Orientation Of Microdomains Research Articles

NANOSCALE PATTERNING IN BLOCK COPOLYMER THIN FILMS

Nanoscale patterning in block copolymer (BCP) thin films is one of the key issues in nanoscience and nanotechnology. To make useful nanoscale patterns in applications such as magnetic media, wire-grid polarizer, and opto-electronics, the control of orientation and lateral ordering of BCP microdomains is required. The generation of highly ordered and oriented cylindrical microdomains in BCP thin films will be discussed. This is followed by a review of applications of nanoscale patterns like fabrication of nanoporous templates, metallic nanowires, and the preparation of nanostructured inorganic materials.

A novel strategy for controlling the orientation of cylindrical domains in thin blend copolymer films via ‘double phase separation’

We discuss a simple and robust approach to control the orientation of cylindrical microdomains in block copolymer films. The approach is based on the so-called double phase separation occurring in the binary blend of incompatible compositionally asymmetric block copolymers and their selective adsorption on a homogeneous surface. As a proof-of-concept, we perform mesoscale simulations, using dynamic density functional theory and dissipative particle dynamics, and show that the target morphology with perpendicularly oriented cylinders can be thermodynamically stable for rather thick films when the intrinsic hexagonal period of the majority bulk phase is commensurate with the surface-induced pattern formed by the strongly adsorbed minority component.

Bending of Lamellar Microdomains of Block Copolymers on Nonselective Surfaces

Symmetric diblock copolymers self-assemble into lamellar microdomains which can be used to create line and space patterns on substrates by controlling the orientation of microdomains. Previously, it has been reported that the orientation of lamellar microdomains can be controlled perpendicular to the surface by modifying the substrate surface to become nonselective to each domain. For practical use of the block copolymer based patterns for nanofabrications, of great interest is precise control over the lateral placement of lamellar microdomains. Commonly predefined guiding patterns containing chemical and/or topographical contrast have been used to direct the lateral placement of lamellae on substrates. In the absence of guiding patterns, the lamella microdomains produce fingerprint-like surface patterns on nonselective substrates as reported previously with a variety of model diblock copolymers, where the most well-studied diblock copolymer is poly(styrene-b-methyl methacrylate) (PS-b-PMMA). The fact that this fingerprint-like pattern contains many regions of high curvature of bent lamellae suggests the cost of lamellae bending may be relatively small. Previously, Nealey and co-workers reported that the lamellar microdomains of PS-bPMMA faithfully follow the lithographically predefined surface patterns of chemical contrast. They showed that bent block copolymer lamellae could be obtained when homopolymers were added to the copolymer. More recently, Wilmes et al. examined the extent to which patterns with different curvatures can be created within the same block copolymer thin films. They showed that the lamellae conform to patterns with radii of curvature equal to the equilibrium domain spacing. In this Note, we report our investigation on the bending characteristics of lamellar microdomains resulting from directed assembly using topographic guiding patterns. We designed the topographic guiding patterns as elbows with varying corner angles. By controlling the surface of guiding patterns nonselective to the microdomains, we rendered the orientation of lamellae perpendicular to the surfaces of bottom and sidewalls of the guiding patterns. Detailed experimental observation of the degree of bending and tilting of lamellae at the sidewall surface is reported along with a theoretical investigation using self-consistent-field calculations, which provide additional knowledge regarding the free energy and interfacial characteristics of the experimentally observed morphologies. We also extended our investigation to a block copolymer hybrid system which is a mixture of an organic diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an organosilicate (OS) resin. The effect of larger orientational correlation length of the hybrid system on the bending property of lamellar microdomains will be discussed.

Surface-independent vertical orientation of cylindrical microdomains in block copolymer thin films directed by comb-coil architecture

Vertically oriented cylindrical microdomains of block copolymer thin films consisting of polystyrene and poly(methyl methacrylate) were fabricated regardless of the substrate by introduction of a grafted architecture into the diblock copolymer chains. A series of comb-coil block copolymers, poly(methyl methacrylate)-b-poly(2-(2-bromopropionyloxy)-ethyl acrylate)-g-polystyrene (PMMA-b-PBPEA-g-PS) with various lengths of PS were synthesized by combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). When the volume fraction of PS was 75%, microphase separation produced cylindrical microdomains of PMMA surrounded by PS matrix in a thin film state after thermal annealing. Analysis of the thin films after subsequent etching of PMMA domains by atomic force microscopy, cross-sectional scanning electron microscope images and grazing incidence X-ray scattering measurements showed that the microdomains were oriented perpendicular to various substrates including metals, silicon, polymers, and even patterned surfaces. Steric repulsion between the grafted chains during the phase separation was attributed to the driving force for perpendicular orientation of the cylindrical microdomains without the aid of external fields.

Mesoscopic simulation of the self-organization of a binary mixture of copolymers in the vicinity of a selective adsorbing surface

A new strategy for the construction of ultrathin structured films with cylindrical microdomains oriented primarily perpendicularly to the surface of a solid substrate is proposed. This approach is based on the phase separation of a binary mixture of incompatible compositionally asymmetric block copolymers, microphase separation of their blocks, and selective adsorption of one of the copolymers on the surface. Preferentially adsorbed copolymer forms a regular pattern on the surface, which serves as an orienting support for another component that dominates in the system and forms the hexagonal mesophase in the bulk of the film. This approach has been verified by the mesoscopic simulation based on the dynamic version of the density functional theory. As a result of the self-organization of macromolecules, the morphology with perpendicularly oriented domains can be thermodynamically stable for the films with a thickness of ∼100 nm. The commensurability of the parameters of the surface pattern and volume mesophase is shown to be the key factor controlling the orientation of cylindrical microdomains.

Orientation of Block Copolymer Resists on Interlayer Dielectrics with Tunable Surface Energy

It has been well established that one of most important factors to control the orientation of microdomains in block copolymer (BCP) films is the wetting behavior of a BCP at the interface (or on a substrate). From this perspective, we studied the wetting behavior of P(S-b-MMA) thin films on organosilicate (OS) interlayer dielectric layers (ILDs), which were employed as the target substrates for patterning by BCP lithography. We controlled the surface energy of OS substrates by varying the cure temperature of OS substrates ranging from 200 to 400 °C. As the cure temperature was increased, the wetting behavior of a P(S-b-MMA) film on the OS substrate changed from the asymmetric to the symmetric wetting, as confirmed by AFM, which, in turn, allowed us to find the optimal cure temperature for neutral wetting behavior. As a result, we obtained perpendicularly oriented BCP microdomains on the OS substrate cured at 360 °C without further surface modification. Finally, we were able to transfer sub-25 nm BCP patterns directly onto the neutral ILD layers through reactive ion etching. The process approach taken here clearly demonstrates that the process involved in BCP lithography could be simplified by eliminating the additional surface modification step on a target substrate.

Transition Behavior of Weakly Interacting PS-b-PMMA Films on Preferential Surfaces: A Direct Observation by GISAXS

The phase transitions for the films of a symmetric polystyrene-b-poly(methyl methacryltate) (PS-b-PMMA) on a modified surface, like the order-to-disorder transition (ODT) in the weak segregation regime, was investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS). The selective interactions at the surface by PS-brush substrate that favors the preferential interactions with the PS component of the block copolymer enhance the parallel orientation of the lamellar microdomains to the film surface. This effect for weakly interacting PS-b-PMMA films leads to a gentle decrease of the transition temperature with increasing film thickness up to 50L0 (lamellar period). We discuss this thickness dependence of transition temperatures for PS-b-PMMA films on preferential surfaces in terms of the temperature dependence of χ between two block components.

Cylindrical Microdomain Orientation of PS-b-PMMA on the Balanced Interfacial Interactions: Composition Effect of Block Copolymers

Cylindrical Microdomain Orientation of PS-<i>b</i>-PMMA on the Balanced Interfacial Interactions: Composition Effect of Block Copolymers

Orientation of cylindrical microdomains of triblock copolymers byin situstress–strain‐ birefringence measurements

AbstractThe orientation behavior of polystyrene‐block‐hydrogenated polyisoprene‐block‐polystyrene (SEPS) with cylindrical microdomains of polystyrene (PS) dispersed in the rubbery segments was investigated by simultaneous measurements of stress and birefringence during uniaxial stretching. The stress increased sharply with strain below the yield strain and then it gradually increased. In contrast, the birefringence changed little below the yield strain, increased sharply with strain above the yield strain up to a strain of 0.5, and then gradually increased. The characteristic birefringence behavior is attributed to the form birefringence induced by the orientation and the parallel arrangement of the cylindrical microdomains associated with the orientation of the rubbery segments. The orientation function of the cylindrical microdomainsfevaluated by analyzing the form birefringence agrees well with that obtained from the SAXS result. Thefwas almost zero below the yield strain and it increased sharply with strain up to a strain of 0.5 and then was constant above a strain of 0.5. These results suggest that the orientation of the cylindrical microdomains occur above the yield strain up to a strain of 0.5 and that the orientation does not increase above a strain of 0.5 in spite of the continuous orientation of the rubbery ethylene–propylene segments. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 715–723, 2009

Lateral Ordering of Cylindrical Microdomains Under Solvent Vapor

The development of the morphology in asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin films in tetrahydrofuran (THF) vapor, a selective solvent for majority PS block, as a function of time was investigated by scanning force microscopy (SFM) and grazing incidence small-angle X-ray scattering (GISAXS). When the PS-b-P4VP films were spin-coated from a toluene/THF mixture onto a silicon substrate, cylindrical microdomains were found to be oriented normal to the surface. By annealing under the THF solvent vapor, the distribution of the size and center-to-center distance between the cylindrical microdomains were significantly narrowed. The orientation and grain size of the cylindrical microdomains in the annealed films were characterized using Moiré analysis obtained from SFM scan. GISAXS was used to characterize the morphology of the entire film.

Enhanced light emission of nano-patterned GaN via block copolymer thin films

We demonstrate that the nanoscopic block copolymer patterns on GaN can enhance light extraction efficiency of GaN-based light emitting diodes. Nanoporous patterns were fabricated on a bare GaN substrate via self-assembly of poly(styrene-b-methyl methacrylate) block copolymers from which PMMA microdomains were selectively removed later on. A bare GaN surface was treated with a photo-crosslinkable thin layer of poly(styrene-r-methyl methacrylate) random copolymers to tune the cylindrical microdomain orientations. The nanoporous block copolymer thin film was controlled to be thicker than its typical repeat period in bulk by incorporating PMMA homopolymer into block copolymer. Consequently, the light extraction efficiency in photoluminescence spectra could be tuned with the thickness of nanopatterned thin film on GaN.

Highly ordered nanoporous thin films by blending of PSt‐b‐PMMA block copolymers and PEO additives as structure directing agents

AbstractHerein, we present a simple method for producing nanoporous templates with a high degree of lateral ordering by self‐assembly of block copolymers. A key feature of this approach is control of the orientation of polymeric microdomains through the use of hydrophilic additives as structure directing agents. Incorporation of hydrophilic poly(ethylene oxide) (PEO) moieties into poly(styrene‐b‐methyl methacrylate) (PSt‐b‐PMMA) diblock copolymers gives vertical alignment of PMMA cylinders on the substrate after solvent annealing. Because of the miscibility between PEO and PMMA, PEO additives were selectively positioned within PMMA microdomains and by controlling the processing conditions, it was found that ordering of PSt‐b‐PMMA diblock copolymers could be achieved. The perpendicular orientation of PMMA cylinders was achieved by increasing the molecular size of the PEO additives leading to an increased hydrophilicity of the PMMA domains and consequently to control the orientation of microdomains in PSt‐b‐PMMA block copolymer thin films. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8041–8048, 2008

Microdomain Orientation of PS-b-PMMA by Controlled Interfacial Interactions

The microdomain orientation in thin films of cylinder- and lamella-forming diblock copolymers, polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), was investigated as a function of the film thickness and the composition of random copolymers composed of styrene (S) and methyl methacrylate (MMA), denoted as P(S-r-MMA), that were anchored to the substrate. Using scanning force microscopy (SFM) and grazing incidence small-angle X-ray scattering (GISAXS), the dependence of the microdomain orientation on film thickness around lattice period (or d-spacing, L0), where the microdomain orientations normal to the film surface could be achieved, showed that the optimal condition for the balanced interfacial interactions (the so-called neutrality in random copolymer) was 0.64 of PS mole fraction (XPS) for the cylindrical microdomain having narrow compositional range of P(S-r-MMA) from 0.52 to 0.72 of XPS, whereas for the lamella microdomain it was observed at XPS = 0.55 ranging extensively from 0.48 to 0.78.

A Simple Route to Highly Oriented and Ordered Nanoporous Block Copolymer Templates

Controlling the orientation and lateral ordering of the block copolymer microdomains is essential to their use as templates and scaffolds for the fabrication of nanostructured materials. In addition, a process must be robust, simple to implement, and rapid, and should not introduce disruptive processing steps that would impede their use. Here, we describe thin films of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymers, spin-coated from mixed solvents that show highly oriented, cylindrical microdomains with a high degree of order on a wide range of substrates, including silicon oxide, polystyrene, germanium, polyimide, and poly(butylene terephthalate). In addition, the preferential solvation of the P4VP block with an alcohol caused a surface reconstruction that resulted in the formation of a nanoporous film upon drying. The evaporation of gold onto the reconstructed films produced thermally stable and reactive ion etching resistant films.

Lamellae Orientation in Block Copolymer Films with Ionic Complexes

Lamellae orientation in lithium-complexed polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer films on natively oxidized silicon wafers is investigated as a function of film thickness and percentage of carbonyl groups coordinated with lithium ions using cross-sectional transmission electron microscopy and grazing incidence small-angle X-ray scattering. For films with a lower percentage of ionic complexes, the strong surface interaction of the PMMA blocks with the substrate is not changed significantly and the orientation of the lamellar microdomains depends on the film thickness and is dictated by a coupling of the interfacial interactions and the degree of microphase separation. For films with a higher percentage of ionic complexes, the surface interaction is mediated. Along with the enhanced immiscibility between the two blocks, which drives the self-assembly into a stronger microphase segregation, an orientation of the lamellar microdomains normal to the surface is seen, independent of film thickness. Thus, by tuning the amount of ionic complexes, the orientation of lamellar microdomain can be controlled from a random arrangement to being oriented parallel or perpendicular to the film surface without any surface modification or use of external fields, which opens a simple and general route for the fabrication of nanostructured materials.

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter EmBn block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E224B114 exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.

Lamellar Orientation in Thin Films of Symmetric Semicrytalline Polystyrene-b-poly(ethylene-co-butene) Block Copolymers: Effects of Molar Mass, Temperature of Solvent Evaporation, and Annealing

Orientation of the lamellar microdomains in thin films of three symmetric polystyrene-b-poly(ethylene-co-butylene) block copolymers (S65E155, S156E358, and S199E452) on mica was investigated via atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The results show that lamellar orientation in the SxEy block copolymers greatly depends on the molar mass of the block copolymers, the temperature of solvent evaporation, and annealing. The nascent thin film of the low molar mass block copolymer, S65E155, shows a multilayered structure parallel to the mica surface with the PS block at both polymer/mica and polymer/air interfaces, but the high molar mass block copolymers, S156E358 and S199E452, exhibit a structure with lamellar microdomains perpendicular to the mica surface. When the solvent is evaporated at a lower temperature, the crystallization rate is fast and a two-dimensional spherulite structure with the lamellar microdomains perpendicular to the mica surface is observed. Annealing of all the thin films with lamellar microdomains perpendicular to the mica surface leads to morphological transformation into a multilayered structure parallel to the mica surface. In all SxEy thin films on mica, the stems of PE crystals are always perpendicular to the interface between the lamellar PE and PS microdomains. A mechanism is proposed for the formation of different microdomain orientations in the thin films of semicrystalline block copolymers. When the thin film is prepared from a homogeneous solution, microdomains perpendicular to the substrate surface are formed rapidly for strongly segregated block copolymers or at a lower crystallization temperature and kinetically trapped by the strong segregation strength or solidification of crystallization, while for weakly segregated block copolymers or at slower crystallization rate, the orientation of the microdomains is dominated by surface selectivity.

Templated nanostructured PS‐b‐PEO nanotubes

AbstractWith anodic aluminum oxide (AAO) membranes as wetting templates, nanotubes of the cylinder‐forming polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) copolymer were generated. The PS‐b‐PEO solution was introduced into the cylindrical nanopores of an AAO membrane by capillary force and polymeric nanotubes formed after solvent evaporation. Because of the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respect to the nanotube walls, the nanotubes were permeable to aqueous media. PS‐b‐PEO nanotubes were also prepared on the interior walls of amorphous carbon nanotubes (a‐CNTs). Because of the unique water permeability of the PEO microdomains, an avenue for functionalizing the interior of the a‐CNTs is enabled. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2912–2917, 2007

Fabrication of Metallized Nanoporous Films from the Self-Assembly of a Block Copolymer and Homopolymer Mixture

Inorganic compound HAuCl4, which can form a complex with pyridine, is introduced into a poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) block copolymer/poly(methyl methacrylate) (PMMA) homopolymer mixture. The orientation of the cylindrical microdomains formed by the P2VP block, PMMA, and HAuCl4 normal to the substrate surface can be generated via cooperative self-assembly of the mixture. Selective removal of the homopolymer can lead to porous nanostructures containing metal components in P2VP domains, which have a novel photoluminescence property.

Surface Modification with Cross-Linked Random Copolymers: Minimum Effective Thickness

Ultrathin films of benzocyclobutene (BCB)-functionalized random copolymer with styrene and methyl methacrylate, P(S-r-BCB-r-MMA), with thicknesses ranging from 0 to 10.5 nm, were thermally cross- linked on Si substrates. The penetration of deuterated PMMA (dPMMA) into the P(S-r-BCB-r-MMA) films and the microdomain orientation of PS-b-PMMA diblock copolymers on P(S-r-BCB-r-MMA) coated substrates were investigated by neutron reflectivity (NR) and scanning force microscopy (SFM), respectively. NR measurements on bilayers of dPMMA on cross-linked P(S-r-BCB-r-MMA) showed that the neutron scattering length density (SLD) at the substrate was equal to that of P(S-r-BCB-r-MMA) if the P(S-r-BCB-r-MMA) film was thicker than 5.5 nm. With decreasing thickness of the P(S-r-BCB-r-MMA) film, the SLD at the substrate increased, characteristic of an increasing penetration of the dPMMA. When thin films of PS-b-PMMA diblock copolymer having cylindrical microdomains are placed on surfaces modified with cross-linked films of P(S-r-BCB-r-MMA) thinner than 5.5 nm, the cylindrical microdomains orient parallel to the surface, whereas for thicker films, the microdomains orient normal to the surface. Both of these results indicate that interfacial interactions are screened when the P(S-r-BCB-r-MMA) film is thicker than 5.5 nm.