Cylinder-forming Block Copolymers Research Articles

Perpendicular Orientation of Domains in Cylinder-Forming Block Copolymer Thick Films by Controlled Interfacial Interactions

We report the induction of perpendicularly oriented cylindrical domains in PS-b-PMMA block copolymer (BCP) films thicker than 100 nm by thermally annealing on a substrate modified with a random copolymer. The effects of annealing temperature, composition of the substrate-modifying random copolymer, and BCP film thickness on the morphology of PMMA cylinder forming PS-b-PMMA were studied. For BCP films thicker than 100 nm, the fabrication of perpendicular PMMA cylinders is highly dependent on both the substrate-modifying random copolymer and the annealing temperature as these two parameters control the interactions of the BCP with the substrate and the free surface, respectively. We found the best perpendicular structures to be created by using a random copolymer brush with a styrene fraction (FSt) near 0.70 and an annealing temperature near 230 °C. Perpendicular cylinder structures were achieved in ∼300 nm thick films using these conditions. When the BCP film was thicker than 300 nm, nucleation and growth of the microdomains proceeded independently from each interface. We present scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM) images of these perpendicular structures and explain the results on the basis of previous simulation reports.

Sphere-forming diblock copolymers in slit confinement: A dynamic density functional theory study

With mean-field dynamic density functional theory, we study the morphologies of sphere-forming diblock copolymers confined between two homogeneous surfaces. The effects of the film thickness and the surface field strength on the phase behavior of sphere-forming copolymer film are investigated. The morphologies deviating from the bulk sphere-forming structure are revealed, including cylinders oriented perpendicular to the surface, cylinders oriented parallel to the surface, perforated lamellae and lamellae by varying the film thickness, and surface field strength. We also construct the phase diagram of surface reconstruction, in which some interesting phase transitions are presented. Besides, we compare the present phase diagram with the relevant phase diagram of cylinder-forming block copolymer film.

Temperature-dependent phase behaviors in cylinder-forming block copolymers.

We demonstrate that the temperature-dependent phase behaviors of parallel and perpendicular cylinder-forming block copolymers are governed by domain-domain segregation forces inherently present in block copolymer material itself. With increasing temperature, a parallel cylinder-forming block copolymer experienced a parallel cylinder straightening process before the order-disorder transition (ODT) and did not show long-range composition fluctuations near the ODT temperature due to the weak segregation forces between the block domains. A perpendicular cylinder-forming block copolymer with a strong segregation force between the block domains displayed cylinder orientation transition from perpendicular to parallel below the ODT temperature. On the other hand, a perpendicular cylinder-forming block copolymer material with an exceptionally strong segregation force between the block domains maintained its initial perpendicular cylinder orientation up to near the ODT temperature. In both cases of perpendicular cylinder-forming block copolymers, submicrometer-scale long-range composition fluctuations were observed well above the ODT temperature due to their intrinsically strong segregation forces between the block domains.

Selective distribution of interacting magnetic nanoparticles into block copolymer domains based on the facile inversion of micelles

We demonstrate a simple methodology to incorporate interacting magnetic nanoparticles (mNPs) into cylinder forming block copolymer templates. Poly(styrene- block-isoprene) (PS- b-PI) with PI cylinders and poly(styrene- block-4vinylpyridine) (PS- b-P4VP) with PS cylinders were used as the block copolymer templates and γ-Fe 2O 3 NPs coated with oleic acids were pre-synthesized for the interacting mNPs. Regardless of the template block copolymers, the selective location of mNPs and the size of mNP aggregates are clearly altered by changing casting solvents. When good solvents for both blocks were used as casting solvents, mNPs are readily aggregated during the solvent evaporation. In contrast, under selective casting solvents for the minor blocks, the mNPs were selectively trapped into the cylinder domains through the facile inversion of micelles during solvent evaporation. The interplay between mNPs and block copolymers was also tested with different molecular weights of block copolymers.

Self-assembling study of a cylinder-forming block copolymer via a nucleation–growthmechanism

Block copolymer materials form self-assembling structures at a nanometric scale, of interest innanotechnology. The organization process of asymmetric poly(styrene-block-methyl methacrylate)(PS-b-PMMA) copolymer thin films is studied. In a first step it is demonstrated that twoconsecutive mechanisms lead to the formation of a well-ordered phase. The first mechanismis the local segregation of blocks, which leads to a metastable disordered cylinder phase(Cd). The second mechanism is a transformation of theCd phase to a vertical cylinder phase via a nucleation–growth mechanism. The influence of film thicknessand surface tension on the organization is also studied. Above the natural cylinder monolayer height,h1,the kinetics of the cylinder organization strongly depends on the initial film thickness, and belowh1 the film splits into terraces. By varying the interactions between the substrate surfaceand the different blocks, a disordered phase can be formed instead of terraces.

Self-Assembly of Cylinder-Forming Block Copolymers on Ultrananocrystalline Diamond (UNCD) Thin Films for Lithographic Applications

AbstractBlock copolymers (BCPs) consist of two or more chemically distinct and incompatible polymer chains (or blocks) covalently bonded. Due to the incompatibility and connectivity constraints between the two blocks, diblock copolymers spontaneously self-assemble into microphase-separated nanoscale domains that exhibit ordered 0, 1, 2 or 3 dimensional morphologies at equilibrium. Commonly observed microdomain morphologies in bulk samples are periodic arrangements of lamellae, cylinders, or spheres. Block copolymer lithography refers to the use of these ordered structures in the form of thin films as templates for patterning through selective etching or deposition. The self-assembly and domain orientation of block copolymers on a given substrate is critical to realize block copolymer lithography as a tool for large throughput nanolithography applications. In this work, we survey the morphology of cylinder-forming block copolymers by atomic force microscopy (AFM). Three kind of block copolymers were studied: a) poly(styrene-block-ferrocenyldimethylsilane), PS-b-PFS b) poly(styrene-block-methylmethacrylate), PS-b-PMMA and c) poly(styrene-block-dimethylsiloxane) PS-b-PDMS. Block copolymers were dissolved in a neutral solvent for both blocks (toluene) in order to obtain solutions of various concentrations (1 and 1.5 wt %). From these solutions, films were prepared by spin casting on ultrananocrystalline diamond (UNCD) thin film substrates. Results indicate that PS-b-PFS exhibits chemical and morphological compatibility to the UNCD surface in terms of wetting and domain control. A systematic comparison of self-assembly of these polymers on silicon nitride substrates demonstrates that UNCD thin films would require pre-treatment to be considered as a substrate for BCP lithography.

Effect of Composition of Substrate-Modifying Random Copolymers on the Orientation of Symmetric and Asymmetric Diblock Copolymer Domains

The ability of random copolymer brushes and cross-linked mats to induce the vertical orientation of domains in overlying films of lamellae- and cylinder-forming block copolymers was investigated as a function of the composition. The substrate-modifying layers consisted of styrene and methyl methacrylate random copolymers and contained either a terminal hydroxyl group or a third polar comonomer of 2-hydroxyethyl methacrylate (HEMA) for grafting brushes to silicon oxide surfaces or glycidyl methacrylate (GMA) for cross-linking the random copolymer into a mat. Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) lamellae- and cylinder-forming block copolymers (both PS and PMMA minority block copolymers) were deposited and annealed on the modified surfaces. In all cases the vertical orientation of domains was observed for a range of random copolymer composition, but the ranges of composition were different for each combination of surface layer and block copolymer. The cylindrical domains of PS exhibited vertical structures for a very narrow range of compositions compared to cylindrical domains of PMMA or lamellae. As expected, the incorporation of polar HEMA or GMA monomers in the surface layers shifted the composition range for the perpendicular orientation of domains to higher fractions of styrene. The results are discussed in terms of the equilibration of the films in the presence of the chemically modified surfaces.

Characterization of Cylinder-Forming Block Copolymers Directed to Assemble on Spotted Chemical Patterns

Films of cylinder-forming polystyrene-block-poly(methyl methacrylate) (bulk intercylinder spacing ao) were directed to assemble on surfaces chemically patterned with a hexagonal array of spots (interspot spacing as). The block copolymer domain assembly was investigated as a function of the film thickness and the commensurability between ao and as. The domains assembled vertically away from and matched the spacing and hexagonal order of the underlying lithographically patterned chemical substrate, provided as was equal to or up to 6% larger than ao and the film thickness was ≤54 nm. In contrast, comparably poor assembly was obtained when as was 8% less than ao. When as was commensurate with ao, the cylinder diameter and eccentricity had a lower standard deviation and was more circular, respectively, than that of self-assembled cylinders on a neutral substrate.

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.

Characterization of the dynamics of block copolymer microdomains with local morphological measures

We investigate the structure formation in thin films of cylinder forming block copolymers. With in situ scanning probe microscopy image sequences can be recorded with high temporal (2 min per frame) and spatial (10 nm) resolution. We compare different image processing methods for quantitative analysis of the large amount of data. Computing local Minkowski functionals yields local geometrical and morphological information about the observed structures and enables us to track their evolution with time. An alternative characterization method is to reduce the gray scale images to their skeleton and to classify and count the branching points of the skeletonized structure. We tracked the temporal evolution of these measures and computed correlation functions.

Directed Assembly of Cylinder-Forming Block Copolymers into Patterned Structures to Fabricate Arrays of Spherical Domains and Nanoparticles

The first demonstration of the directed assembly of cylindrical microdomains of a block copolymer into nonregular, patterned structures, as well as the subsequent formation of registered 1D arrays of spherical microdomains and nanoparticles, is presented. Directed assembly of pure cylinder-forming polystyrene-block-poly(t-butylacrylate) copolymer, which can undergo a cylinder-to-sphere morphological transition via thermolysis, on lithographically defined linear, circular, or angled chemical nanopatterns resulted in long-range-ordered cylindrical domains that were registered to the underlying nanopattern. Thermolysis of the cylindrical domains yields registered arrays of spherical domains, which are difficult to pattern with advanced lithographic techniques. The spherical domains maintain uniform size in all patterns studied, including 45° corners. In situ synthesis of metal and metal oxide nanoparticles within the spherical microdomains created linear, circular, and angled nanoparticle 1D arrays, register...

Square Arrays of Vertical Cylinders of PS-b-PMMA on Chemically Nanopatterned Surfaces

Cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films were directed to assemble on chemically nanopatterned surfaces consisting of a square array of spots with varying diameters of lattice spacing and spot size. In all cases the patterned spots were preferentially wet by the PMMA block in a continuum of surface wet by the PS block. On this chemical pattern, cylindrical domains oriented perpendicular to the surfaces formed a square array of vertical cylinders, instead of a hexagonal array typically formed in the bulk morphology of cylinder-forming block copolymers. When the lattice spacing of chemical spot patterns was incommensurate with the natural dimension of diblock copolymer in a square array, surface reconstruction led to the formation of microdomain structures that do not appear naturally in the bulk state, such as a loop structure of PMMA cylinders connecting two neighboring PMMA cylindrical sections, and semicylinder domains located at the center of the square unit c...

Nanoscaling of Microdomain Spacings in Thin Films of Cylinder-Forming Block Copolymers

We present first quantitative measurements of the characteristic lateral dimensions in thin films of cylinder-forming block copolymers. Using a metrological scanning force microscope and tailor-made image analysis, we map out lateral distances with subnanometer accuracy. Microdomain spacings change in a systematic way as a function of the film thickness and as a function the lateral cylinder bending. We show that in very thin films the unit cell is stretched perpendicular to the plane of the film resulting in lateral distances smaller than those in bulk. The changes are distinct, although small, and can be rationalized within the framework of the strong segregation theory of block copolymers.

Application of Cylinder Forming Block Copolymers as Templates for Formation of Bit Patterned and Graded Media

AbstractBit patterned media, including media fabricated with a gradient in composition, is being developed as a potential path to higher information storage density. The formation of metal nanopillars with 20-30 nm repeat spacing and precisely controlled magnetic properties presents a significant challenge to current fabrication methods. We have been developing cylinder forming block copolymer phases as a method to generate the desired patterns coupled with the processing steps necessary to transfer the pattern into magnetic material. This involves spin coating of the polymer on an appropriate orienting layer, annealing to allow the pattern to form by self-organization of the block copolymer, solvent processing to remove the minority domain, electrodeposition to form a hard mask, followed by ion-milling to transfer the pattern to the magnetic material. We have demonstrated each step in this process and report on the quality of the pattern achieved.

Development of Strategies to Improvement Ordering and Perpendicular Alignment of Cylinder Phase Block Copolymers Used as Templates for Bit Patterned Media

AbstractBit patterned media, including media fabricated with a gradient in composition, is being developed as a potential path to higher information storage density. The noise level in such media is significantly impacted by the precision of the ordering of the individual bits and by the narrowness of their size distribution. Block copolymers that phase separate on the appropriate length scale are one method of pattern generation that is receiving considerable attention. For cylinder forming block copolymer phases the ordering and degree of perpendicular alignment is largely determined by the matching of the substrate surface to the block copolymer. If the chemical properties of the substrate surface match the average for the block copolymer, then thin films of the block copolymer align perpendicularly on annealing. Although there are a number of examples where the substrate surface fortuitously matches the block copolymer, in general an orienting layer is necessary to provide the appropriate match. The most popular approach has been to synthesize a random copolymer with the same average composition as the block copolymer. In order to produce suitably thin orienting layers it has been necessary to chemically tether the random copolymer to the substrate. Previously used chemistry has not been suitable for noble metal substrates such as platinum. We have been developing an alternate approach using thiol functional groups which we anticipate will be more suitable for Pt capped substrates.

Effect of Chain Architecture and Surface Energies on the Ordering Behavior of Lamellar and Cylinder Forming Block Copolymers

We investigate the effect of surface energy and chain architecture on the orientation of microdomains in relatively thick films (600−800 nm) of lamellar and cylindrical block copolymers of poly(cyclohexylethylene) (C) and poly(ethylene) (E). The E block has 26 ethyl branches per 1000 backbone carbon atoms. Melt surface energies of the C and E blocks are 22.3 and 20.9 mJ/m2, respectively. Grazing-incidence small-angle X-ray scattering (GISAXS), scanning force microscopy (SFM), and cross-sectional transmission electron microscopy (TEM) show that cylindrical and lamellar CEC triblock copolymers orient their microdomains normal to the surface throughout the film thickness. However, a lamellar CE diblock copolymer prefers a parallel orientation of the lamellae relative to the surface with an E surface layer. Moreover, a cylindrical CEBC triblock copolymer where the EB block has 125 ethyl branches per 1000 backbone carbon atoms leads to EB cylinder domains that always orient parallel to the surface. In this cas...

Salt Complexation in Block Copolymer Thin Films

Ion complexation within cylinder-forming block copolymer thin films was found to affect the ordering process of the copolymer films during solvent annealing, significantly enhancing the long-range positional order. Small amounts of alkali halide or metal salts were added to PS-b-PEO, on the order of a few ions per chain, where the salt complexed with the PEO block. The orientation of the cylindrical microdomains strongly depended on the salt concentration and the ability of the ions to complex with PEO. The process shows large flexibility in the choice of salt used, including gold or cobalt salts, whereby well-organized patterns of nanoparticles can be generated inside the copolymer microdomains. By further increasing the amount of added salts, the copolymer remained highly ordered at large degrees of swelling and demonstrated long-range positional correlations of the microdomains in the swollen state, which holds promise as a route to addressable media.

Long-Range Order and Orientation of Cylinder-Forming Block Copolymers on Chemically Nanopatterned Striped Surfaces

Cylinder-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) thin films were directed to assemble on chemically nanopatterned surfaces consisting of alternating stripes that were preferentially wet by the two blocks of the copolymer. The cylindrical domains oriented in the plane of the film and formed defect-free periodic arrays over large areas in registration with the underlying chemical surface pattern if three constraints were satisfied: the substrate pattern period (LS) was commensurate with the bulk intercylinder period of the block copolymer (LO), the initial film thickness was quantized with respect to the thickness of a half layer (L/2) or single layer of cylinders (L), and the widths of adjacent stripes of the chemical surface pattern were nearly equal.

Perpendicular Domain Orientation in Thin Films of Polystyrene−Polylactide Diblock Copolymers

Block copolymer thin films are ideal templates for a wide range of technologies where large area patterns of nanoscale features are desired. One of the main challenges in using lamellae- and cylinder-forming block copolymers for this purpose is to induce the block copolymer domains to orient perpendicular to the film surface. We here show that perpendicular domain orientation can be easily achieved with polystyrene-b-polylactide (PS−PLA) thin films. Cylinder-forming PS−PLA films were prepared by spin-coating on a variety of substrates followed by thermal annealing. The molecular weight, film thickness, annealing temperature, and annealing time were varied. When the film thickness was larger than the repeat spacing of the bulk morphology, the domains oriented perpendicular to the surface independent of the substrate/film interface. The films were then used to prepare nanoporous templates by a combination of hydrolytic PLA degradation and oxygen reactive ion etching (O2-RIE). The template pattern was then transferred to the substrate using CF4-RIE to form an array of nanoscale pits.

Nanoordering of Fluorinated Side-Chain Liquid Crystalline/Amorphous Diblock Copolymers

The role of anchoring the mesogenic side chains at the internal and boundary interfaces in the thin film morphology has been investigated for two fluorinated side-chain liquid crystalline/amorphous diblock copolymers. Initial bulk investigations indicate that the two polymers strongly microphase segregate into a cylindrical and a lamellar morphology, respectively. Inside the liquid-crystal block, the cylinder-forming block copolymer shows both a crystalline-B and a smectic-A phase, whereas the lamellae-forming polymer gives only a smectic-A phase, with the side chains organized in double layers with a spacing of 3.33 nm. In both situations the smectic layers are oriented perpendicular to the block interfaces, which state is retained in films. In thin films of the cylinder-forming block copolymer, the smectic layering and the orientational wetting properties of the fluorinated side chains stabilize the minority cylindrical domains normal to the substrate. For films of the lamellar system, however, an evolv...