Alignment Of Block Copolymers Research Articles

Chiral Plasmonic Nanowaves by Tilted Assembly of Unidirectionally Aligned Block Copolymers with Buckling-Induced Microwrinkles.

Chiral-structured nanoscale materials exhibit chiroptical properties with preferential absorptions of circularly polarized light. The distinctive optical responses of chiral materials have great potential for advanced optical and biomedical applications. However, the fabrication of three-dimensional structures with mirrored nanoscale geometry is still challenging. This study introduces chiral plasmonic nanopatterns in wavy shapes based on the unidirectional alignment of block copolymer thin films and their tilted transfer, combined with buckling processes. The cylindrical nanodomains of polystyrene-block-poly(2-vinylpyridine) thin films were unidirectionally aligned over a large area by the shear-rolling process. The aligned block copolymer thin films were transferred onto uniaxially prestrained polydimethylsiloxane films at certain angles relative to the stretching directions. The strain was then released to induce buckling. The aligned nanopatterns across the axis of the formed microwrinkles were selectively infiltrated with gold ions. After reduction by plasma treatment, chiral plasmonic nanowave patterns were fabricated with the presence of mirror-reflected circular dichroism spectra. This fabrication method does not require any lithography processing or innately chiral biomaterials, which can be advantageous over other conventional fabrication methods for artificial nanoscale chiral materials.

Hyperbolic Optical Metamaterials from Shear‐Aligned Block Copolymer Cylinder Arrays

Hyperbolic metamaterials behave similarly to either metals or dielectrics depending on the polarization of incident light. This behavior is reflected in permittivity tensors having both negative and positive components, which leads to interesting optics such as negative refraction and super‐resolution imaging. To achieve this polarization‐dependent electromagnetic response, hyperbolic metamaterials require a strongly anisotropic nanostructure made from noble metals. Herein, a nanostructured array of aligned gold and silver nanocylinders that exhibit a strongly anisotropic optical response both in reflection and transmission are investigated. The plasmonic material is manufactured using films of aligned block copolymer (BCP) cylinders as template, where a uniform alignment is achieved by annealing in the presence of an external shear force. Metal deposition into a nanoporous polymer template resulted in free‐standing metal nanostructures consisting of a hexagonally packed in‐plane cylinder array supported by occasional interconnections between adjacent cylinders. 3D full‐wave simulations confirm the experimentally measured anisotropic response of the metallic nanocylinders and show that the interconnections between cylinders affect the optical response while maintaining the hyperbolic nature of the metamaterial. The large‐scale alignment of anisotropic BCP nanostructures provides useful templates for the fabrication of metal‐based optical materials with potential applications as polarizers, sensors, or angle‐dependent broadband absorbers.

Orthogonally Aligned Block Copolymer Line Patterns on Minimal Topographic Patterns.

We demonstrate the generation of block copolymer (BCP) line patterns oriented orthogonal to a very small (minimal) topographic trench pattern over arbitrarily large areas using solvent-vapor annealing. Increasing the thickness of BCP films induced an orthogonal alignment of the BCP cylindrical microdomains, where full orthogonal alignment of the cylindrical microdomains with respect to the trench direction was obtained at a film thickness corresponding to 1.70 L0. A capillary flow of the solvent across the trenches was a critical factor in the alignment of the cylindrical microdomains. Grazing incidence small-angle X-ray scattering was used to determine the orientation function of the microdomains, with a value of 0.997 being found reflecting a nearly perfect orientation. This approach to produce orthogonally aligned BCP line patterns could be extended to the nanomanufacturing and fabrication of hierarchical nanostructures.

Block copolymer-based nanocomposites with exotic self-assembled structures induced by a magnetic field

Structural control of polymer nanocomposites is important for their applications in organic semiconductors, lithographic nanopatterning, separation membranes, and nanofabrication templates. However, manufacturing nanocomposite materials with novel structures in a highly efficient yet precise manner remains a great challenge. To create nanocomposite structures, we combined self-assembly processing of block copolymer (BCP)-metal complex nanocomposites with an applied magnetic field. Here, we describe in detail the mechanism of magnetic alignment of block copolymers doped with metal complexes; specifically, we investigated the effect of the applied magnetic field on the phase behavior of the assembled block copolymer-metal complex nanocomposites with various molecular weights and with different molecular structures. We show that our combination of self-assembly processing and application of a magnetic field yielded lamellar structures of alternating multilayers with different layer thicknesses. This self-assembled structure is not included in phase diagrams of BCPs. The influence of the block copolymers’ molecular structures on the nanocomposites’ phase transformation behavior is also discussed. Our results provide a route to manufacturing nanocomposite materials in a highly efficient yet precise manner, which could lead to improvement in the material properties of nanocomposites.

In-depth TEM characterization of block copolymer pattern transfer at germanium surfaces

Dry plasma etching for the pattern transfer of mask features is fundamental to semiconductor processing and the development of device and electrically conducting elements becomes more challenging as features reach the deep nanoscale regime. In this work, high resolution transmission electron microscopy (TEM) coupled with energy dispersive x-ray (EDX) characterization were used to analyze the pattern transfer of graphoepitaxially aligned block copolymer (BCP) features to germanium (Ge) substrates as a function of time. The BCP patterns were converted into metal oxide hardmasks in order to affect good aspect ratios of the transferred features. An unusual interface layer between metal oxide nanowires and the germanium-on-insulator substrate was observed. EDX analysis shows that the origin of this interface layer is a result of the presence of a negative tone e-beam resist material, HSQ (hydrogen silsesquioxane). HSQ was employed as a guiding material to align line-space features of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP with 16 nm half-pitch topography. Additionally, the existence of a metal oxide layer (from the initial PS-b-P4VP film) is also shown through ex situ TEM and EDX characterization. Three dimensional modeling of features is also provided giving a unique insight into the arrangement and structure of BCP features prior to and after the pattern transfer process. The results presented in this article highlight the accuracy of high resolution electron microscopy and elemental mapping of BCP generated on-chip etch masks to observe and understand through-film features affecting pattern transfer.

Reversible Switching of Block Copolymer Nanopatterns by Orthogonal Electric Fields.

It is demonstrated that the orientation of striped patterns can be reversibly switched between two perpendicular in-plane orientations upon exposure to electric fields. The results on thin films of symmetric polystyrene-block-poly(2-vinyl pyridine) polymer in the intermediate segregation regime disclose two types of reorientation mechanisms from perpendicular to parallel relative to the electric field orientation. Domains orient via grain rotation and via formation of defects such as stretched undulations and temporal phase transitions. The contribution of additional fields to the structural evolution is also addressed to elucidate the generality of the observed phenomena. In particular solvent effects are considered. This study reveals the stabilization of the meta-stable in-plane oriented lamella due to sequential swelling and quenching of the film. Further, the reorientation behavior of lamella domains blended with selective nanoparticles is addressed, which affect the interfacial tensions of the blocks and hence introduce another internal field to the studied system. Switching the orientation of aligned block copolymer patterns between two orthogonal directions may open new applications of nanomaterials as switchable electric nanowires or optical gratings.

Enabling complex nanoscale pattern customization using directed self-assembly.

Block copolymer directed self-assembly is an attractive method to fabricate highly uniform nanoscale features for various technological applications, but the dense periodicity of block copolymer features limits the complexity of the resulting patterns and their potential utility. Therefore, customizability of nanoscale patterns has been a long-standing goal for using directed self-assembly in device fabrication. Here we show that a hybrid organic/inorganic chemical pattern serves as a guiding pattern for self-assembly as well as a self-aligned mask for pattern customization through cotransfer of aligned block copolymer features and an inorganic prepattern. As informed by a phenomenological model, deliberate process engineering is implemented to maintain global alignment of block copolymer features over arbitrarily shaped, 'masking' features incorporated into the chemical patterns. These hybrid chemical patterns with embedded customization information enable deterministic, complex two-dimensional nanoscale pattern customization through directed self-assembly.

Single-molecule tracking studies of flow-induced microdomain alignment in cylinder-forming polystyrene-poly(ethylene oxide) diblock copolymer films.

Flow-based approaches are promising routes to preparation of aligned block copolymer microdomains within confined spaces. An in-depth characterization of such nanoscale morphologies within macroscopically nonuniform materials under ambient conditions is, however, often challenging. In this study, single-molecule tracking (SMT) methods were employed to probe the flow-induced alignment of cylindrical microdomains (ca. 22 nm in diameter) in polystyrene-poly(ethylene oxide) diblock copolymer (PS-b-PEO) films. Films of micrometer-scale thicknesses were prepared by overlaying a benzene solution droplet on a glass coverslip with a rectangular glass plate, followed by solvent evaporation under a nitrogen atmosphere. The microdomain alignment was quantitatively assessed from SMT data exhibiting the diffusional motions of individual sulforhodamine B fluorescent probes that preferentially partitioned into cylindrical PEO microdomains. Better overall microdomain orientation along the flow direction was observed near the substrate interface in films prepared at a higher flow rate, suggesting that the microdomain alignment was primarily induced by shear flow. The SMT data also revealed the presence of micrometer-scale grains consisting of highly ordered microdomains with coherent orientation. The results of this study provide insights into shear-based preparation of aligned cylindrical microdomains in block copolymer films from solutions within confined spaces.

Effects of metal loading and magnetic field strength on alignment of noncrystalline block copolymers doped with metal complexes

Control of the orientation of block copolymers in self-assembled nanostructures is important for their applications in organic semiconductors, lithographic nanopatterning, separation membranes, and nanofabrication templates. We recently reported that addition of magnetically sensitive metal complexes to block copolymers can be used to align the block copolymers under the influence of a magnetic field. In the present study, we investigated the mechanism of magnetic alignment of block copolymers doped with metal complexes. Specifically, we used small-angle X-ray scattering analysis to evaluate the effects the metal complex molar ratio and the strength of the applied magnetic field have on magnetic alignment in block copolymer–metal composites. Two Fe precursors, tricarbonyl(cyclooctatetraene) iron and acetylacetonate iron(III), and one Pt precursor, platinum dimethylcyclooctadiene, were selectively introduced into separate polymer blocks of a block copolymer, polystyrene-block-poly(2-vinylpyridine) (PS-P2VP, 102 k/97 k) or polystyrene-block-poly(4-vinylpyridine) (PS-P4VP, 40 k/5.6 k), and the resulting films were annealed in a magnetic field. We found that magnetic alignment of the block copolymers was enhanced by high metal complex molar ratios and high magnetic field strength. The lamellar structures of the self-assembled PS-P2VP(102 k/97 k) composites were disturbed when the amounts of the metal complexes were increased, and magnetic alignment of the lamellar structures was enhanced when the strength of the applied magnetic field was increased. Magnetic alignment induced shrinkage of the cylindrical structures of the self-assembled PS-P4VP(40 k/5.6 k) composites with high metal complex ratios.

Photochemical Reactions for Replicating and Aligning Block Copolymer Thin Film Patterns

A photochemical process for transfer printing patterns formed by block copolymer thin films is described here. An unpatterned, transparent substrate coated in liquid conformal layer is pressed to an epitaxially aligned block copolymer thin film. Irradiation through the sample stack by UV light drives a benzophenone-mediated grafting reaction which covalently bonds the top surface of the block copolymer film to the photopolymerized conformal layer. Separation of the sample stack recovers the original guiding pattern as well a new chemically patterned substrate replicated from the original pattern. Using a single lithographically directed chemical pattern, 10 replicas are generated, each possessing 28 nm periodicity, perpendicularly oriented microdomains, and long-range epitaxial alignment.

Dynamics of Magnetic Alignment in Rod–Coil Block Copolymers

The dynamics associated with magnetic field alignment of a model rod–coil block copolymer poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylenevinylene)-b-polyisoprene (PPV–PI) have been investigated using a combination of time-resolved in situ small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Alignment is observed over a wide range of field strengths (0.2–7 T); however, the highest field strengths studied produce the highest degree of alignment. Experiments examining alignment of a disordered sample, cooled into the ordered state in the presence of a magnetic field, show that alignment mostly occurs during nucleation and growth of the block copolymer nanostructure. The slower secondary processes of defect annihilation and grain rotation progress are necessary in producing extremely highly aligned samples. At the highest field strength, due to the increased order–disorder transition temperature (TODT), selective ordering is likely observed at temperatures near the order–disorder transition leading to nucleation of aligned block copolymer grains, resulting in faster and a higher degree of alignment. Additionally, at these high field strengths the alignment process appears to have a more complex defect production and removal process than at low field strengths. At low field strengths isotropic nucleation occurs, and then preferential growth of aligned block copolymer grains is primarily responsible for alignment. Finally, an optimum alignment temperature is observed where the thermodynamic driving force for alignment, thermal disordering processes, and the kinetic effects governing block copolymer growth and defect removal are balanced.

Highly Aligned Block Copolymer Thin Films by Synergistic Coupling of Static Graphoepitaxy and Dynamic Thermal Annealing Fields.

Directed self-assembly of cylinder forming block copolymer (c-BCP) thin films via a dynamic thermal field on multidimensional symmetric graphoepitaxy channels is reported. A synergy of dynamic thermal and static boundary fields induces highly aligned c-BCP cylinders inside the channels with a power law dependence of orientational order parameter f, on trench width, f ∼ d-0.3, analogous to dual-field alignment of semiconducting metals and liquid crystals on graphoepitaxy surfaces, f' ∼ d-1. Static thermal annealing of identical films in a vacuum oven for several days fails to produce comparable results. Furthermore, we demonstrate global c-BCP cylinder alignment over mesas and trenches by tuning the synergy between the dynamic thermal field and asymmetry of the graphoepitaxy static field.

Directed self-assembly of PS-b-PMMA block copolymer using HSQ lines for translational alignment

We report here the graphoepitaxial alignment of a lamellar forming PS-b-PMMA block copolymer (BCP) for directed self-assembly using topographical patterns (simple line structures) of hydrogen silsesquioxane (HSQ). The system demonstrates the importance of the sidewall chemistry on translational alignment of BCP domains. A method was developed where a silicon substrate was precoated with a hydroxyl-terminated random copolymer brush of PS-r-PMMA prior to the HSQ feature formation process. The brush ordains the vertical (to the substrate plane) alignment of the BCP lamellar microdomains. Translational BCP alignment is a result of PMMA selectively wetting the HSQ. The formed BCP pattern was selectively etched to remove the PMMA domain allowing direct imaging and to demonstrate capability in forming an on-chip mask.

Large-scale parallel arrays of silicon nanowires via block copolymer directed self-assembly

Extending the resolution and spatial proximity of lithographic patterning below critical dimensions of 20 nm remains a key challenge with very-large-scale integration, especially if the persistent scaling of silicon electronic devices is sustained. One approach, which relies upon the directed self-assembly of block copolymers by chemical-epitaxy, is capable of achieving high density 1 : 1 patterning with critical dimensions approaching 5 nm. Herein, we outline an integration-favourable strategy for fabricating high areal density arrays of aligned silicon nanowires by directed self-assembly of a PS-b-PMMA block copolymer nanopatterns with a L(0) (pitch) of 42 nm, on chemically pre-patterned surfaces. Parallel arrays (5 × 10(6) wires per cm) of uni-directional and isolated silicon nanowires on insulator substrates with critical dimension ranging from 15 to 19 nm were fabricated by using precision plasma etch processes; with each stage monitored by electron microscopy. This step-by-step approach provides detailed information on interfacial oxide formation at the device silicon layer, the polystyrene profile during plasma etching, final critical dimension uniformity and line edge roughness variation nanowire during processing. The resulting silicon-nanowire array devices exhibit Schottky-type behaviour and a clear field-effect. The measured values for resistivity and specific contact resistance were ((2.6 ± 1.2) × 10(5)Ωcm) and ((240 ± 80) Ωcm(2)) respectively. These values are typical for intrinsic (un-doped) silicon when contacted by high work function metal albeit counterintuitive as the resistivity of the starting wafer (∼10 Ωcm) is 4 orders of magnitude lower. In essence, the nanowires are so small and consist of so few atoms, that statistically, at the original doping level each nanowire contains less than a single dopant atom and consequently exhibits the electrical behaviour of the un-doped host material. Moreover this indicates that the processing successfully avoided unintentional doping. Therefore our approach permits tuning of the device steps to contact the nanowires functionality through careful selection of the initial bulk starting material and/or by means of post processing steps e.g. thermal annealing of metal contacts to produce high performance devices. We envision that such a controllable process, combined with the precision patterning of the aligned block copolymer nanopatterns, could prolong the scaling of nanoelectronics and potentially enable the fabrication of dense, parallel arrays of multi-gate field effect transistors.

Beyond Orientation: The Impact of Electric Fields on Block Copolymers

AbstractSince the first report on electric field‐induced alignment of block copolymers (BCPs) in 1991, electric fields have been shown not only to direct the orientation of BCP nanostructures in bulk, solution, and thin films, but also to reversibly induce order–order transitions, affect the order–disorder transition temperature, and control morphologies' dimensions with nanometer precision. Theoretical and experimental results of the past years in this very interesting field of research are summarized and future perspectives are outlined.

Anisotropic Proton Conduction in Aligned Block Copolymer Electrolyte Membranes at Equilibrium with Humid Air

The effect of alignment of proton-conducting domains in hydrated poly(styrenesulfonate-b-methylbutylene) copolymer films on conductivity was studied by impedance spectroscopy. Pressing isotropic sa...

Mesoscopic crystallography of shear-aligned soft materials

The development of a novel device for examining the three-dimensional structure of shear-aligned soft materials using small-angle X-ray scattering is reported. Following alignmentviaoscillatory shear, the shear plates are fixed together and the whole assembly mounted on a miniature goniometer placed within the oven of the rheometer. It is then possible to perform two-axis crystallography, rotating around the shear direction, and tilting with respect to this axis. Preliminary results are presented that highlight the information on aligned block copolymer nanostructures that can be accessed using the goniometer. Diffraction patterns obtained from a shear-aligned hexagonal close-packed micellar structure formed in an aqueous solution of an amphiphilic diblock are discussed, as well as those of the metastable hexagonal perforated lamellar structure formed in a diblock copolymer melt.

Viscoelastic Properties of Aligned Block Copolymer Lamellae

Structure−property relationships of aligned block copolymer lamellae in the vicinity of the order−disorder transition are studied by a combination of optical birefringence and rheology. Minimizing ...