Direct Nanoimprint Lithography Research Articles

Chemically amplified molecular resins for shrinkage-controlled direct nanoimprint lithography of functional oxides: an application towards dark-light dual-mode antibacterial surfaces

The imprinting studies using epoxy/oxetane-based bifunctional monomers of Ti, Zr and Nb showed a reduced feature size shrinkage as low as ∼50%. The TiO2/AgBr nanocomposites displayed high antibacterial efficacy under dark-light dual-mode conditions.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here,wedemonstrate a nanoimprinted nonlinear barium titanate2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonicdevices.

Refractive Index Tuning of All-Inorganic TiO2 Nanocrystal-Based Films and High Aspect Ratio Nanostructures Using Atomic Layer Deposition: Implications for High-Throughput Fabrication of Metalenses

High refractive index (RI) components and nanostructures are of great interest for compact optics, waveguides, photonics, and metamaterials. Herein, we demonstrate that atomic layer deposition (ALD) can be used as a short postfabrication step to dramatically increase the RI of nanocrystal (NC)-based films and patterned nanostructures in conjunction with a single-step, direct nanoimprint lithography (NIL). The initial RI of TiO2 NC-based films was n = 1.95 at 543 nm which can then be increased to 2.00 by post-calcination and further up to 2.15 by TiO2 ALD. Fifteen cycles of ALD were sufficient to achieve this rapid increase of RI, and any intermediate RI value can be tuned by adjusting the number of ALD cycles accordingly. Nanoscale interstitial gaps between TiO2 NCs allowed a uniform diffusion of ALD precursors, resulting in a much denser structure and the RI increase. An array of 4 mm-sized metalenses were fabricated to demonstrate the effects of the RI tuning on the performance of optical devices. The focusing efficiencies of the as-imprinted metalenses (smallest dimension ∼ 80 nm, highest aspect ratio ∼ 8) were 61% on average, but the post-treatments including calcination and ALD significantly increased the average efficiency to 67% and up to 75% for the best-performing lens. This approach combines the fast, scalable, and versatile solvent-assisted NIL method to pattern optical nanostructures with a short post-patterning deposition and densification step that significantly enhances optical performance.

Smart Systems for Material and Process Designing in Direct Nanoimprint Lithography Using Hybrid Deep Learning.

A hybrid smart process and material design system for nanoimprinting is proposed, which is combined with a learning system based on experimental and numerical simulation results. Instead of carrying out extensive learning experiments for various conditions, the simulation learning results are partially complimented when the results can theoretically be predicted by numerical simulation. In other words, the data that are lacking in experimental learning are complimented by simulation-based learning results. Therefore, the prediction of nanoimprint results without experimental learning could be realized under various conditions, even for unknown materials. In this study, material and process designs are demonstrated for a low-temperature nanoimprint process using glycerol-containing polyvinyl alcohol. The experimental results under limited conditions were learned to investigate the optimum glycerol concentrations and process temperatures. Simulation-based learning was used to predict the dependence on press pressure and shape parameters. The prediction results for unknown glycerol concentrations agreed well with the follow-up experiments.

Extreme-Pressure Imprint Lithography for Heat and Ultraviolet-Free Direct Patterning of Rigid Nanoscale Features.

Nanoimprint lithography (NIL) is typically performed by filling up of molds by heated polymers or UV-curable liquid resists, inevitably requiring subsequent pattern-transfer processes. Although direct NIL techniques have been suggested alternatively, they usually require precursors or ink-type resists containing undesired organic components. Here, we demonstrate extreme-pressure imprint lithography (EPIL) that effectively produces well-defined multiscale structures with a wide range from 10 nm to 10 mm on diverse surfaces even including pure or alloy metals without using any precursors, heating, UV exposure, or pattern transfer. In particular, EPIL is accomplished through precise control of room-temperature plastic deformation in nanoscale volumes, which is elucidated by finite element analyses and molecular dynamics simulations. In addition to scalability to macroscopic areas, we confirm the outstanding versatility of EPIL via its successful applications to Ni, Cu, steel, and organics. We expect that the state-of-the-art EPIL process combined with other emerging nanopatterning technologies will be extendable to the future large-area nanofabrication of various devices.

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing.

Metal-assisted chemical etching (MACE) has emerged as an effective method to fabricate high aspect ratio nanostructures. This method requires a catalytic mask that is generally composed of a metal. Here, we challenge the general view that the catalyst needs to be a metal by introducing oxide-assisted chemical etching (OACE). We perform etching with metal oxides such as RuO2 and IrO2 by transposing materials used in electrocatalysis to nanofabrication. These oxides can be solution-processed as polymers exhibiting similar capabilities of metals for MACE. Nanopatterned oxides can be obtained by direct nanoimprint lithography or block-copolymer lithography from chemical solution on a large scale. High aspect ratio silicon nanostructures were obtained at the sub-20 nm scale exclusively by cost-effective solution processing by halving the number of fabrication steps compared to MACE. In general, OACE is expected to stimulate new fundamental research on chemical etching assisted by other materials, providing new possibilities for device fabrication.

Direct Nanoimprint Lithography of Polyethersulfone Using Cellulose‐Based Mold

AbstractPolyethersulfone (PES) with a glass transition temperature of 224 °C is a class of high‐performance super engineering plastics with excellent heat resistance, as well as mechanical, electrical, and optical properties. However, the nanopatterning of PES using thermal nanoimprint lithography at a high temperature can induce technical difficulties. High‐accuracy 900‐nm line patterning of PES is fabricated 50 times using both a solvent‐permeable cellulose‐based mold and dimethylformamide as a dilution solvent in direct thermal nanoimprint lithography. The use of dilution solvents in PES has yet been limited in the nanoimprint lithography, and the developed processes expand the surface patterning of super engineering plastic.

Study on induced strain in direct nanoimprint lithography

The induced shear strain distribution in a polymer film is investigated by computational study in a direct nanoimprint process. The effects of the polymer thickness, mold pattern shape such as rectangular, triangular or overcut pattern shape, and the coefficient of friction between the mold and the polymer are studied by computational work. As the coefficient of friction increases, the induced shear strain increases along the mold surface. Depending on the polymer thickness, the shear strain is induced in the residual and/or pattern area. In the triangular pattern, the strain is induced in the pattern central area. The results suggest that shear stress remains in the triangular pattern area in the direct nanoimprint process. On the other hand, the rectangular pattern is suitable for suppressing the induced strain inside the pattern.

Direct soft imprint of chalcogenide glasses

The authors explored an approach for nanostructuring of surfaces of chalcogenide glasses by direct soft nanoimprint lithography. The authors produced soft nanoimprint molds with microsized and nanosized relief features using polydimethylsiloxane. To enable the direct replication of the mold pattern on the surface of bulk chalcogenide glasses, the authors engineered a thermal imprint tool that prevents the substrate deformation using a physical confinement. The authors optimized the imprint process parameters to achieve an unprecedented full transfer micropattern onto a bulk chalcogenide glass. Furthermore, the authors explored pattern replication of nanosized structures by thermal imprint. This process paves the way for a facile and cost-effective fabrication of nanostructures on chalcogenide glasses and their numerous applications in optical and photonic devices.

Contact electrification induced interfacial reactions and direct electrochemical nanoimprint lithography in n-type gallium arsenate wafer.

Although metal assisted chemical etching (MacEtch) has emerged as a versatile micro-nanofabrication method for semiconductors, the chemical mechanism remains ambiguous in terms of both thermodynamics and kinetics. Here we demonstrate an innovative phenomenon, i.e., the contact electrification between platinum (Pt) and an n-type gallium arsenide (100) wafer (n-GaAs) can induce interfacial redox reactions. Because of their different work functions, when the Pt electrode comes into contact with n-GaAs, electrons will move from n-GaAs to Pt and form a contact electric field at the Pt/n-GaAs junction until their electron Fermi levels (EF) become equal. In the presence of an electrolyte, the potential of the Pt/electrolyte interface will shift due to the contact electricity and induce the spontaneous reduction of MnO4- anions on the Pt surface. Because the equilibrium of contact electrification is disturbed, electrons will transfer from n-GaAs to Pt through the tunneling effect. Thus, the accumulated positive holes at the n-GaAs/electrolyte interface make n-GaAs dissolve anodically along the Pt/n-GaAs/electrolyte 3-phase interface. Based on this principle, we developed a direct electrochemical nanoimprint lithography method applicable to crystalline semiconductors.

Nanoimprint of a 3D structure on an optical fiber for light wavefront manipulation

Integration of complex photonic structures onto optical fiber facets enables powerful platforms with unprecedented optical functionalities. Conventional nanofabrication technologies, however, do not permit viable integration of complex photonic devices onto optical fibers owing to their low throughput and high cost. In this paper we report the fabrication of a three-dimensional structure achieved by direct nanoimprint lithography on the facet of an optical fiber. Nanoimprint processes and tools were specifically developed to enable a high lithographic accuracy and coaxial alignment of the optical device with respect to the fiber core. To demonstrate the capability of this new approach, a 3D beam splitter has been designed, imprinted and optically characterized. Scanning electron microscopy and optical measurements confirmed the good lithographic capabilities of the proposed approach as well as the desired optical performance of the imprinted structure. The inexpensive solution presented here should enable advancements in areas such as integrated optics and sensing, achieving enhanced portability and versatility of fiber optic components.

Study on Induced Stress and Strain in Direct Nanoimprint Lithography

Study on Induced Stress and Strain in Direct Nanoimprint Lithography

Negative index fishnet with nanopillars formed by direct nano-imprint lithography

In this paper we demonstrate the ability to fabricate fishnets by nanoimprinting directly into a pre-deposited three layer metal–dielectric–metal stack, enabling us to pattern large areas in two minutes. We have designed and fabricated two different fishnet structures of varying dimensions using this method and measured their resonant wavelengths in the near-infrared at 1.45 μm and 1.88 μm. An important by-product of directly imprinting into the metal–dielectric stack, without separation from the substrate, is the formation of rectangular nanopillars that sit within the rectangular apertures between the fishnet slabs. Simulations complement our measurements and suggest a negative refractive index real part with a magnitude of 1.6. Further simulations suggest that if the fishnet were to be detached from the supporting substrate a refractive index real part of 5 and FOM of 2.74 could be obtained.

Invited Presentation: Electrolyte Breakthrough to Advance Flexible Batteries

Flexible batteries have garnered a great deal of attention as a promising power source for versatile-shaped electronic devices including bendable smart phones, roll-up displays and wearable electronics, which play crucial roles in facilitating the advent of wearable electronics era. One formidable challenge for development of the flexible batteries arises from difficulties in securing solid-state electrolytes with reliable electrochemical/mechanical properties. Currently available liquid electrolytes show good electrochemical performance suitable for practical applications, however, they often suffer from safety failures due to unwanted leakage problems and flammable characteristics. Moreover, liquid electrolytes limit choices in cell design due to their fluidic attributes, which essentially require the use of separator membranes and packaging canisters with fixed dimension in cell assembly. These shortcomings of liquid electrolytes motivate us to develop advanced solid-state electrolytes that can easily conform to complex-structured substrates such as three-dimensional (3D) electrodes and also ensure sufficient mechanical deformability for reliable use.Herein, as electrolyte breakthrough to advance flexible batteries, a new class of bendable, shape-conformable and thermally-stable composite polymer electrolyte is demonstrated and also its potential application to flexible batteries featuring shape diversity is explored. More notably, the composite polymer electrolyte can be directly writable and printable onto complicated/contoured electrodes, without the aid of processing solvents. The unique structural/compositional designs and well-tuned rheological properties, in collaboration with direct UV-assisted nanoimprint lithography, allow for successful fabrication of solid-state composite polymer electrolytes in geometries that lie far beyond those accessible with conventional electrolytes. As an alternative electrolyte to exceed conventional carbonate-based liquid electrolytes, plastic crystal electrolytes (PCEs) have been investigated. The PCEs, which are composed of lithium salts and plastic crystals bearing good solvation capability, are characterized with unusual thermal stability and ionic transport behaviors. Among various plastic crystal candidates, succinonitrile (SN) is known to show excellence in thermal stability and ionic transport due to high boiling point and structural defects (i.e., trans-gauche isomerism) of plastic crystal phase. Despite of these advantageous characteristics, application of the PCEs to flexible batteries is staggered due to their poor mechanical properties. Most of PCEs are excessively plastic and even more show liquid-like behavior at room temperature. In this talk, we present a wide variety of material strategies to improve the physicochemical properties (in particular, focusing on mechanical flexibility) of SN-based PCEs, which are mainly devoted to integrating with functional polymer matrix that serves as a mechanical framework. The structural design concept demonstrated herein for the synthesis of plastic crystal polymer electrolytes can be suggested as a versatile and scalable electrolyte platform to bring unprecedented opportunities in progress of next-generation flexible energy storage/conversion devices. References Keun-Ho Choi, Sung-Ju Cho, Se-Hee Kim, Yo Han Kwon, Je Young Kim, Sang-Young Lee, "Thin, deformable, and safety-reinforced plastic crystal polymer electrolytes for high-performance flexible lithium-ion batteries", Adv. Funct. Mater. online published.Sang-Young Lee, Keun-Ho Choi, Yo Han Kwon, Hye-Ran Jung, Heon-Cheol Shin, Je Young Kim, "Progress in flexible energy storage and conversion systems, with a focus on cable-type lithium ion battery", Energy Environ. Sci. 2013, 6, 2414. (featured as a back cover image).Eun-Hye Kil, Keun-Ho Choi, Hyo-Jeong Ha, Sheng Xu, John A. Rogers, Mi Ri Kim, Young-Gi Lee, Kwang-Man Kim, Kuk Young Cho, Sang-Young Lee, "Imprintable, bendable, and shape-conformable polymer electrolytes for versatile-shaped lithium-ion batteries", Adv. Mater. 2013, 25, 1395. (featured as a back cover image).

Roll-to-roll nanoimprint lithography for patterning on a large-area substrate roll

In this paper, we present a process for direct nanoimprint lithography from one roll to another. The basic concept of roll-to-roll nanoimprint lithography (R2R-NIL) is illustrated and the possibility of pattern-transfer between two cylindrical, curved surfaces is evaluated. For the replication of nanopatterns to a large-area roll from a small-area roll, the R2R-NIL process has to be conducted in a step-and-repeat manner. We were finally able to fabricate nanopatterns on a substrate roll of 250-mm diameter and 366-mm width. There exist seams and stitches of more than 1-mm in width on the patterned area of the substrate roll. For such a large roll, pre- and post-processes, including spin-coating, are also required. Even so, R2R-NIL is expected to be an effective tool to fabricate large-area rolls that can be used for the production of nanopatterned films.

Flexible Palladium-Based H2 Sensor with Fast Response and Low Leakage Detection by Nanoimprint Lithography

Flexible palladium-based H2 sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H2 sensor that is capable of detecting H2 gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H2 sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H2 sensors with improved durability, sensitivity, and response time in comparison to palladium thin films. At ambient pressure and temperature, the device showed a fast response time of 18 s at a H2 concentration of 3500 ppm. At 50 ppm concentration, the response time was found to be 57 s. The flexibility of the sensor does not appear to compromise its performance.

Metal hierarchical patterning by direct nanoimprint lithography.

Three-dimensional hierarchical patterning of metals is of paramount importance in diverse fields involving photonics, controlling surface wettability and wearable electronics. Conventionally, this type of structuring is tedious and usually involves layer-by-layer lithographic patterning. Here, we describe a simple process of direct nanoimprint lithography using palladium benzylthiolate, a versatile metal-organic ink, which not only leads to the formation of hierarchical patterns but also is amenable to layer-by-layer stacking of the metal over large areas. The key to achieving such multi-faceted patterning is hysteretic melting of ink, enabling its shaping. It undergoes transformation to metallic palladium under gentle thermal conditions without affecting the integrity of the hierarchical patterns on micro- as well as nanoscale. A metallic rice leaf structure showing anisotropic wetting behavior and woodpile-like structures were thus fabricated. Furthermore, this method is extendable for transferring imprinted structures to a flexible substrate to make them robust enough to sustain numerous bending cycles.

Imprintable, Bendable, and Shape‐Conformable Polymer Electrolytes for Versatile‐Shaped Lithium‐Ion Batteries

A class of imprintable, bendable, and shape-conformable polymer electrolyte with excellent electrochemical performance in a lithium battery system is reported. The material consists of a UV-cured polymer matrix, high-boiling point liquid electrolyte, and Al2 O3 nanoparticles, formulated for use in lithium-ion batteries with 3D-structured electrodes or flexible characteristics. The unique structural design and well-tuned rheological characteristics of the UV-curable electrolyte mixture, in combination with direct UV-assisted nanoimprint lithography, allow the successful fabrication of polymer electrolytes in geometries not accessible with conventional materials.

Direct nanoimprint lithography of Al2O3 using a chelated monomer-based precursor

Nanostructuring of Al2O3 is predominantly achieved by the anodization of aluminum film and is limited to obtaining porous anodized aluminum oxide (AAO). One of the main restrictions in developing approaches for direct fabrication of various types of Al2O3 patterns, such as lines, pillars, holes, etc, is the lack of a processable aluminum-containing resist. In this paper, we demonstrate a stable precursor prepared by reacting aluminum tri-sec-butoxide with 2-(methacryloyloxy)ethyl acetoacetate, a chelating monomer, which can be used for large area direct nanoimprint lithography of Al2O3. Chelation in the precursor makes it stable against hydrolysis whilst the presence of a reactive methacrylate group renders it polymerizable. The precursor was mixed with a cross-linker and their in situ thermal free-radical co-polymerization during nanoimprinting rigidly shaped the patterns, trapped the metal atoms, reduced the surface energy and strengthened the structures, thereby giving a ∼100% yield after demolding. The imprinted structures were heat-treated, leading to the loss of organics and their subsequent shrinkage. Amorphous Al2O3 patterns with line-widths as small as 17 nm were obtained. Our process utilizes the advantages of sol–gel and methacrylate routes for imprinting and at the same time alleviates the disadvantages associated with both these methods. With these benefits, the chelating monomer route may be the harbinger of the universal scheme for direct nanoimprinting of metal oxides.

Recent progress in direct patterning technologies based on nano-imprint lithography

Nano-imprint lithography (NIL) is one of the most promising patterning technologies, in which nano- and micro-patterns are fabricated on various substrates. NIL provides high throughput and low cost in fabricating nano-structures due to its simple process and allows resolution below 10 nm without issues of light diffraction with conventional lithographic techniques. Its patterning mechanism is based on mechanical deformation of a polymer resist, which is simply done by pressing with a mold. This patterning mechanism also enables inorganic and organic-inorganic hybrid materials to be directly patterned by NIL. This article covers the recent progress of NIL-based direct patterning techniques and their applications to devices. Recently, functional nano- and micro-patterns have been applied to various electronic devices for the enhancement of overall performance. Fabrication methods of these devices are difficult using convention lithographic techniques due to complex processes, high cost and low throughput. Direct NIL technique using functional resist can simply fabricate functional nano- and micro-structures and thus can be usefully applied to various industries.