Session A45: Focus Session: Thin Film Block Copolymers - Directed Assembly

Directed Ordering of Block Copolymer Thin Films with Flexible Interfaces for Functional Materials

Orientation control of block copolymer (BCP) films is important for advanced technological applications. We present studies on directed ordering of block copolymer thin films on rigid substrates such as quartz to elastomeric PDMS and flexible Kapton substrates for tunable orientation of microphase separated poly (styrene) -- block -poly (methylmethacrylate) (PS-PMMA) cylinder and lamellae forming BCP films. Although the crosslinked PDMS has low surface energy, its surface energy can be tuned by exposing to UV-Ozone (UVO) that presents an opportunity to change BCP-PDMS interfacial energy to control BCP orientation across full range of orientation and film wettability. On the other hand, Kapton offers a near neutral surface for PS-PMMA without surface modification. Via a modified version of a dynamic thermal processing termed cold zone annealing-sharp (CZA-S), we obtain a wide range of orientations of the block copolymer films in unfilled and nanoparticle filled systems with an interest in photovoltaic systems. With CZA-S, vertical orientation of PS-PMMA can be obtained in films as thick as 1 micron with etchable PMMA domains for membrane applications. GISAXS characterization of these etched BCP membranes reveals up to 5 orders of diffraction indicating hexagonally packed vertical nanopores that extend throughout the film. Under similar thermal gradient, but static conditions, temporally stable vertical cylinders form only within a narrow zone of maximum temperature gradient. Primary CZA-S ordering mechanism thus involves propagating this narrow vertically oriented zone of BCP cylinders created at the maximum thermal gradient section, across the film. An optimal speed is needed since the process competes with preferential surface wetting dynamics that favors parallel orientation. These results are reproduced on large area flexible films on a prototype dynamic R2R assembly platform with incorporated multi-CZA gradient for thin (100 nm) BCP films currently.   

Facile Route to Vertically Aligned High-Aspect Ratio Block Copolymer Films via Dynamic Zone Annealing

Directed assembly of block copolymers (BCP) can be used to fabricate a diversity of nanostructures useful for nanotech applications. The ability to vertically orient etchable high aspect ratio ($\sim $30) ordered BCP domains on flexible substrates via continuous processing methods are particularly attractive for nanomanufacturing. We apply sharp dynamic cold zone annealing (CZA-S) to create etchable, and predominantly vertically oriented 30nm cylindrical domains in 1 $\mu $m thick poly(styrene-b-methylmethacrylate) films on low thermal conductivity rigid (quartz) and flexible (PDMS {\&} Kapton) substrates. Under similar static conditions, temporally stable vertical cylinders form within a narrow zone above a critical temperature gradient. Primary ordering mechanism of CZA-S involves sweeping this vertically orienting zone created at maximum thermal gradient. An optimal speed is needed since the process competes with preferential surface wetting dynamics that favors parallel orientation. GISAXS of etched BCP films confirms internal morphology.   

Controlling Domain Orientation in Thin Films of Lamellar AB and ABA Block Copolymers

Thin films of block copolymers are very popular for low cost, large area nanopatterning. To generate a nanoporous template, the domains must be oriented perpendicular to the substrate. This talk will discuss the effects of copolymer architecture on surface energetics and domain orientations. We consider lamellar copolymers based on polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks. All films are cast on ``neutral'' substrates, and the resulting structures are evaluated with microscopy and grazing-incidence small-angle X-ray scattering. For PS-PMMA diblock copolymers, domain orientations are very sensitive to process conditions. The desired perpendicular orientation is most reliably obtained at high annealing temperatures where PS and PMMA have similar melt surface tensions. For PMMA-PS-PMMA triblock copolymers, the perpendicular domain orientation is stable for all film thicknesses and annealing temperatures that were studied, consistent with recent works that consider architectural effects when calculating the copolymer surface tension. These data suggest that triblocks are easier to use for nanopatterning. However, we also find that diblock and triblock films contain a high density of tilted domains, and such defects should be minimized for most applications.   

Diblock and Triblock Copolymer Thin Films on a Substrate with Controlled Selectivity

We study the morphology developments in linear ABC triblock and also AB diblock copolymer films on neutral and selective substrates, using a self-consistent field theory (SCFT). For the ABC copolymer films, various nanopatterns with tunable square morphologies evolved due to the effects of the substrate preferable to interior (B) block. The domain patterns became diversified from those parallel to the substrate with substrate selectivity for end-block or those vertical to the substrate without substrate selectivity. Furthermore, in order to figure out an economical and efficient way to fabricate useful passive pattern transfer layers potentially applicable to microelectronic processes and ultrahigh density storage media, we scrutinized conditions for generating square symmetries using symmetric AB diblock copolymers deposited on substrates created from ABC triblock copolymer films. It was found that a thinner film with relatively weak incompatibility can produce square patterns.   

Microdomain Orientation of PS-b-PMMA Thin Films on the PS grafted Substrates

The phase transitions and morphologies, specifically the orientation of lamellar microdomains of symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) were investigated with grazing incidence small-angle x-ray scattering (GISAXS) and transmission electron microscopy (TEM). The microdomain orientation in thin films of PS-b-PMMAs on the substrates with brushed (or grafted) polymers was influenced by the interfacial interactions, where grafting density of underlying PS layers are controlled by the reaction parameters such as time and temperature during grafting to the substrates. When grafting density range of PS brush is adjusted, such a simple and facile route provides the perpendicular orientation of cylindrical and lamellar microdomains in PS-b-PMMA films.   

Orientation control of cold zone annealed Block copolymer films on tunable gradient surface energy substrates using combinatorial methods

Microphase morphologies of poly(styrene)-block-poly(methylmethacrylate) (PS-PMMA) block co-polymer (BCP) films coated on various tunable surface energy gradient (SEG) substrates were compared. Substrates were prepared by coating silane self assembled monolayer (SAM) and hydrophobic sol-gel based layer of silica (xerogel) on quartz and exposed to UV-ozone radiation by placing them on an accelerating stage that oxidizes the surface to generate SEG. The combinatorial thickness gradient samples of BCP film were prepared by flow coating the BCP solution orthogonal to the SEG. Samples were annealed using novel cold zone annealing (CZA) method with a sharp thermal gradient (50 $^{o}$C/mm) to obtain highly ordered BCP morphologies. Effect of CZA annealing rate and film thickness on BCP morphologies of the SAM treated and untreated quartz as well as xerogel substrates were compared. It was observed that BCP films coated on the untreated quartz substrates exhibited hexagonally packed perpendicular cylindrical morphologies whereas higher area fraction of parallel cylinders was observed for SEG xerogel substrates for higher surface energies ($>$40 mJ/m$^{2})$. BCP 2D surface morphologies studied using AFM, were confirmed to extend to the interior of the film (3D) by GISAXS.   

Block Copolymer Ordering on Soft, Patternable and Flexible Substrates

Directed assembly of cylinder and lamellae forming block copolymer films via flexible PDMS substrate is examined to investigate the ordering properties of polystyrene-$b$-poly(methyl methacrylate) (PS-$b$-PMMA) films. We study the cases where the PS-$b$-PMMA films are either directly coated on patterned PDMS flexible substrates, or coated on a flat PDMS substrate with a top patterned and flexible PDMS confinement. The surface energy of the PDMS substrates was modified to vary from 20 to 68 mJ/m$^{2}$ by exposing them to UV-ozone (UVO) for controlled wettability and orientation control. We replicated different patterned media and observed perpendicular lamellar orientation and parallel cylindrical orientation on patterned flexible substrate at higher surface energies in preliminary measurements. Characterization of orientation was investigated with Grazing-Incidence Small Angle X-ray Scattering (GISAXS) measurement as well as with Atomic Force Microscope (AFM) results. Optical Microscope (OM) was also used to study of the surface morphology of the BCP films.   

Controlling the alignment and the morphology of bilayer BCP films using templated self-assembly

Templated self-assembly of block copolymer (BCP) thin films can control the alignment of BCP microdomains in a single layer using chemical and topographical methods. However, controlling the alignment and the morphology of BCP microdomains in two different layers simultaneously and fabricating complicated three-dimensional (3D) structures is relatively unexplored. This control is useful for the fabrication of multilevel thin film devices. Also, the forces and energetics governing BCP self-assembly are better understood at the bulk scale. This paper discusses how to control the BCP in two different layers by using a majority-block-functionalized post template. We showed by using an array of majority block functionalized posts, we could fabricate very complicated three-dimensional structures and we were able to control the BCP in two different layers. We fabricated three dimensional junctions and bends in two different levels of the BCP, bottom and top. Moreover, we showed we could fabricate periodic superstructures as well as changing the morphology of the BCP in one of the layers from the original cylinders to ellipsoids, spheres, and bicontinuous cylinders and having two different morphologies on top of each other.   

Controlling the Block Copolymer Patterns via Combining Graphoepitaxy and Surface Guidance

One of the key issues for the block copolymer patterning is controlling the pattern array. Using directed assembly of block copolymer on the chemically patterned surfaces guarantee highly uniform array or controlled non-regular array. In this system, the chemically patterned surface can induce well resisted array over arbitrary large areas. However, this method requires expensive and complicated e-beam lithography and thus is not readily applicable to mass production process. Alternatively, another method is the graphoepitaxy method. In this case, the self-assembly of block copolymer is guided by the topographical wall of lithographically pre-patterned substrates. But, it has limitation on the pitch size of wall to obtain the highly ordered patterns. In this work, we demonstrate a new type of fabrication method to achieve highly controlled and uniform block copolymer patterns. Our approach is to combine the graphoepitaxy method and hexagonally surface guiding patterns from crosslinkable block copolymers. When the lamellar forming block copolymers were prepared on hexagonal patterns, a highly aligned stripe was obtained and the alignment was significantly improved comparing to the case when no hexagonal surface patterns were used.   

Macroscopic Ordering of Block Copolymers into Sequenced Patterns on Topographically Corrugated Surface

For the guided block copolymer assembly we used corrugated SiCN ceramic substrates which were fabricated by a facile replication process using non-lithographic PDMS masters. During thermal annealing of polystyrene-b-polybutadiene diblock copolymer, the material transport was guided by the wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphological behavior of block copolymers, the film surface appears as sequenced patterns of alternative microphase separated structures. The ordering process is attributed to the formation of reverse terraces which match the substrate topography, so that the resulting surface patterns are free from the surface relief structures within macroscopically-large areas. The issues of the film thickness, the substrate surface energy and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers towards complex hierarchically-structured patterned surfaces.   

Block copolymer thin films with long-range lateral order

We investigated thin block copolymer films on silicon wafers as well as faceted surfaces of sapphire. Thin films were prepared by spin-coating block copolymer solutions on the corresponding substrates. Subsequent annealing in organic solvent vapors served as a means to induce lateral long-range order in the thin films. The resulting block copolymer structures were analyzed by AFM and GISAXS.   

Combining block copolymer lithography with nano-imprinting lithography: Mass-producing sub-20nm unidirectional line patterns

A facile, simple pathway for mass production of highly aligned sub 20 nm unidirectional line patterns over arbitrarily large areas is reported. The directed self-assembly of block copolymers and nano-imprint lithography technique are combined together for this purpose. Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was self-assembled on the faceted sapphire substrates to generate unidirectionally aligned cylindrical microdomains oriented parallel to the substrate. The nanoporous trench patterns were achieved by selective reconstruction of the cylindrical P4VP microdomains followed by oxygen plasma etching. The resulting 1D trench nanopatterns were characterized by scanning force microscopy, scanning electron microscopy, and grazing incident small angle x-ray scattering (GISAXS), yielding an orientation parameter of 0.974. A cyclic-siloxane was cast onto the polymeric trench nanopatterns and cured to make a negative replica. After demolding, the cross-linked siloxane replica was used as a second master to mass-produce unidirectionally aligned nanolines on various substrates with high fidelity, quality, and yield by nano-imprinting lithography technique. Importantly, this strategy works for both thermal and UV-assisted imprinting. The methodology may afford an easy approach for mass production of ideal templates for the fabrication of magnetic storage media, optical devices, and arrays of conducting wires.   

Offset block copolymer printing using brushless substrates and elastomeric stamps

The block copolymer (BCP) films have been utilized as the useful templates for the nanopatterning because of their ability to form quite small feature size with a relatively easy and cheap method. Recently, the control of orientation as well as the perfect registration with long range order of nanosturctures in BCP films has been achieved by directing the assembly of BCP films with the chemical or topographical templates. However, such control of nanostructures and utilization of BCP films are limited on Si wafers since the templates are usually prepared by the semiconductor fabrication process. In this study, we will introduce a technique that overcomes such limitation on the application of BCP films. The chemical pattern prepared by e-beam lithography on the substrate without polymer brushes or mats was used as a template for the directed assembly of BCP films. Because the adhesion between this template and a BCP film is fairly weak, the directed assembled BCP films on this template could be easily transferred to the other substrates with adhesion-controlled elastomeric stamps. We believe that this transfer method, or printing method, would expand the applications of BCP films to areas where the nanotemplates are required.