600 ± 420 kJ mol^-1, and a transition from a fast ordering regime with a growth exponent of Φ = 0.30 at lower orientational order parameters (ψ₂ < 0.36) to a slow ordering regime with Φ < 0.05 at ψ₂ > 0.36, where well-aligned lamellae restrict defect annihilations to enthalpically unfavorable glide mechanisms that require BCP intermixing.

Rapid solvo-microwave annealing for optimizing ordered nanostructures and crystallization of regioregular polythiophene-based block copolymers

Solvo-microwave annealing is an effective method for producing thin films of polythiophene-based block copolymers with ordered structures and high crystallinity in a very short processing time (∼3 min).

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.

Self-assembly of gradient copolymers synthesized in semi-batch mode by nitroxide mediated polymerization

The effect of diffuse compositional interfaces on copolymer self-assembly was studied via gradient copolymers (GCP). Poly(methyl methacrylate)-grad-(styrene) (PMMA-grad-PSt) copolymers were synthesized in semi-batch mode using nitroxide-mediated polymerization (NMP) with varied monomer injection protocols to produce varied diffuse interfaces (number average molecular weights (M_n) ranged from 62 000 g mol^-1 to 94 000 g mol^-1 with dispersities (Đ) between 1.35 and 1.59). The GCPs were spun into thin films on substrates made neutral by (St-ran-MMA-ran-hydroxyethyl methacrylate) terpolymers and annealed at elevated temperature to produce vertically oriented microphase-separated domains. The GCPs were found to have domain spacing larger than equivalent monodisperse BCPs, due to their polydisperse nature. This effect was partially offset by the decrease in χ due to the gradient. GCPs synthesized with a single-injection protocol (i.e. less diffuse interfaces) were found to self-assemble into ordered domains. However, GCPs synthesized with long injection times (i.e. more diffuse interfaces) exhibited poor self-assembly attributed to their predicted statistical-copolymer-like middle sequence, which caused a reduction of the effective enthalpic interaction parameter.

Influence of Grafting Point Distribution on the Surface Structures of Y-Shaped Polymer Brushes in Solution

We report a simulated annealing study of surface structures of the Y-shaped copolymers grafted onto a planar substrate in nonselective solvents. The influences of the lateral size of the grafting surface and the distribution manner of the grafting point on the order degree of the ripple structures are investigated. Under uniformly distribution conditions, it is found that the well-defined ripple structures can be formed when the lateral size less than a threshold which depends on the solvent quality and grafting density. However, introducing a density fluctuation into the uniformly distribution grafting points in different ways, the defects with different degrees are observed in the ripple structures. The influence of the density fluctuations on the ripple phase are studied quantitatively. Furthermore, the possibility of the formation of surface structures with long-range order induced by directed self-assembly is investigated. The findings provide guidelines for fabricating patterned surfaces with highly ordered structures.

Strip-Pattern-Spheres Self-Assembled from Polypeptide-Based Polymer Mixtures: Structure and Defect Features

We found that poly(γ-benzyl-L-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) rod-coil block copolymers and polystyrene (PS) homopolymers can cooperatively self-assemble into nano-spheres with striped patterns on their surfaces (strip-pattern-spheres) in aqueous solution. With assistance of dissipative particle dynamics simulation, it is discovered that the PS homopolymers form a spherical template core and the PBLG-b-PEG block copolymers assemble into striped patterns on the spherical surface. The hydrophobic PBLG rods are packed orderly in the strips, while the hydrophilic PEG blocks stabilize the strip-pattern-spheres in solution. Defects such as dislocations and disclinations can be observed in the striped patterns. Self-assembling temperature and sphere radius are found to affect defect densities in the striped patterns. A possible mechanism is proposed to illustrate how PBLG-b-PEG and PS cooperatively self-assemble into hierarchical spheres with striped patterns on surfaces.

Molecular pathways for defect annihilation in directed self-assembly

Over the last few years, the directed self-assembly of block copolymers by surface patterns has transitioned from academic curiosity to viable contender for commercial fabrication of next-generation nanocircuits by lithography. Recently, it has become apparent that kinetics, and not only thermodynamics, plays a key role for the ability of a polymeric material to self-assemble into a perfect, defect-free ordered state. Perfection, in this context, implies not more than one defect, with characteristic dimensions on the order of 5 nm, over a sample area as large as 100 cm(2). In this work, we identify the key pathways and the corresponding free energy barriers for eliminating defects, and we demonstrate that an extraordinarily large thermodynamic driving force is not necessarily sufficient for their removal. By adopting a concerted computational and experimental approach, we explain the molecular origins of these barriers and how they depend on material characteristics, and we propose strategies designed to overcome them. The validity of our conclusions for industrially relevant patterning processes is established by relying on instruments and assembly lines that are only available at state-of-the-art fabrication facilities, and, through this confluence of fundamental and applied research, we are able to discern the evolution of morphology at the smallest relevant length scales-a handful of nanometers-and present a view of defect annihilation in directed self-assembly at an unprecedented level of detail.

Defects in the Self-Assembly of Block Copolymers and Their Relevance for Directed Self-Assembly

Block copolymer self-assembly provides a platform for fabricating dense, ordered nanostructures by encoding information in the chemical architecture of multicomponent macromolecules. Depending on the volume fraction of the components and chain topology, these macromolecules form a variety of spatially periodic microphases in thermodynamic equilibrium. The kinetics of self-assembly, however, often results in initial morphologies with defects, and the subsequent ordering is protracted. Different strategies have been devised to direct the self-assembly of copolymer materials by external fields to align and perfect the self-assembled nanostructures. Understanding and controlling the thermodynamics of defects, their response to external fields, and their dynamics is important because applications in microelectronics either require extremely low defect densities or aim at generating specific defects at predetermined locations to fabricate irregular device-oriented structures for integrated circuits. In this review, we discuss defect morphologies of block copolymers in the bulk and thin films, highlighting (a) analogies to and differences from defects in other crystalline materials, (b) the stability of defects and their dynamics, and (c) the influence of external fields.

Determination of the internal morphology of nanostructures patterned by directed self assembly

The directed self-assembly (DSA) of block copolymers (BCP) is an emerging resolution enhancement tool that can multiply or subdivide the pitch of a lithographically defined chemical or topological pattern and is a resolution enhancement candidate to augment conventional lithography for patterning sub-20 nm features. Continuing the development of this technology will require an improved understanding of the polymer physics involved as well as experimental confirmation of the simulations used to guide the design process. Both of these endeavors would be greatly facilitated by a metrology, which is capable of probing the internal morphology of a DSA film. We have developed a new measurement technique, resonant critical-dimension small-angle X-ray scattering (res-CDSAXS), to evaluate the 3D buried features inside the film. This is an X-ray scattering measurement where the sample angle is varied to probe the 3D structure of the film, while resonant soft X-rays are used to enhance the scattering contrast. By measuring the same sample with both res-CDSAXS and traditional CDSAXS (with hard X-rays), we are able to demonstrate the dramatic improvement in scattering obtained through the use of resonant soft X-rays. Analysis of the reciprocal space map constructed from the res-CDSAXS measurements allowed us to reconstruct the complex buried features in DSA BCP films. We studied a series of DSA BCP films with varying template widths, and the internal morphologies for these samples were compared to the results of single chain in mean-field simulations. The measurements revealed a range of morphologies that occur with changing template width, including results that suggest the presence of mixed morphologies composed of both whole and necking lamella. The development of res-CDSAXS will enable a better understanding of the fundamental physics behind the formation of buried features in DSA BCP films.

Deconvoluting the mechanism of microwave annealing of block copolymer thin films

The self-assembly of block copolymer (BCP) thin films is a versatile method for producing periodic nanoscale patterns with a variety of shapes. The key to attaining a desired pattern or structure is the annealing step undertaken to facilitate the reorganization of nanoscale phase-segregated domains of the BCP on a surface. Annealing BCPs on silicon substrates using a microwave oven has been shown to be very fast (seconds to minutes), both with and without contributions from solvent vapor. The mechanism of the microwave annealing process remains, however, unclear. This work endeavors to uncover the key steps that take place during microwave annealing, which enable the self-assembly process to proceed. Through the use of in situ temperature monitoring with a fiber optic temperature probe in direct contact with the sample, we have demonstrated that the silicon substrate on which the BCP film is cast is the dominant source of heating if the doping of the silicon wafer is sufficiently low. Surface temperatures as high as 240 °C are reached in under 1 min for lightly doped, high resistivity silicon wafers (n- or p-type). The influence of doping, sample size, and BCP composition was analyzed to rule out other possible mechanisms. In situ temperature monitoring of various polymer samples (PS, P2VP, PMMA, and the BCPs used here) showed that the polymers do not heat to any significant extent on their own with microwave irradiation of this frequency (2.45 GHz) and power (∼600 W). It was demonstrated that BCP annealing can be effectively carried out in 60 s on non-microwave-responsive substrates, such as highly doped silicon, indium tin oxide (ITO)-coated glass, glass, and Kapton, by placing a piece of high resistivity silicon wafer in contact with the sample-in this configuration, the silicon wafer is termed the heating element. Annealing and self-assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCPs into horizontal cylinder structures were shown to take place in under 1 min, using a silicon wafer heating element, in a household microwave oven. Defect densities were calculated and were shown to decrease with higher maximum obtained temperatures. Conflicting results in the literature regarding BCP annealing with microwave are explained in light of the results obtained in this study.

Local reorganization of diblock copolymer domains in directed self-assembly monitored by in situ high-temperature AFM

In situ high-temperature AFM was used to locally follow dynamic processes, leading to directed self-assembly of copolymers in the context of graphoepitaxy. We focused on the effect of heating for temperatures much higher than the Tg of the used PS-b-PMMA polymer. We showed that such conditions favors the block rearrangement, leading to very regular and perfectly aligned structures in limited times. The use of in situ AFM allowed us to locally investigate the self-organization process at high temperature, thus bringing new insights into self-assembly of block copolymers by graphoepitaxy. In particular, we demonstrate that a slight increase of temperature between 180 and 200 °C allowed overpassing an energy barrier and considerably improves the long distance arrangement, even for relatively short times.

Topologically Distinct Lamellar Block Copolymer Morphologies Formed by Solvent and Thermal Annealing

Solvent annealing produces ordered assemblies in thin films of block copolymers and, in contrast to uniform thermal annealing, can be used to tune the self-assembled morphology, control the domain orientation with respect to the substrate, and, as demonstrated here, reduce the defect density. The two-dimensional network topology of lamellae self-assembled by polystyrene-block-poly(methyl methacrylate) block copolymers in thin films was compared when processed by solvent and thermal annealing techniques. The mixed solvent annealing method described here reduced the overall defect density (e.g., dislocations with PMMA or PS cores) and thus the connectivity of the lamellar domains compared to thermal annealing; however, the long-range continuity of the networks was maintained and depended primarily on the copolymer composition. In addition, the persistence length of the lamellar domains for solvent annealed films was found to be 2-3 times that of the corresponding thermally annealed systems.

Effect of film thickness and domain spacing on defect densities in directed self-assembly of cylindrical morphology block copolymers

Directed assembly of block copolymer thin films is recognized as a high-throughput, low-cost complement to optical lithography with the ability to overcome the 32 nm natural resolution limit of conventional lithographic techniques. For bulk block copolymer systems, desired feature sizes ranging from 5 to 100 nm can be obtained by controlling the molecular weight and composition of a block copolymer, as long as the bulk order-disorder temperature (ODT) is such that the copolymer is well-segregated at the processing conditions. However, our studies on graphoepitaxially aligned cylindrical morphology block copolymer monolayer and bilayer films demonstrate that, as domain sizes are reduced, the block copolymer becomes increasingly susceptible to an unacceptably high density of thermally generated defects, resulting in a significant reduction of the ODT. Thus, in thin films, the minimum feature spacing accessible is limited by thermal defect generation and not by the bulk ODT. Our experimental studies on monolayer films of cylindrical morphology polystyrene-b-poly(2-vinyl pyridine) with microdomain spacings approaching 20 nm reveal that defect densities and the ODT are surprisingly sensitive to variations as small as 2 nm in the microdomain spacing. Additionally, the monolayer and bilayer ODT differ by nearly 100 °C when the monolayer domain spacing is 20 nm, while the difference is only 20 °C when the monolayer domain spacing is 22 nm. We explain this behavior using a quantitative estimation of the energetic cost of defect production in terms of the domain spacing, χN, and block copolymer composition. These studies reveal unexpected consequences on the equilibrium defect densities of thin film block copolymers which must be accounted for when designing a block-copolymer-based directed-assembly process.

Ultralarge-area block copolymer lithography enabled by disposable photoresist prepatterning

We accomplished truly scalable, low cost, arbitrarily large-area block copolymer lithography, synergistically integrating the two principles of graphoepitaxy and epitaxial self-assembly. Graphoepitaxy morphology composed of highly aligned lamellar block copolymer film that self-assembled within a disposable photoresist trench pattern was prepared by conventional I-line lithography and utilized as a chemical nanopatterning mask for the underlying substrate. After the block copolymer film and disposable photoresist layer were removed, the same lamellar block copolymer film was epitaxially assembled on the exposed chemically patterned substrate. Highly oriented lamellar morphology was attained without any trace of structure directing the photoresist pattern over an arbitrarily large area.

Controlling Order in Block Copolymer Thin Films for Nanopatterning Applications

An attractive “unconventional” lithographic technique to pattern periodic, sub-100 nm features uses self-assembled block copolymer thin films as etch masks. Unfortunately, as-cast films lack the orientational and positional order of the microphase-separated domains that are necessary for many desired applications. Reviewed herein are techniques developed to guide the self-assembly process in thin films, which permit varying degrees of control over the patterns formed by the microdomains. Techniques that can control the out-of-plane order of the microdomains are first summarized. Then, techniques that control the lateral ordering are reviewed, beginning with those that generate large defect-free grains, then those that impart orientational order to the microdomains, and finally those that can control both the orientation and position of individual microdomains. Each technique is summarized with experimental examples and discussions regarding the mechanism of the guided self-assembly process.

Specific features of defect structure and dynamics in the cylinder phase of block copolymers

We present a systematic study of defects in thin films of cylinder-forming block copolymers upon long-term thermal or solvent annealing. In particular, we consider in detail the peculiarities of both classical and specific topological defects, and conclude that there is a strong "defect structure-chain mobility" relationship in block copolymers. In the systems studied, representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or classical disclinations with incorporated alternative, nonbulk structures with planar symmetry. In solvent-annealed films with enhanced chain mobility, the neck defects (bridges between parallel cylinders) were observed. This type of nonsingular defect has not been identified in block copolymer systems before. We argue that topological arguments and 2D defect representation, sufficient for lamellar systems, are not sufficient to determine the stability and mobility of defects in the cylindrical phase. In-situ scanning force microscopy measurements are compared with the simulations based on the dynamic self-consistent mean field theory. The close match between experimental measurements and simulation results suggests that the lateral defect motion is diffusion-driven. In addition, 3D simulations demonstrated that the bottom (wetting) layer is only weakly involved into the structure ordering at the free surface. Finally, the morphological evolution is considered with the focus on the motion and interaction of the representative defect configurations.

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

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

Unconventional methods for forming nanopatterns

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