Lamella-forming Block Copolymers Research Articles

Solvent Processing and Ionic Liquid-Enabled Long-Range Vertical Ordering in Block Copolymer Films with Enhanced Film Stability

Rapid and reliable processing methods for forming ordered block copolymer (BCP) materials with low defect density in a thin film geometry are required for many nanotechnology applications. Vertically aligned BCP structures, in particular, have applications ranging from nanolithography for electronics and photonics applications to nanoporous membranes for water remediation and novel batteries for flexible electronics. However, the attainment of a nearly complete vertical orientational order of the BCP-ordered phase remains challenging. Solvent-based techniques, such as direct immersion annealing (DIA) and solvent vapor annealing (SVA), have immense potential for these applications of BCP films, as it allows for a ‘tuning’ of their thermodynamic, structural, and chain mobility-driven kinetic properties. We first demonstrate that DIA, using a judicious choice of binary solvent mixtures, along with a relatively hydrophobic ionic liquid (IL), induces the rapid vertical ordering in polystyrene-b-poly(methyl methacrylate) (PS-PMMA) in lamellae-forming block copolymer films. The IL in a binary solvent mixture with toluene and heptane apparently gives rise to a near-neutral solvent for the BCP for itself and the boundary that is useful for attaining the vertical microstructure within the BCP film. Next, we show that an IL can moreover suppress the dewetting of the PS-PMMA films to achieve long-range order using SVA in cylindrical films after long annealing times. Both vertical and horizontal morphologies are attained in these films by selecting different solvent conditions. Attaining enhanced vertical and horizontal BCP structures with long-range defect-free order by tuning solvent quality and using additives like ILs can render them useful for many nanotech applications.

Block copolymer directed self-assembly defect modes induced by localized errors in chemoepitaxial guiding underlayers: A molecular simulation study

Block copolymer (BCP) directed self-assembly (DSA) has been presented as a potential economically attractive enhancement to extend the capabilities of optical lithography for semiconductor manufacturing. One concern in DSA is the level of defectivity that can be achieved in such a process. Although entropic effects will always lead to some degree of defectivity, highly ordered structures with a low theoretical equilibrium defect density can be produced by guiding the ordering and placement of the BCP domains using a patterned underlayer. Recent experimental studies have shown that while DSA processes can significantly reduce the observed defect density, defectivity levels are generally still higher than allowable for high-volume manufacturing and higher than what would be anticipated from free energy estimates of the observed defect modes. In particular, bridge defects are one of the most commonly observed defect modes in experimental DSA studies. A number of hypotheses have been proposed to explain the origins of these defects. One hypothesis is that so-called affinity defects present in the underlayer can spawn bridge defects in the overlying BCP film. The goal of the work reported here was to investigate the extent to which bridge defects can be generated or further reinforced in lamellae-forming block copolymer films due to affinity defects in the underlayer pattern. Coarse-grained molecular dynamics simulations were used to simulate the chemoepitaxial DSA of monodisperse block copolymer films atop underlayers with varying affinity defect sizes. Affinity defects were simulated by creating circular regions of a single polymer block type (which is the opposite block type of that used to pattern the underlayer guiding stripes) in the nominally neutral background region of the underlayer. These affinity defects were positioned in regions of the underlayer where they were the incorrect type to match the overlying block copolymer pattern. It was observed that the presence of an affinity defect in the neutral region of the underlayer caused the energetically preferential polymer block to wet the affinity defect, thus creating the nucleus of what could potentially become a bridge defect—even when the affinity defects were very small. As the radius of the underlayer affinity defect (RoD) increased, the amount of block copolymer of incorrect type (with respect to a perfectly assembled copolymer pattern) that assembled above the affinity defect increased; but, in general, the thickness of the wetting layer in contact with the affinity defect was only roughly one polymer chain thick. These data suggest that an affinity defect in the underlayer alone is unlikely to be noticeably enhanced by significant bridge defect formation in a monodisperse block copolymer film.

Characterisation of the PS-PMMA Interfaces in Microphase Separated Block Copolymer Thin Films by Analytical (S)TEM.

Block copolymer (BCP) self-assembly is a promising tool for next generation lithography as microphase separated polymer domains in thin films can act as templates for surface nanopatterning with sub-20 nm features. The replicated patterns can, however, only be as precise as their templates. Thus, the investigation of the morphology of polymer domains is of great importance. Commonly used analytical techniques (neutron scattering, scanning force microscopy) either lack spatial information or nanoscale resolution. Using advanced analytical (scanning) transmission electron microscopy ((S)TEM), we provide real space information on polymer domain morphology and interfaces between polystyrene (PS) and polymethylmethacrylate (PMMA) in cylinder- and lamellae-forming BCPs at highest resolution. This allows us to correlate the internal structure of polymer domains with line edge roughnesses, interface widths and domain sizes. STEM is employed for high-resolution imaging, electron energy loss spectroscopy and energy filtered TEM (EFTEM) spectroscopic imaging for material identification and EFTEM thickness mapping for visualisation of material densities at defects. The volume fraction of non-phase separated polymer species can be analysed by EFTEM. These methods give new insights into the morphology of polymer domains the exact knowledge of which will allow to improve pattern quality for nanolithography.

Mechanisms of Ion Transport in Block Copolymeric Polymerized Ionic Liquids.

We present the results of a multiscale simulation framework investigating the ion transport mechanisms in multicomponent polymerized ionic liquids. Three different classes of polymeric ionic liquid systems, namely, random copolymers, lamellae forming block copolymers, and homopolymers, are constructed at the coarse-grained scale, and their atomistic counterparts are derived by using a reverse mapping method. Using such a framework, we investigate the influence of morphology on ion transport properties of such polymerized ionic liquids. Our results for ion mobilities are in qualitative agreement with experimental observations. Further analysis of random copolymer and block copolymer systems reveal that the reduced ion mobilities in such systems arise from the influence of architecture and morphology on ion coordination and intramolecular hopping events.

Effects of Helical Chain Shape on Lamellae-Forming Block Copolymer Self-Assembly

Understanding the effects of nonideal polymer chain shapes on block copolymer self-assembly is important for designing functional materials, such as biopolymers or conjugated polymers, with control...

Block copolymer ordering on elastomeric substrates of tunable surface energy

We report a range of surface characteristics from strong and partial dewetting to complete wetting for thin films of cylinder and lamellae forming block copolymers (c-BCP and l-BCP, respectively) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) on flexible substrates. BCP of various molecular weights (Mw) was applied on elastomeric polydimethylsiloxane (PDMS) substrates of distinct cross-link densities and various surface energy (SE) obtained by exposure to UV-ozone. We demonstrate that lower Mw c-BCP films dewet faster than l-BCP films of higher Mw applied on PDMS of lowest cross-link density and SE < 25 mJ/m2. Partial dewetting was observed for c-BCP films applied on the lowest cross-link density substrates with 25 mJ/m2 < SE < 32 mJ/m2, with a sharper transition from dewetting to wetting for l-BCP films. Finally, complete wetting (stable films) occurred for both c-BCP and l-BCP films on PDMS substrates with lowest cross-link density and SE > 32 mJ/m2, as well as on PDMS of the highest cross-link density with SE > 25 mJ/m2. In this extended SE wetting regime, c-BCP show vertical orientation when applied on both types of cross-linked PDMS in a narrow neutral SE range (39–40) mJ/m2, while being oriented parallel to the substrate when SE > 40 mJ/m2. The ratio of vertically oriented l-BCP however decreases gradually when applied on substrates with a higher SE range (40–65 mJ/m2) in comparison to the sharply transitioning c-BCP orientation, reflecting the intrinsic stability of vertical lamellae over a wider range of substrate SE, consistent with theoretical estimates. These results have important ramifications for design of next-generation flexible electronics utilizing BCP thin films.

Role of Penetrability into a Brush-Coated Surface in Directed Self-Assembly of Block Copolymers.

High-density and high-resolution line and space patterns on surfaces are obtained by directed self-assembly of lamella-forming block copolymers (BCPs) using wide-stripe chemical guiding patterns. When the width of the chemical pattern is larger than the half-pitch of the BCP, the interaction energy between each BCP domain and the surface is crucial to obtain the desired segregated film morphology. We investigate how the intermixing between BCPs and polymer brush molecules on the surface influences the optimal surface and interface free energies to obtain a proper BCP alignment. We have found that computational models successfully predict the experimentally obtained guided patterns if the penetrability of the brush layer is taken into account instead of a hard, impenetrable surface. Experiments on directed self-assembly of lamella-forming poly(styrene- block-methyl methacrylate) using chemical guiding patterns corroborate the models used in the simulations, where the values of the surface free energy between the BCP and the guiding and background stripes are accurately determined using an experimental method based on the characterization of contact angles in droplets formed after dewetting of homopolymer blends.

A Detailed Examination of the Topological Constraints of Lamellae-Forming Block Copolymers

A microscopic molecular model of polymeric molecules that captures the effects of topological constraints is used to consider how microphase segregation can alter the distribution of entanglements both in space and along chain contours. Such topological constraints are obtained by using the Z1 algorithm, and it is found that for diblock copolymers in the lamellar morphology they are not homogeneously distributed, but instead exhibit a spatial dependence as a consequence of the self-organization of the polymer blocks. The specific shape of the inhomogeneous distribution is affected by the molecular weight of the copolymer. The microscopic information obtained by these calculations is then compared with the corresponding results generated from a coarser description of entangled block copolymers that includes soft intermolecular interactions and slip-springs, whose role is to incorporate the effects of entanglements that are lost during coarse-graining. This comparison is helpful for improving coarse-grained...

Well-ordered self-assembled nanostructures of block copolymer films via synergistic integration of chemoepitaxy and zone annealing.

It is an extremely challenging task to fabricate macroscopically well-ordered nanostructures of block copolymers in a limited annealing time. In this work, we propose a novel integrated strategy of chemoepitaxy and zone annealing to direct the self-assembly of lamella-forming block copolymers. Large-scale numerical simulations corroborate that the integrated strategy has the capability to generate well-aligned and well-oriented lamellae over a macroscopic area via the synergy between the alignment guidance of the chemical template and the defect annihilation of zone annealing, even though the guiding stripes of the chemical template are extremely sparse. It is further demonstrated that the effective annealing time to achieve the well-ordered nanostructures is strongly dependent upon the pitch of guiding stripes and the thickness of block copolymer films. This work provides an instructive example for boosting the directing efficiency of chemoepitaxy through the integrated strategy, and lays the groundwork for rapidly fabricating well-ordered nanostructures of block copolymer nanolithography.

Ordering kinetics of lamella-forming block copolymers under the guidance of various external fields studied by dynamic self-consistent field theory.

Self-consistent field theory with a dynamic extension is exploited to investigate the kinetics of the lamellar formation of symmetric block copolymers under the direction of external fields. In particular, three types of directed self-assembly methods - a permanent field for chemo-epitaxy, a dynamic field for zone annealing and an integrated permanent/dynamic field - are examined. For the chemo-epitaxy involving sparsely prepatterned substrates or zone annealing, the block copolymers generally develop into polycrystalline nanostructures with multiple orientations due to the lack of strong driving forces for eliminating the long-lived imperfections in a limited time. As the integrated chemo-epitaxy and zone annealing method is applied to the block copolymer systems, single-crystalline nanostructures with precisely registered orientations are achieved in a short annealing time owing to the mutual acceleration of defect annihilations, which cannot be produced by the conventional techniques alone. Furthermore, the integrated method allows the rapid fabrication of well-ordered nanostructures on the extremely sparse prepatterned substrates. Our theoretical work may serve to rationalize the faster modern nanolithographic fabrication of smaller microelectronic components using lower-spatial-frequency templates.

Structure, Stability, and Reorganization of 0.5 L0 Topography in Block Copolymer Thin Films.

The structure, stability, and reorganization of lamella-forming block copolymer thin film surface topography ("islands" and "holes") were studied under boundary conditions driving the formation of 0.5 L0 thick structures at short thermal annealing times. Self-consistent field theory predicts that the presence of one perfectly neutral surface renders 0.5 L0 topography thermodynamically stable relative to 1 L0 thick features, in agreement with previous experimental observations. The calculated through-film structures match cross-sectional scanning electron micrographs, collectively demonstrating the pinning of edge dislocations at the neutral surface. Remarkably, near-neutral surface compositions exhibit 0.5 L0 topography metastability upon extended thermal treatment, slowly transitioning to 1 L0 islands or holes as evidenced by optical and atomic force microscopy. Surface restructuring is rationalized by invoking commensurability effects imposed by slightly preferential surfaces. The results described herein clarify the impact of interfacial interactions on block copolymer self-assembly and solidify an understanding of 0.5 L0 topography, which is frequently used to determine neutral surface compositions of considerable importance to contemporary technological applications.

Three-Tone Chemical Patterns for Block Copolymer Directed Self-Assembly.

Chemical patterns for directed self-assembly (DSA) of lamellae-forming block copolymers (BCP) with density multiplication can be fabricated by patterning resist on a cross-linked polystyrene layer, etching to create guide stripes, and depositing end-grafted brushes in between the stripes as background. To date, two-tone chemical patterns have been targeted with the guide stripes preferentially wet by one block of the copolymer and the background chemistry weakly preferentially wet by the other block. In the course of fabricating chemical patterns in an all-track process using 300 mm wafers, it was discovered that the etching process followed by brush grafting could produce a three-tone pattern. We characterized the three regions of the chemical patterns with a combination of SEM, grazing-incidence small-angle X-ray scattering (GISAXS), and assessment of BCP-wetting behavior, and evaluated the DSA behavior on patterns over a range of guide stripe widths. In its best form, the three-tone pattern consists of guide stripes preferentially wet by one block of the copolymer, each flanked by two additional stripes that wet the other block of the copolymer, with a third chemistry as the background. Three-tone patterns guide three times as many BCP domains as two-tone patterns and thus have the potential to provide a larger driving force for the system to assemble into the desired architecture with fewer defects in shorter time and over a larger process window.

Morphology control of three-dimensional nanostructures in porous templates using lamella-forming block copolymers and solvent vapors.

The microphase separation behavior of block copolymers confined in cylindrical nanopores has been extensively investigated. Recently, the solvent-annealing-induced nanowetting in templates (SAINT) method has been demonstrated to be a versatile approach for the infiltration of block copolymers into the nanopores of porous templates. The function of the annealing solvents, however, is still not well understood, especially in the morphology control of the fabricated block copolymer nanostructures. In this work, we elucidate the function of the annealing solvents in the SAINT method using a lamella-forming block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and anodic aluminum oxide (AAO) templates. By changing the composition of the annealing solvents, different morphologies such as the concentric lamellar morphology, the winding cylinder morphology, and the irregular hybrid morphology are observed, mainly caused by the annealing-solvent-induced volume change. The morphology of the block copolymer nanostructures can be further confirmed using an HF solution to remove the PDMS domain selectively.

Simulation study of the effect of molar mass dispersity on domain interfacial roughness in lamellae forming block copolymers for directed self-assembly

A coarse-grained molecular dynamics model was used to study the thin film self-assembly and resulting pattern properties of block copolymer (BCP) systems with various molar mass dispersities. Diblock copolymers (i.e. A–b–B type) were simulated in an aligned lamellar state, which is one of the most common patterns of potential use for integrated circuit fabrication via directed self-assembly of BCPs. Effects of the molar mass dispersity (Ð) on feature pitch and interfacial roughness, which are critical lithographic parameters that have a direct impact on integrated circuit performance, were simulated. It was found that for a realistic distribution of polymer molecular weights, modeled by a Wesslau distribution, both line edge roughness (LER) and line width roughness (LWR) increase approximately linearly with increasing Ð, up to ∼45% of the monodisperse value at Ð = 1.5. Mechanisms of compensation for increased A–A and B–B roughness were considered. It was found that long and short chain positions were not correlated, and that long chains were significantly deformed in shape. The increase in LWR was due to the increase in LER and a constant correlation between the line edges. Unaligned systems show a correlation between domain width and local molecular weight, while systems aligned on an alternating pattern of A and B lines did not show any correlation. When the volume fraction of individual chains was allowed to vary, similar results were found when considering the Ð of the block as opposed to the Ð of the entire system.

Evolution of Crystal Orientation in One-Dimensionally Confined Space Templated by Lamellae-Forming Block Copolymers

Polymer crystallites may exhibit preferred orientation when the crystallization is allowed to occur under the influence of spatial confinement. Using time-resolved wide-angle X-ray scattering (WAXS), we explore the time evolution of the preferred crystal orientation within one-dimensionally confined space constructed by the lamellar microdomains of two crystalline block copolymers, polyethylene-block-poly(dl-lactide) (PE-b-PDLLA) and poly(l-lactide)-block-polyethylene (PLLA-b-PE), where the developments of the parallel and the perpendicular orientation of PE and PLLA crystallites, respectively, were monitored from the early stage of crystallization. Both types of crystallites were randomly oriented at the early stage of formation. As crystallization proceeded further, the ensemble-average orientation progressively improved toward the preferred orientation type, and the rate of establishing the orientation exhibited the same dependence on crystallization temperature (Tc) as the crystallization kinetics. Fu...

Birefringence Analysis of the Effect of Electric Fields on the Order–Disorder Transition Temperature of Lamellae Forming Block Copolymers

A detailed birefringence analysis of the effect of strong dc electric fields on the order–disorder transition temperature (TODT) of lamella forming block copolymers is reported. The setup presented here enabled the measurement of the TODT with high temperature resolution while the birefringence measurements were nondestructive and straightforward compared to alternative methods. A downward shift in the transition temperature was found for all samples upon application of the electric field. The data indicate that the dominating parameter that evokes the mixing of block copolymers when exposed to electric fields is the difference in dielectric permittivity Δϵ between the block copolymer constituents. The extent to which the TODT is shifted is furthermore influenced by the degree of polymerization N. Shifts in the transition temperature of up to 7 °C were found upon application of an electric field of 5 kV/mm.

Defect annihilation in chemo-epitaxial directed self-assembly: Computer simulation and Self-Consistent Field Theory

ABSTRACTExcept at the order-disorder transition, defects in lamella-forming block copolymers have a free energy of several hundreds kBT where kBT denotes the thermal energy scale. Thus, they cannot be conceived as equilibrium fluctuations around a perfectly ordered state. Instead, defects, which are observed in experiments, are formed in the course of self-assembly. Their behavior is dictated by the kinetics of structure formation, in particular, the kinetic pathways of defect motion and annihilation.Computational modeling can contribute to optimize materials parameters such as film thickness, interaction between copolymer blocks and substrate, geometry of confinement, in order to avoid the formation of defects in the early stages of structure formation or facilitate defect annihilation. Computations also provide fundamental insights into the universal physical mechanisms of directing the self-assembly, addressing the equilibrium structure and thermodynamics as well as the kinetics of self-assembly.We present computer simulation of highly coarse-grained particle-based models and self-consistent field calculations that allow us to access the long time and large length scales associated with self-assembly. These calculations provide information about the free-energy landscape and mechanisms of defect annihilation in thin films. Additionally, opportunities for directing the kinetics of self-assembly by temporal changes of thermodynamic conditions are discussed.

Photonic Block Copolymer Films Swollen with an Ionic Liquid

Nonvolatile solvent swollen 1D periodic films were fabricated from lamellae-forming block copolymers with medium molecular weight by infiltrating an ionic liquid. A mixture of imidazole and imidazolium bis(trifluoromethanesulfonyl)imide as a room temperature ionic liquid was added after spin-coating of thin films of polystyrene-b-poly(2-vinylpyridine) (PS–P2VP) block copolymers having an approximately 50/50 composition to create photonic films reflecting in the visible regime. Under normal conditions of temperature and humidity, the films maintained their photonic properties for more than 100 days without perceptible change, stemming from the nonvolatility of the ionic liquid. Transmission electron microscopy revealed the selective swelling of the P2VP nanodomains by the IL and ultrasmall angle X-ray scattering measurements provided quantitative nanostructure information on the periodicities of the films. The wavelength of reflected light from photonic films was tunable by using different molecular weight...

Fabrication of chemical patterns from graphoepitaxially assembled block copolymer films by molecular transfer printing

Fabrication of chemical patterns by electron beam lithography or extreme ultraviolet interference lithography is either low-throughput or prohibitively expensive to practice at sub-50 nm feature dimension. Here we report a new high-throughput approach that combines the advantages of both graphoepitaxy and molecular transfer printing (MTP) to fabricating chemical patterns at low cost for directed assembly of block copolymers. In this new approach, a cylinder-forming block copolymer ternary blend film is directed to assemble on a topographic HSQ substrate with sub-Lo, where Lo is the natural period of the block copolymer, relief structures to increase the feature density by a factor of ∼20, the surface domain pattern is replicated using MTP to create 1:1 chemical pattern, and then a lamellae-forming block copolymer is directed to assemble on the chemical pattern to realize high-aspect ratio Manhattan type nanostructures. This combined strategy allows us to fabricate chemical patterns with feature dimension below the resolution limit of current lithographic tools.

Interfacial Design for Block Copolymer Thin Films

Top coat design, coating, and optimization methodologies are introduced that facilitate the synthesis, application, and identification of neutral top coats for block copolymer (BP) thin films. Polymeric top coat composition, controlled via synthesis, determines interfacial wetting characteristics. Trimethylammonium salts of top coats improve solubility and coating uniformity. A “confined” island and hole test conveniently establishes (non)preferential wetting at the top coat/BP interface, which depends upon top coat composition. The utility of these three concepts was demonstrated with two high-χ, lamella-forming BPs, poly(styrene-block-4-trimethylsilylstyrene) (PS-b-PTMSS) having two periodicities L0 = 18 and 22 nm and poly(styrene-block-methyltrimethylsilylmethacrylate) (PS-b-PTMSM) with L0 = 15 nm. The combination of neutral top and bottom interfaces resulted in a perpendicular orientation of lamellae independent of BP film thickness (1–3 L0) when thermally annealed for 60 s or less.