Self-assembled Block Copolymer Thin Films Research Articles

Controlled Orientation of Silicon-Containing Diblock Copolymer Thin Films by Substrate Functionalization Under Vacuum

This work demonstrates a simple approach to control the orientation of the self-assembled nanostructured block copolymer thin films of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) by functionalization of the oxide layer (SiO2) on the Si substrate followed by thermal annealing under low-pressure environmental conditions. The substrate can be functionalized through two-step grafting of hydroxy-terminated polystyrene brush (PS–OH brush) followed by hydroxy-terminated polydimethylsiloxane brush (PDMS–OH brush) onto the wafer substrate. By controlling the grafting ratio of PS–OH and PDMS–OH brushes, the affinities of the PS and PDMS blocks with the substrates can be fine-tuned to provide a neutral substrate in order to form perpendicular cylinders from the bottom after thermal annealing. Owing to the vacuum-driven orientation [i.e., thermal annealing under low-pressure environment conditions (∼10–4 Pa)], the orientation of the cylinders can be controlled at the air/polymer interface. Interestingly, by combining the vacuum-driven approach with substrate functionalization, perpendicular cylinders from the air/polymer interface and substrate/polymer interface can be generated, respectively. Consequently, well-aligned perpendicular cylinders with long-range ordering can be fabricated by the self-alignment process.

(Invited) Vapor-Phase Infiltration for Microelectronics Applications

Vapor-phase infiltration (VPI) is an emerging organic-inorganic materials hybridization technique derived from atomic layer deposition (ALD) wherein gaseous organometallic precursors and co-reactants diffuse into starting organic templates in a sequential and cyclic manner and become hybridized with the organic matrix. The resulting hybrids feature uniquely enhanced materials properties, including but not limited to mechanical, chemical, dielectric, optical, and electrical properties, which are controllable by the type and amount of infiltrated inorganic species. When combined with polymer templates with either dimensional confinement (e.g., lithographically defined polymer patterns) or intrinsic spatial chemical contrast (e.g., self-assembled block copolymer (BCP) thin films), the technique also yields a site-specific infiltration, analogous to area-selective ALD, creating a spatially localized hybridization. Furthermore, the organic matrix of such infiltrated hybrids can be selectively removed by plasma ashing and thermal annealing to generate inorganic nanostructures that inherit the morphology of starting polymer templates, providing an alternative inorganic nanopatterning methodology. In this talk, I will showcase how these features of VPI can be utilized for the applications in microelectronics, including: (a) arbitrary patterning ultrahigh aspect-ratio metal oxide nanostructures; (b) generation of metal-oxide-based nanowire and nanomesh structures for conductometric sensing; and (c) VPI-derived metal-oxide-infiltrated hybrid photoresists for extreme ultraviolet (EUV) lithography.

Microdomain homogeneity evaluation of perpendicular lamellar structures in block copolymer films: X-ray scattering and IR nanospectroscopy analyses

Self-assembled block copolymer (BCP) thin films necessarily require a distinct chemical contrast between the blocks (or microdomain homogeneity) for the purpose of high-resolution pattern transfer applications. Despite its importance, however, there has been difficulty in measuring the chemical distribution associated with the self-assembling processes, i.e. solvent vapor annealing (SVA) or thermal annealing (TA). Herein, we present a simple yet effective method to probe the chemical distribution over polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films that underwent the SVA or SVA + TA processes. An IR nanospectroscopy using scattering-type scanning near-field optical microscopy (s-SNOM), in concert with X-ray scattering analyses, provides the relative chemical concentration of the perpendicular lamellar patterns. The chemical mapping for the components based on the s-SNOM reveals that the SVA process leaves a considerable portion of the intermixed zones of PS and PMMA blocks, whereas the subsequent TA process facilitates the chain segregation towards high-contrast chemical distribution. Interestingly, such a microdomain homogeneity critically affects the quality of the unidirectional Al2O3 lines that were substituted from topographically guided lamellae. Our results substantiate an important insight into understanding the chain redistribution behavior in the self-assembled BCP films using non-destructive s-SNOM technique.

Block Copolymer Templated Platinum Electrocatalysts from Various Fabrication Pathways: A Comparative Study

PGMs (Platinum Group Metals) are state-of-the-art electrocatalysts for electrochemical energy conversion technologies such as fuel cells and electrolyzers. Fabrication of PGM electrocatalysts templated from self-assembled block copolymer thin films is interesting because they allow precise control over nanoscale feature size and morphology. In our previous work, the mass activity of such electrocatalysts, developed via incipient wetness impregnation, were probed as a function of their feature sizes (~ 11 – 35 nm). This talk presents a comparative electrochemical study of block copolymer templated platinum electrocatalysts prepared from three different pathways (i) incipient wetness impregnation (ii) physical vapor deposition (iii) atomic layer deposition. A combination of electron microscopy, atomic force microscopy, grazing incidence x-ray scattering, x-ray photoelectron spectroscopy, and inductively coupled plasma – optical emission spectrometry was used to characterize PGM geometry and chemical composition. It was found that all studied methods lead to high-fidelity pattern transfer from the block copolymer thin film to the PGM mesostructures. Electrocatalytic activity for HOR/HER and water-splitting were assessed for the various samples prepared. The versatility of fabrication of thin film meso-structured electrocatalysts from self-assembled block copolymers allows for systematic evaluation how PGM deposition method and thin film morphology influence electrochemical reactivity.

Mesoscale Control of PGM Electrocatalysts Using Self-Assembled Block Copolymer Templates

Platinum Group Metals (PGMs) are state-of-the-art electrocatalysts for electrochemical energy conversion technologies. This talk presents a templating approach for the fabrication of dense and periodic platinum and iridium oxide nanowires and standing cylinders with tuned feature sizes (e.g., 12 to 40 nm) and long-range order from self-assembled block copolymer thin films. Unlike previous studies, we show that a starting template utilizing an ion-containing block copolymer electrolyte over a non-ionic block copolymer yields thicker and denser metal/metal oxide nanostructures. PGM geometry and chemical composition were characterized using electron microscopy, atomic force microscopy, grazing incidence x-ray scattering, x-ray photoelectron spectroscopy, and inductively coupled plasma – optical emission spectrometry. The structural characterization tools show high-fidelity pattern transfer from the block copolymer template to the PGM nanostructure. Additionally, the templating process was conducive for preparing the nanostructured PGMs on glassy carbon disks and interdigitated electrodes (IDEs) for electrochemical reactivity assessments. The former substrates were used to assess HER and ORR in a conventional rotating disk electrode setup, while the latter platform was amenable for hydrogen pump and water electrolysis demonstrations with thin film low-temperature and high-temperature polymer electrolytes. Electrochemical measurements revealed a commensurate trend between PGM surface area and mass activity. In summary, the block copolymer templating approach represents a unique platform for high throughput comparative analyses of the intrinsic catalytic activity as a function of meso-scale geometry. It is also an effective platform to understand electrocatalytic reactivity differences for a given catalyst with different types of electrolytes.

Block Copolymer Derived Vertically Coupled Plasmonic Arrays for Surface-Enhanced Raman Spectroscopy.

A surface-enhanced Raman spectroscopy sensing template consisting of gold-covered nanopillars is developed. The plasmonic slab consists of a perforated gold film at the base of the nanopillars and a Babinet complementary dot array on top of the pillars. The nanopillars were fabricated by the incorporation of an iron salt precursor into a self-assembled block copolymer thin film and subsequent reactive ion etching. The preparation is easy, scalable, and cost-effective. We report on the increase in surface-enhanced Raman scattering efficiency for smaller pillar heights and stronger coupling between the dot array and perforated gold film with average enhancement factors as high as 107. In addition, the block copolymer-derived templates show an excellent relative standard deviation of 8% in the measurement of the Raman intensity. Finite difference time domain simulations were performed to investigate the nature of the electromagnetic near-field enhancement and to identify plasmonic hot spots.

Nanoscale Mapping of Thermal and Mechanical Properties of Bare and Metal-Covered Self-Assembled Block Copolymer Thin Films

We report on the structural, mechanical, and thermal analysis of 40 nm thick polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) films coated with evaporated chromium layers of ...

Enhanced Hybridization and Nanopatterning via Heated Liquid-Phase Infiltration into Self-Assembled Block Copolymer Thin Films.

Organic-inorganic hybrids featuring tunable material properties can be readily generated by applying vapor- or liquid-phase infiltration (VPI or LPI) of inorganic materials into organic templates, with resulting properties controlled by type and quantity of infiltrated inorganics. While LPI offers more diverse choices of infiltratable elements, it tends to yield smaller infiltration amount than VPI, but the attempt to address the issue has been rarely reported. Here, we demonstrate a facile temperature-enhanced LPI method to control and drastically increase the quantity and kinetics of Pt infiltration into self-assembled polystyrene-block-poly(2-vinylpyridine) block copolymer (BCP) thin films. By applying LPI at mildly elevated temperatures (40-80 °C), we showcase controllable optical functionality of hybrid BCP films along with conductive three-dimensional (3D) inorganic nanostructures. Structural analysis reveals enhanced metal loading into the BCP matrix at higher LPI temperatures, suggesting multiple metal ion infiltration per monomer of P2VP. Combining temperature-enhanced LPI with hierarchical multilayer BCP self-assembly, we generate BCP-metal hybrid optical coatings featuring tunable antireflective properties as well as scalable conductive 3D Pt nanomesh structures. Enhanced material infiltration and control by temperature-enhanced LPI not only enables tunability of organic-inorganic hybrid nanostructures and properties but also expands the application of BCPs for generating uniquely functional inorganic nanostructures.

High refractive index in low metal content nanoplasmonic surfaces from self-assembled block copolymer thin films.

Materials with a high and tunable refractive index are attractive for nanophotonic applications. In this contribution, we propose a straightforward fabrication technique of high-refractive index surfaces based on self-assembled nanostructured block copolymer thin films. The selective and customizable metal incorporation within out-of-plane polymer lamellae produces azimuthally isotropic metallic nanostructures of defined geometries, which were analysed using microscopy and small-angle X-ray scattering techniques. Variable-angle spectroscopic ellipsometry was used to relate the geometrical parameters of the metallic features and the resulting refractive index of the patterned surfaces. In particular, nanostructured gold patterns with a high degree of homogeneity and a gold content as low as 16 vol% reach a refractive index value of more than 3 in the visible domain. Our study thus demonstrates a new route for the preparation of high refractive index surfaces with a low metal content for optical applications.

Three-dimensional electroactive ZnO nanomesh directly derived from hierarchically self-assembled block copolymer thin films.

Three-dimensional (3D) nanoarchitectures can offer enhanced material properties, such as large surface areas that amplify the structures' interaction with environments making them useful for various sensing applications. Self-assembled block copolymers (BCPs) can readily generate various 3D nanomorphologies, but their conversion to useful inorganic materials remains one of the critical challenges against the practical application of self-assembled BCPs. This work reports the vapor-phase infiltration synthesis of optoelectrically active, 3D ZnO nanomesh architectures by combining hierarchical successive stacking of self-assembled, lamellar-phase polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) BCP thin films and a modified block-selective vapor-phase material infiltration protocol. The 3D ZnO nanomesh exhibits optoelectrical functionality, featuring stack-layer-number-dependent electrical conductance resembling the percolative transport originating from the intrinsic morphological network connectivity of the lamellar BCP pattern with symmetric block ratio. The results not only illustrate the first demonstration of electrical functionality based on the ZnO nanoarchitecture directly generated by the infiltration synthesis in self-assembled BCP thin films but also present a new, large-area scalable, metal oxide thin film nanoarchitecture fabrication method utilizing industry-compatible polymer solution coating and atomic layer deposition. Given the large surface area, three-dimensional porosity, and readily scalable fabrication procedures, the generated ZnO nanomesh promises potential applications as an efficient active medium in chemical and optical sensors.

Highly Ordered Titanium Dioxide Nanostructures via a Simple One-Step Vapor-Inclusion Method in Block Copolymer Films

Nanostructured crystalline titanium dioxide (TiO2) finds applications in numerous fields such as photocatalysis or photovoltaics where its physical and chemical properties depend on its shape and crystallinity. We report a simple method of fabricating TiO2 nanowires by selective area deposition of titanium tetraisopropoxide (TTIP) and water in a CVD-based approach at low temperature by utilizing PS-b-PEO self-assembled block copolymer thin film as a template. Parameters such as exposure time to TTIP (minutes to hours), working temperature (~18 to 40 °C) and relative humidity (20 to 70 RH%) were systemically investigated through GISAXS, SEM and XPS and optimized for fabrication of TiO2 nanostructures. The resulting processing conditions yielded titanium dioxide nanowires with a diameter of 24 nm. An extra calcination step (400 – 700 °C) was introduced to burn off the remaining organic matrix and introduce phase change from amorphous to anatase in TiO2 nanowires without any loss in order.

Simultaneous In-Film Polymer Synthesis and Self-Assembly for Hierarchical Nanopatterns.

A key requirement for practical applications of nanostructured block copolymer (BCP) self-assembly is the ability to generate complex geometries including different shapes and diverse sizes across one substrate surface. This has been difficult because spatial control over the underlying chemistry of the BCP has been limited. Here, we demonstrate a photocontrolled in-film polymerization process in the presence of monomer vapor for synthesizing homopolymers in self-assembled BCP films. The homopolymers blend with BCPs and alter the nanopatterns by changing the underlying polymer chemistry and composition. We apply this technique to a variety of BCPs including polystyrene-b-polyisoprene-b-polystyrene, polystyrene-b-poly(methyl methacrylate), and polystyrene-b-poly(4-vinylpyridine). The region of in-film polymerization can be modulated by the location of irradiation using photomasks for obtaining distinct morphologies on one substrate, providing a new platform for hierarchically manipulating nanopatterns within the self-assembled BCP thin film as well as opening up a new area for radical polymerizations of monomers within such geometrically confined, swollen films.

Directed Self-Assembly of Star-Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism.

Exploring the ordering mechanism and dynamics of self-assembled block copolymer (BCP) thin films under confined conditions are highly essential in the application of BCP lithography. In this study, it is aimed to examine the self-assembling mechanism and kinetics of silicon-containing 3-arm star-block copolymer composed of polystyrene (PS) and poly(dimethylsiloxane) blocks as nanostructured thin films with perpendicular cylinders and controlled lateral ordering by directed self-assembly using topographically patterned substrates. The ordering process of the star-block copolymer within fabricated topographic patterns with PS-functionalized sidewall can be carried out through the type of secondary (i.e., heterogeneous) nucleation for microphase separation initiated from the edge and/or corner of the topographic patterns, and directed to grow as well-ordered hexagonally packed perpendicular cylinders. The growth rate for the confined microphase separation is highly dependent upon the dimension and also the geometric texture of the preformed pattern. Fast self-assembly for ordering of BCP thin film can be achieved by lowering the confinement dimension and also increasing the concern number of the preformed pattern, providing a new strategy for the design of BCP lithography from the integration of top-down and bottom-up approaches.

Exploiting Molecular Weight Distribution Shape to Tune Domain Spacing in Block Copolymer Thin Films.

We report a method for tuning the domain spacing ( Dsp) of self-assembled block copolymer thin films of poly(styrene- block-methyl methacrylate) (PS- b-PMMA) over a large range of lamellar periods. By modifying the molecular weight distribution (MWD) shape (including both the breadth and skew) of the PS block via temporal control of polymer chain initiation in anionic polymerization, we observe increases of up to 41% in Dsp for polymers with the same overall molecular weight ( Mn ≈ 125 kg mol-1) without significantly changing the overall morphology or chemical composition of the final material. In conjunction with our experimental efforts, we have utilized concepts from population statistics and least-squares analysis to develop a model for predicting Dsp based on the first three moments of the MWDs. This statistical model reproduces experimental Dsp values with high fidelity (with mean absolute errors of 1.2 nm or 1.8%) and provides novel physical insight into the individual and collective roles played by the MWD moments in determining this property of interest. This work demonstrates that both MWD breadth and skew have a profound influence over Dsp, thereby providing an experimental and conceptual platform for exploiting MWD shape as a simple and modular handle for fine-tuning Dsp in block copolymer thin films.

Gas Transport Selectivity of Ultrathin, Nanoporous, Inorganic Membranes Made from Block Copolymer Templates

We report the fabrication of ultrathin, nanoporous silicon nitride membranes made from templates of regular, nanoscale features in self-assembled block copolymer thin films. The inorganic membranes feature thicknesses less than 50 nm and volume porosities over 30%, with straight-through pores that offer high throughout for gas transport and separation applications. As fabricated, the pores are uniformly around 20 nm in diameter, but they can be controllably and continuously tuned to single-digit nanometer dimensions by atomic layer deposition of conformal coatings. A deviation from expected Knudsen diffusion is revealed for transport characteristics of saturated vapors of organic solvents across the membrane, which becomes more significant for membranes of smaller pores. We attribute this to capillary condensation of saturated vapors within membrane pores, which reduces membrane throughput by over 1 order of magnitude but significantly improves the membrane’s selectivity. Between vapors of acetone and eth...

Ozone-Based Sequential Infiltration Synthesis of Al2O3 Nanostructures in Symmetric Block Copolymer.

Sequential infiltration synthesis (SIS) provides an original strategy to grow inorganic materials by infiltrating gaseous precursors in polymeric films. Combined with microphase-separated nanostructures resulting from block copolymer (BCP) self-assembly, SIS selectively binds the precursors to only one domain, mimicking the morphology of the original BCP template. This methodology represents a smart solution for the fabrication of inorganic nanostructures starting from self-assembled BCP thin films, in view of advanced lithographic application and of functional nanostructure synthesis. The SIS process using trimethylaluminum (TMA) and H2O precursors in self-assembled PS-b-PMMA BCP thin films was established as a model system, where the PMMA phase is selectively infiltrated. However, the temperature range allowed by polymeric material restricts the available precursors to highly reactive reagents, such as TMA. In order to extend the SIS methodology and access a wide library of materials, a crucial step is the implementation of processes using reactive reagents that are fully compatible with the initial polymeric template. This work reports a comprehensive morphological (SEM, SE, AFM) and physicochemical (XPS) investigation of alumina nanostructures synthesized by means of a SIS process using O3 as oxygen precursor in self-assembled PS-b-PMMA thin films with lamellar morphology. The comparison with the H2O-based SIS process validates the possibility to use O3 as oxygen precursor, expanding the possible range of precursors for the fabrication of inorganic nanostructures.

Au nanocluster arrays on self-assembled block copolymer thin films as highly active SERS substrates with excellent reproducibility

We demonstrate the fabrication of uniform Au nanocluster arrays utilizing a self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin film as the template and their application as a surface-enhanced Raman scattering (SERS) substrate.

Assessing the Local Nanomechanical Properties of Self-Assembled Block Copolymer Thin Films by Peak Force Tapping.

The mechanical properties of several types of block copolymer (BCP) thin films have been investigated using PeakForce quantitative nanomechanical mapping. The samples consisted of polystyrene/poly(methylmethacrylate) (PS/PMMA)-based BCP thin films with different pitches both randomly oriented and self-assembled. The measured films have a critical thickness below 50 nm and present features to be resolved of less than 22 nm. Beyond measuring and discriminate surface elastic modulus and adhesion forces of the different phases, we tuned the peak force parameters in order to reliably image those samples, avoiding plastic deformation. The method is able to detect the changes in mechanical response associated with the orientation of the PMMA cylinders with respect to the substrate (parallel versus vertical). The nanomechanical investigation is also capable of recognizing local stiffening due to the preferential growth of alumina deposited by atomic layer deposition on BCP samples, opening up new possibilities in the field of hard mask materials characterization.

Effect of annealing and UV-radiation time over micropore architecture of self-assembled block copolymer thin film

Block copolymers have been recognized as versatile materials to prepare nanoporous polymer films or mem- branes, but their potential has not been completely explored. This study focuses on the formation and characterization of nanoporous polymer films based on poly(styrene)-block-(methylmethacrylate/methacrylic acid); (PS-b-MMA/MAA) were obtained through atom transfer radical polymerization (ATRP), by using two different protocols: annealing and annealing- irradiation; for improving the formation of microporous surface. The composition, crystallinity and structural order of the films were studied by Raman spectroscopy. The film polymer thickness was obtained through very high resolution ellip- sometry (VHRE). Finally, atomic force microcopy (AFM) and scanning electron microscopy (SEM) techniques were used to detect changes in the porous-structure. These results show that the morphological properties of the block copolymer were affected via the modification of two variables, UV-radiation time and annealing. SEM and AFM micrographs showed that the morphology exhibit a porous ordered structure. Contact angle measurement suggests additional interactions between hydrophilic functional groups that influence the film wettability.

Nanostructured organic radical cathodes from self-assembled nitroxide-containing block copolymer thin films

Nanostructured thin film organic radical cathodes have been prepared from poly(styrene)-block-poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) diblock copolymers.