UV Nanoimprint Lithography Research Articles

Electroless deposition of silver grids flexible transparent electrode integrated by ultra-violet nanoimprint lithography

Abstract Among recent explorations of flexible transparent electrodes (FTEs) applied in photoelectric devices, Ag nanowires (AgNWs) are widely used due to the advantages of high conductivity, large-scale manufacture and superior flexibility. However, relatively high roughness and contact resistance at junctions of Ag nanowire electrodes still affect the optoelectronic performance. Herein, adopting the ultraviolet nanoimprint lithography technology, the flexible Ag grids-based electrode without contact junctions has been prepared by electrolessly depositing metallic Ag on the exposed polydopamine (PDA) through coordination-bond interactions, which shows better flexibility and interfacial adhesion with comparison to the commercial PET/ITO. The light-emitting diode lamp bulb connected with PET/PDA@Ag grids electrode is still brilliantly lit after 200 continuous bending cycles with radius of 3 mm, suggesting robust mechanical stability. In addition, the PET/PDA@Ag grids-based polymer solar cells yield a power conversion efficiency of 11.6%.

Approach to fabricating high-performance cooler with near-ideal emissive spectrum for above-ambient air temperature radiative cooling

Radiative cooling is a passive cooling method that can radiate heat into the universe through transparent atmospheric windows. A good radiative cooler should possess not only a suitable emissive spectrum profile but also characteristics such as flexibility, large-area, low-cost, and angle-independency. To satisfy these demands, we propose an approach to fabricate a photonic crystal-structured UV-curable adhesive (PCSUVA) on a polyethylene terephthalate (PET) substrate using UV nanoimprint lithography. The PCSUVA-based radiative cooler has a near-ideal emissive spectrum in the wavelength range of 0.4–25 μm and the cooler is successful in cooling its object, no matter the object needs sunlight or not. When the PCSUVA is coated on a transparent substrate (i.e. PET), it can reduce the temperature of the underlying silicon wafer by 1.2 ± 0.2 °C, and when the PCSUVA is coated on PET backed with an Ag layer, it can reduce the temperature of the underlying silicon wafer by 7.7 ± 0.2 °C on a summer day. The achieved cooling effect can be attributed to the reflection of sunlight by the Ag layer and the thermal emission of the PCSUVA. The approach is cost-effective, making it a suitable candidate for a flexible, large-area, low-cost, and angle-independent radiative cooler.

One-dimensional surface wrinkling for twisted nematic liquid crystal display based on ultraviolet nanoimprint lithography

Surface wrinkling method is used to fabricate a 1-dimensional nanostructure. The structure is transferred to an ultraviolet cured polymer which is used as an alignment layer. The anisotropic geometry serves as a guide for aligning liquid crystal molecules uniformly without defects. The TN-LC cell showed a successful LC switching, with a response time of 20.5 ms, and a threshold voltage of 2.00 V. It also exhibited high thermal stability above 180°C. The proposed UV-cured polymers with 1-D nano wrinkle geometry can be a candidate for alternative alignment techniques, for advanced liquid crystal devices with high thermal budgets.

Impact of Imprint Pressure on Residual Layer Thickness in UV Nanoimprint Lithography

Ultraviolet nanoimprint lithography (UV-NIL) is an important micro/nano fabrication approach. UV-NIL has the advantage of high solution, high throughput and low cost. However, some inner resist flow mechanism is not completely understood by experimental methods. This contribution used the simulation method based on contact mechanics to analyze the impact of imprint pressure on residual layer thickness (RLT). Seven different pressure, from 0.1 MPa to 0.7 MPa, was applied to imprint the same stamp containing grating patterns with various protrusion density. It was found that RLT is not uniform and the areas with high protrusion density has much higher RLT. The average RLT decreases with imprint time and tends to be stable when the imprint process is completed. Both the maximum and minimum RLT decreases with the increase of imprint pressure. These findings can help engineers to design proper stamp pattern distribution and process parameters. Introduction

Design and Fabrication of Wafer-Level Microlens Array with Moth-Eye Antireflective Nanostructures.

Wafer-level packaging (WLP) based camera module production has attracted widespread industrial interest because it offers high production efficiency and compact modules. However, suppressing the surface Fresnel reflection losses is challenging for wafer-level microlens arrays. Traditional dielectric antireflection (AR) coatings can cause wafer warpage and coating fractures during wafer lens coating and reflow. In this paper, we present the fabrication of a multiscale functional structure-based wafer-level lens array incorporating moth-eye nanostructures for AR effects, hundred-micrometer-level aspherical lenses for camera imaging, and a wafer-level substrate for wafer assembly. The proposed fabrication process includes manufacturing a wafer lens array metal mold using ultraprecise machining, chemically generating a nanopore array layer, and replicating the multiscale wafer lens array using ultraviolet nanoimprint lithography. A 50-mm-diameter wafer lens array is fabricated containing 437 accurate aspherical microlenses with diameters of 1.0 mm; each lens surface possesses nanostructures with an average period of ~120 nm. The microlens quality is sufficient for imaging in terms of profile accuracy and roughness. Compared to lenses without AR nanostructures, the transmittance of the fabricated multiscale lens is increased by ~3% under wavelengths of 400–750 nm. This research provides a foundation for the high-throughput and low-cost industrial application of wafer-level arrays with AR nanostructures.

Investigation on the neighbouring effect of filling micro‐ and nanoscale cavities in ultraviolet nanoimprint lithography

Micro-/nanoscale patterns are popular in many applications, and several techniques to transfer multiscale patterns have been developed. Ultraviolet nanoimprint lithography (UV-NIL) is the most promising method to quickly produce complex structures, and there is a need to understand the resist-flowing and cavity-filling processes because partial cavity filling may cause defects. One crucial element of a stamp is the layout of the multiscale cavities. They can induce some difficulties during the filling process. Five different stamps with various micro-/nanoscale features (one nano feature, one micro feature, one nano feature, and one micro feature, two nano features and one micro feature) were employed in this simulation study. The imprint results show that the neighbouring effect has major issues in the microscale cavity filling and minor issues in the nanoscale. The layout of complex cavities results in poor filling proportions, and longer imprint times and a symmetrical layout were needed to improve the imprint quality. The contact pressure between the stamp and the resist generally declined when the location approached the centre from both sides of the stamp. The overall physical performance of neighbouring micro-/nanoscale cavities was obtained, and this is essential for further understanding and improvements in the UV-NIL process.

Fabrication of a hydrophobic three-dimensional hybrid structure using an UV-curable electron beam resist

Biomimetic techniques are being studied for use in many fields. The hydrophobic surface is among the biomimetic features used in many industrial products, such as liquid-crystal display monitors and windows. The surface has a fine hybrid three-dimensional (3D) structure, comprising micro- and nanoscale structures that can be seen on the surfaces of the lotus leaf and rose petal. Many methods of fabricating 3D hybrid structures have been developed. In this study, a new fabrication method, employing UV nanoimprint lithography (UV-NIL) and electron beam lithography (EBL), was developed to prepare a polymeric hydrophobic surface. When employing this method, EBL forms a nanoscale pattern directly on the microscale pattern generated by UV-NIL. A UV-curable electron-beam resist polymer was prepared with a positive-tone EBL pattern on a UV-NIL pattern. The fabrication of a hydrophobic structure was demonstrated.

Ultra-violet nanoimprint lithography-compatible positive-tone electron beam resist for 3D hybrid nanostructures

A fabrication technique to produce a 3D hybrid structure having different scale complex patterns is studied. A fabrication method is proposed and an ultra-violet nanoimprint lithography (UV-NIL) compatible positive-tone electron beam (EB) resist is prepared. The resist can be shown to have microstructures and nanostructures on the same surface by UV-NIL and positive-tone EB lithography (EBL). The resist can behave as a positive-tone EB resist after the UV-NIL process, exhibits a high UV-NIL and EBL resolution, and a sensitivity sufficient to fabricate 3D hybrid nanopatterns with a high aspect ratio. The fabrication of a micro lens array with nano anti-reflection structure is then performed. The resist can accurately duplicate a microscale concave lens array with a diameter of 3.8 μm. Nano anti-reflection structures are then directly fabricated by EBL on the lens array. The positive-tone dot pattern, as anti-reflection structures with diameters of 220 nm, are fabricated with an EB dose of 1000 μC/cm2. UV-NIL is used to duplicate the fabricated sample. The resist products can successfully be used as the master mold for UV-NIL. A convex micro lens array, with a 200 nm anti-reflection structure, is obtained. These methods using the resist can provide complex 3D hybrid nanostructures. The usefulness of this method and resist are discussed.

Fabrication of micro-lens array with antireflection structure

Optical lens performance is hindered by reflections from the lens surface. Thus, there is a need for antireflection structures (ARSs) for optical lenses. A typical antireflection film is multilayered. There are two drawbacks of using multilayer films: they require complex manufacturing processes and the films do not suppress the reflection of incident light. Herein, we focused on single films with moth-eye structures to achieve low reflectance over a wide range of wavelengths and incident angles. The moth-eye structure is used for anti-reflection films and can be easily fabricated by ultraviolet nanoimprint lithography (UV-NIL). In a previous study, we formed ARSs on complex substrates, such as aspheric lenses, by the reverse replica mold method.In this study, we fabricated the reverse replica mold using two micro-lens arrays, which investigated the limited minimum scale applicable for the flexible mold in order to use the reverse process as previously reported. We considered changing to heat-resistant resin. As a result, we were able to investigate the compliance limits of flexible molds. In addition, we successfully fabricated an ARS on the micro-lens array by using a reverse replica mold and the previous process. However, fabrication was not possible using OrmoClear resin. Therefore, we undertook a process of forming the micro-lens array and ARS with a single resin instead of using the reverse replica mold process. This process was similar to injection molding. As a result, we successfully fabricated the micro-lens array combined with the ARS by using OrmoClear. This micro-lens array with ARS maintained its nanopatterns after heating for 30 min at 250 °C. This process is suitable for fabricating a micro-lens array.

Recent Progress in Ultraviolet Nanoimprint Lithography and Its Applications

Recent Progress in Ultraviolet Nanoimprint Lithography and Its Applications

Pulsed Laser Deposition of Indium Tin Oxide Thin Films on Nanopatterned Glass Substrates

Indium tin oxide (ITO) thin films were grown on nanopatterned glass substrates by the pulsed laser deposition (PLD) technique. The deposition was carried out at 1.2 J/cm2 laser fluence, low oxygen pressure (1.5 Pa) and on unheated substrate. Arrays of periodic pillars with widths of ~350 nm, heights of ~250 nm, and separation pitches of ~1100 nm were fabricated on glass substrates using UV nanoimprint lithography (UV-NIL), a simple, cost-effective, and high throughput technique used to fabricate nanopatterns on large areas. In order to emphasize the influence of the periodic patterns on the properties of the nanostructured ITO films, this transparent conductive oxide (TCO) was also grown on flat glass substrates. Therefore, the structural, compositional, morphological, optical, and electrical properties of both non-patterned and patterned ITO films were investigated in a comparative manner. The energy dispersive X-ray analysis (EDX) confirms that the ITO films preserve the In2O3:SnO2 weight ratio from the solid ITO target. The SEM and atomic force microscopy (AFM) images prove that the deposited ITO films retain the pattern of the glass substrates. The optical investigations reveal that patterned ITO films present a good optical transmittance. The electrical measurements show that both the non-patterned and patterned ITO films are characterized by a low electrical resistivity (<2.8 × 10−4). However, an improvement in the Hall mobility was achieved in the case of the nanopatterned ITO films, evidencing the potential applications of such nanopatterned TCO films obtained by PLD in photovoltaic and light emitting devices.

Long-term analysis of PV module with large-area patterned anti-reflective film

Due to the PV modules are exposed to the external environment, it is necessary to clean or replace the protective glass upon contamination. In this study, we demonstrate a large-area and flexible anti-reflective film with moth-eye pattern fabricated by using a highly straightforward process: nanoimprint lithography. In addition, the fabricated anti-reflective film exhibits self-cleaning capability owing to the hydrophobic self-assembled-monolayer-coated surface. The fabricated anti-reflective film was attached to the surface of photovoltaic (PV) modules and placed outdoor to conduct a seven-month field test in order to observe the increase in the conversion efficiency of the PV modules as a result of the effect of the anti-reflective film. Two types of patterns, moth-eye pattern and micro-cone pattern, were used as the anti-reflective patterns; these were conveniently fabricated by the ultraviolet-nanoimprint lithography process, which does not use vacuum or high-temperature process. In the seven-month analysis, the moth-eye patterned film was observed to exhibit a transmittance that is 5.0% higher than that of the flat film, whereas the PV module with the moth-eye patterned film on its surface exhibited a conversion efficiency that is 2.85% higher than that of the PV module with the flat film.

Nano pattern transfer on acrylic polymers with UV irradiation for liquid crystal alignment

Ultra-violet nanoimprint lithography was utilized to transfer a one-dimensional nano pattern onto a hydrophilic ultra-violet cured polymer surface for use in liquid crystal displays. A master mold with the one-dimensional nano pattern was initially constructed using laser interference lithography, which is a fast and maskless process that can be employed over a large area. The UV cured polymer showed a hydrophilic characteristic, and was used as a homogeneous alignment layer. The nano-patterned ultra-violet polymer successfully aligned the liquid crystal molecules without defects. Additionally, the liquid crystal cell was characterized by high thermal stability due to the high thermal endurance of UV cured polymer. Therefore, the proposed ultra-violet nanoimprint lithography technique is beneficial for advanced liquid crystal devices with high brightness and resolution with numerous switching components that require a high thermal cost.

UV-curable nanoimprint resist with liquid volume-expanding monomers

For ultraviolet nanoimprint lithography (UV-NIL), the resist volume shrinkage during curing not only influences the pattern fidelity but also induces defects in the demolding process due to strong adhesion. To address this issue, a novel nanoimprint resist was formulated and characterized in this work. The new resist formulation contains 3,9-diethyl- 3,9-bis(allyloxymethyl)-1,5,7,11- tetraoxastetraoxaspiro [5] undecane (DB-TOSU), which is a liquid spiroorthocarbonate monomer that undergoes volume expansion upon acid-catalyzed polymerization. By mixing DB-TOSU with conventional volume-shrinking epoxy monomers at various weight ratios, the formulated resists had much reduced or even zero volume shrinkage. The resist volume shrinkage, elastic modulus, and demolding force decreased with increasing DB-TOSU weight ratio in resist formulation. When DB-TOSU reached 50 wt%, the demolding force was reduced by 69% with an adequate elastic modulus (75 MPa) and low shrinkage (1.86%). This novel resist formulation has the potential to allow high-fidelity pattern replication and reduce demolding defects in UV-NIL.

Experimental and numerical investigation of biosensors plasmonic substrates induced differences by e-beam, soft and hard UV-NIL fabrication techniques

This paper compares plasmonic substrates manufactured by three lithography methods: E-beam, soft and hard UV NanoImprint Lithography. The different plasmonic modes existing in samples made of an array of gold nanostructures on gold film are investigated for biochemical detections taking advantage of Surface Plasmon Resonance Imaging (SPRI) and Surface-Enhanced Raman Scattering (SERS). Recently, it has been shown that this geometry of substrate is of great interest for both SPRI and SERS measurements. A comparison of their performances obtained by the different lithographic methods is provided. In particular, due to limitations in NanoImprint Lithographic techniques, the impact of sidewall geometry of nanostructures is investigated in regard to plasmonic properties. Thus, experimental optical characterization analyses have been carried out on samples and compared with the numerical simulations.

Visualization of organic/inorganic hybridization of UV-cured films with trimethylaluminum by scanning transmission electron microscopy and energy dispersive x-ray spectroscopy

The organic/inorganic hybridization of photopolymerized resist films for ultraviolet nanoimprint lithography (UV-NIL) through chemical vapor modifications of atomic layer deposition (ALD), sequential infiltration synthesis (SIS), and saturated vapor infiltration (SVI) with an inorganic precursor of trimethylaluminum (TMA) and an oxidant of water was investigated. The hybridization of the bisphenol A-based polymethacrylate resist films was compared between resin-A comprising a monomer with hydroxy groups and resin-B comprising another monomer without hydroxy groups. The elemental depth profiles by scanning transmission electron microscopy and energy dispersive x-ray spectroscopy revealed the following three things. ALD and SIS caused the hybridization of the organic resist films with inorganic alumina near the film surfaces, while SVI caused the hybridization of the resist films entirely. The hydroxy-free resin-B physically adsorbed and chemically fixed more TMA molecules than the hydroxy-containing resin-A. Although SIS progressed the entire hybridization of the resist films, different behaviors of segregation of alumina between the hydroxy-containing resin-A and hydroxy-free resin-B films were confirmed near the film surface, inside, and interface with a silicon substrate. The organic/inorganic hybridization enabled the tuning of the etching rate of the NIL resist masks with a thickness of no thicker than 20 nm in oxygen reactive ion etching often used for the removal of residual layers from imprint patterns in UV-NIL processes.

Pt Nanogap Electrode Fabrication by Two-Layer Lift-Off UV-NIL and Nanowire Breakdown

Nanogap electrodes are expected to aid the study of the electrical properties of single molecules and nanoparticles, and also have been applied to nonvolatile memory. In this study, we developed a process to fabricate nanogap electrodes by combining ultraviolet nanoimprint lithography (UV-NIL) and a nanowire-breakdown technique. A pattern of Pt/Ti nanowires with a width of 300 nm was formed on a silicon oxide layer by two-layer lift-off UV-NIL using an alkali-soluble polymer. A Pt nanogap with a distance of couple of nanometers was fabricated by controlling the step voltage applied to the nanowire electrodes. We demonstrated the switching behavior of the Pt nanogap. It was confirmed that switching operations could be performed for 100 cycles and a high resistance ratio of over 102 was obtained.

Fabrication of Cr2O3 Nano-to-Microscale Structures Using UV Nanoimprint Lithography

Fabrication of Cr₂O₃ Nano-to-Microscale Structures Using UV Nanoimprint Lithography

Synthesis and nanoimprinting of high refractive index and highly transparent polythioethers based on thiol‐ene click chemistry

ABSTRACTThis work demonstrates the UV nanoimprinting lithography (UV‐NIL) of high refractive index and highly transparent polythioethers based on thiol‐ene click chemistry. Herein, 9,9‐bis(3‐mercaptopropylphenylether)fluorene (BMPF) is designed as a new thiol monomer with a high refractive index, high transparency, and good processability for UV‐NIL. Colorless polythioethers are synthesized from BMPF and ene monomers under mild thiol‐ene click reaction conditions. Excellent transmittance (96%) of 400 nm light is observed in all the polymer films and high refractive index values of 1.5972–1.6382 are attained. UV‐NIL using thiol‐ene photopolymerization affords polymer nanoimprinting patterns with various features on the order of 100–500 nm without any fractures. To the best of our knowledge, this is the first report on UV‐NIL of high refractive index and highly transparent polymers. Through proper monomer and polymer design, novel polythioethers with suitable glass transition temperature (T g) values are developed with high refractive index, high transparency, and good UV‐NIL processability. Furthermore, UV‐NIL based on thiol‐ene click chemistry is accomplished at the nanoscale. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 2175–2182

Nanoindentation of crystalline silicon pillars fabricated by soft UV nanoimprint lithography and cryogenic deep reactive ion etching

Nanomechanical properties of bulk and micro-pillared monocrystalline silicon are experimentally investigated using nanoscale depth-sensing indentation technique. Silicon pillar arrays with different crystal orientations, i.e., Si<100>, Si<110>, and Si<111>, are prepared by means of a top-down approach consisting of nanoimprint lithography and cryogenic dry etching. A three-sided Berkovich tip used during the experiments has provided highly resolved load-depth-curves and imprinted hardness impressions on the silicon materials. Depending on the aspect ratio of the pillars, the axial stiffness of the objects under measurement has to be considered. Corrected values of the elastic modulus are determined revealing direct nanomechanical comparison between silicon bulk and pillar structures. In summary, enhanced comprehension of elastic-plastic-response occurred within micro-structured silicon materials is shown, providing the potential to use silicon pillar arrays as fundamental material for accurate force measurements.