UV Nanoimprint Lithography Research Articles

Continuous Fabrication of Multiscale Compound Eyes Arrays With Antireflection and Hydrophobic Properties

The multiscale hierarchical structures inspired by moth's compound eyes offer multifunctional properties in optoelectronic devices. However, it is still a major challenge to fabricate these hierarchical structures on large-area substrates using a simple and cost-effective technique. The roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the continuous fabrication of multiscale compound eyes arrays due to its high-speed, large-area, high-resolution, and high-throughput. In this paper, the R2R UV-NIL technique was used to fabricate the multiscale compound eyes arrays on the PET substrate. The mold used in the R2R UV-NIL process was acquired by anodic aluminum oxide process and then the multiscale compound eyes arrays were directly obtained via one-step R2R imprinting. The obtained multiscale compound eyes arrays exhibit excellent antireflective performance within the wavelength 400-800 nm. Besides, the compound eyes arrays also equip the surface of the microlens with excellent hydrophobic characteristics. These multifunctional properties enable the multiscale compound eyes arrays to retain their superior optical properties in real-time environmental conditions. This report can provide a beneficial direction for the continuous production and widespread applications of the multiscale compound eyes arrays.

Size-Dependent Filling Behavior of UV-Curable Di(meth)acrylate Resins into Carbon-Coated Anodic Aluminum Oxide Pores of around 20 nm.

Ultraviolet (UV) nanoimprint lithography is a promising nanofabrication technology with cost efficiency and high throughput for sub-20 nm size semiconductor, data storage, and optical devices. To test formability of organic resist mask patterns, we investigated whether the type of polymerizable di(meth)acrylate monomer affected the fabrication of cured resin nanopillars by UV nanoimprinting using molds with pores of around 20 nm. We used carbon-coated, porous, anodic aluminum oxide (AAO) films prepared by electrochemical oxidation and thermal chemical vapor deposition as molds, because the pore diameter distribution in the range of 10-40 nm was suitable for combinatorial testing to investigate whether UV-curable resins comprising each monomer were filled into the mold recesses in UV nanoimprinting. Although the UV-curable resins, except for a bisphenol A-based one, detached from the molds without pull-out defects after radical photopolymerization under UV light, the number of cured resin nanopillars was independent of the viscosity of the monomer(s) in each resin. The number of resin nanopillars increased and their diameter decreased as the number of hydroxy groups in the aliphatic diacrylate monomers increased. It was concluded that the filling of the carbon-coated pores having diameters of around 20 nm with UV-curable resins was promoted by the presence of hydroxy groups in the aliphatic di(meth)acrylate monomers.

Diazoketo-functionalized POSS resists for high performance replica molds of ultraviolet-nanoimprint lithography

Novel polyhedral oligomeric silsesquioxane (POSS) resists, which are based on a new photo-crosslinking system via Wolff rearrangement, are developed as ideal replica mold materials for ultraviolet-nanoimprint lithography. These POSS resist materials are synthesized by incorporating diazoketo and hydroxyl groups into the POSS core. The resist materials have exhibited a variety of desirable properties as replica molds, such as high modulus, low shrinkage ratio, high transparency, low surface energy, and resistance to organic solvents. The resultant replica molds exhibit a high resolution patterning capacity. These economic fabrication methods of replica molds with high mechanical durability and good releasing properties are potentially useful for versatile applications in the area of mold-based lithography.

Upright nanopyramid structured cover glass with light harvesting and self-cleaning effects for solar cell applications

In this paper, we demonstrate the effect of upright nanopyramid (UNP) structured cover glass with light harvesting and self-cleaning functions on the device performance of monocrystalline Si solar cells. The UNP structures were fabricated on the surface of the glass substrate by simple, high throughput and low cost UV nanoimprint lithography, using a Si master mold with inverted nanopyramid (INP) structures. The diffuse transmittance and haze ratio values were significantly increased for UNP patterned glass, especially in the wavelength range 300–600 nm compared to the bare glass; this implies that antireflection and strong light scattering are due to the UNP structures. By replacing a bare cover glass with UNP patterned glass, the power conversion efficiency of the monocrystalline Si solar cell was substantially enhanced by about 10.97%; this is mainly due to the increased short-circuit current density JSC of 32.39 mA cm−2 compared to the reference cell with bare cover glass (i.e. JSC = 31.60 mA cm−2). In addition, unlike the bare cover glass (i.e. θCA ~ 36°), the fluorinated UNP structured cover glass exhibited a hydrophobic surface with a water contact angle (θCA) of ~132° and excellent self-cleaning of dust particles by rolling down water droplets.

Efficient light trapping nanopyramid structures for solar cells patterned using UV nanoimprint lithography

In this paper, we demonstrate that periodic inverted nanopyramid structures can enhance the power conversion efficiency of monocrystalline silicon solar cells by minimizing reflections, improving light trapping process in addition to its self-cleaning functionality. The periodic inverted nanopyramid structures were fabricated on monocrystalline silicon solar cell surfaces using a UV nanoimprint lithography. By introducing inverted nanopyramid structures on the front side of the monocrystalline silicon solar cells, the power conversion efficiency was improved by 11.73% compared to identical solar cells without the texturing. The inverted nanopyramid coating decreased the reflectance and increased the external quantum efficiency over a broad wavelength range. Moreover, the surface of the solar cells exhibited hydrophobic properties due to increased contact angle caused by the nanostructure patterns and the self-assembled monolayer coating. The enhanced hydrophobicity provided the solar cells with an added self-cleaning functionality. These results suggest that the periodic inverted nanopyramid structures has high potential in improving the performance of silicon solar cells and may be applied to different types of solar cells.

The polarization modulation and fabrication method of two dimensional silica photonic crystals based on UV nanoimprint lithography and hot imprint.

Based on a silica sol-gel technique, highly-structurally ordered silica photonic structures were fabricated by UV lithography and hot manual nanoimprint efforts, which makes large-scale fabrication of silica photonic crystals easy and results in low-cost. These photonic structures show perfect periodicity, smooth and flat surfaces and consistent aspect ratios, which are checked by scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition, glass substrates with imprinted photonic nanostructures show good diffraction performance in both transmission and reflection mode. Furthermore, the reflection efficiency can be enhanced by 5 nm Au nanoparticle coating, which does not affect the original imprint structure. Also the refractive index and dielectric constant of the imprinted silica is close to that of the dielectric layer in nanodevices. In addition, the polarization characteristics of the reflected light can be modulated by stripe nanostructures through changing the incident light angle. The experimental findings match with theoretical results, making silica photonic nanostructures functional integration layers in many optical or optoelectronic devices, such as LED and microlasers to enhance the optical performance and modulate polarization properties in an economical and large-scale way.

Bubble-free patterning with low line edge roughness by ultraviolet nanoimprinting using trans-1,3,3,3-tetrafluoropropene condensable gas

Ultraviolet nanoimprint lithography (UV-NIL) in pentafluoropropane (PFP) is recognized as one of the most promising methods to achieve ultrahigh-speed UV-NIL without air bubble defects. Although liquefied PFP dissolved in the resin reduces the resin viscosity and demolding force, it also causes large shrinkage of the pattern and degrades the pattern quality compared with that achieved in air or He environments. In this work, UV-NIL in trans-1,3,3,3-tetrafluoropropene (TFP), which has a higher vapor pressure than that of PFP, was investigated with respect to throughput and pattern quality characteristics to find an alternative to PFP. Bubble-free filling of UV-NIL was demonstrated in an TFP atmosphere: the cavity-filling time was about four times shorter than that in He, and the fine values of line edge roughnesses comparable to those achieved in air or He were obtained.

Large-Scale Uniform Silver Nanocave Array for Visible Light Refractive Index Sensing Using Soft UV Nanoimprint

In this paper, a wafer-scale uniform silver nanocave array is fabricated by soft ultraviolet nanoimprint lithography. We investigate its plasmonic effects using far-field and near-field experimental approaches and illuminate the physics inside by theoretical analysis and computational simulation. The array shows robust multispectral features for various surrounding media and possesses the sensitivity up to 514.7 nm/RIU in the visible range, which is promising for the mass production of high-performance plasmonic refractive index sensors.

Influence of discrete drop locations on film thickness uniformity in UV- nanoimprint lithography

Multi-nozzle inkjetting is the material deposition approach for a variant of UV-nanoimprint lithography called Jet-and-Flash Imprint Lithography (J-FIL). J-FIL has several advantages with respect to addressing pattern density variations in the template as well as achieving low material waste when compared with spin-coating. In this work, the influence of discrete drop placement in J-FIL on residual layer thickness uniformity has been investigated with the help of a novel nonlinear lubrication theory model, which takes into account the compliant nature of the template in the presence of a moving contact line. This work is directed at thin residual layers (sub-20nm), where a correlation exists between minimum drop resolution, inkjet nozzle pitch and residual layer thickness uniformity. Numerical simulations reveal that the non-uniformity resulting from discrete drop placement can exceed desired process tolerances when low film thicknesses are desired with relatively larger drop volumes. The results have been verified experimentally to reveal the same trend. This work can be useful in determining the correlation of inkjet drop resolution with inkjet nozzle pitch to meet process tolerances in the J-FIL process.

Bubble-free high-speed UV nanoimprint lithography using condensable gas with very low global warming potential

Bubble-free filling needs to be achieved to realize high-throughput mass production in ultraviolet nanoimprint lithography (UV-NIL). Although bubble-free filling can be accomplished by performing UV-NIL under vacuum, nonvacuum processes can lower equipment and operation costs. UV-NIL in 1,1,1,3,3-pentafluoropropane (PFP) has been recognized as a promising method of realizing ultrahigh-speed UV-NIL; however, the global warming potential (GWP) of PFP of 1030 might restrict its industrial use. In this work, UV-NIL of a spin-coated UV-curable resin in trans-1-chloro-3,3,3-trifluoropropene (CTFP), which has a low GWP of <5, was studied with the aim of identifying an alternative to PFP. The cavity filling speed of resin and mold release force in CTFP were comparable to those in PFP, and superior to those in helium atmosphere. Sub-100 nm patterns were successfully fabricated by UV-NIL in CTFP, although the line width shrinkage ratio of patterns fabricated in CTFP was slightly larger than that of patterns fabricated in PFP.

ChemInform Abstract: Directed Self‐Assembly of Block Copolymers for Use in Bit Patterned Media Fabrication

Reduction of the bit size in conventional magnetic recording media is becoming increasingly difficult due to the superparamagnetic limit. Bit patterned media (BPM) has been proposed as a replacement technology as it will enable hard disk areal densities to increase past 1 Tb in−2. Block copolymer directed self-assembly (BCP DSA) is the leading candidate for forming BPM due to its ability to create uniform patterns over macroscopic areas. Here we review the latest research into two different BCP DSA techniques: graphoepitaxy and chemoepitaxy (or chemical prepatterning). In addition to assessing their potential for forming high density bit patterns, we also review current approaches using these techniques for forming servo patterns, which are required for hard disk drive (HDD) operation. Finally, we review the current state of UV nanoimprint lithography, which is the favoured technique for enabling mass production of BPM HDDs.

Durability to oxygen reactive ion etching enhanced by addition of synthesized bis(trimethylsilyl)phenyl-containing (meth)acrylates in ultraviolet nanoimprint lithography

We investigated the selection of bis(trimethylsilyl)phenyl-containing (meth)acrylates as additives to improve the durability to oxygen reactive ion etching (O2 RIE) of sub-50 nm imprint resist patterns suitable for bubble-defect-free UV nanoimprinting with a readily condensable gas. 2,5-Bis(2-acryloyloxyethoxy)-1,4-bis(trimethylsilyl)benzene, which has a diacrylate chemical structure similar to that of glycerol 1,3-diglycerolate diacrylate used as a base monomer, and 3-(2-methacryloyloxyethoxy)-1-(hydroxylethoxy)-2-propoxy-3,5-bis(trimethylsilyl)benzene, which has a hydroxy group similar to the base monomer, were synthesized taking into consideration the Ohnishi and ring parameters, and the oxidization of the trimethylsilyl moiety to inorganic species during O2 RIE. The addition of the latter liquid additive to the base monomer decreased etching rate owing to the good miscibility of the additive in the base monomer, while the addition of the former crystalline additive caused phase separation after UV nanoimprinting. The latter additive worked as a compatibilizer to the former additive, which is preferred for etching durability improvement. The coexistence of the additives enabled the fabrication of a 45 nm line-and-space resist pattern by UV nanoimprinting, and its residual layer could be removed by O2 RIE.

Discharge of viscous UV-curable resin droplets by screen printing for UV nanoimprint lithography

We demonstrated a coating method of screen printing for discharging droplets of a high-viscosity resin on a substrate for ultraviolet (UV) nanoimprint lithography (NIL). Compared with a spin-coated resin film on a silicon substrate, discharged resin droplets on a silicon substrate were effective in terms of the uniformity of residual layer thickness (RLT) in contact with a mold with various pattern densities. Fluorescence microscope observations with a fluorescent-dye-containing UV-curable resin enabled the evaluation of the shapes of resin droplets discharged on a substrate surface. Widely used screen mesh plates composed of a stainless mesh covered with a patterned emulsion film caused defects of undischarged parts, whereas defects-free resin droplets with a narrow size distribution were discharged by mesh-free plates prepared with laser ablation. The pitch-to-diameter ratio in the configuration of 10-µm-diameter holes needs to be larger than 2.5 times for printing a resin having a viscosity of 12,800 mPa s.

Sub-70 nm resolution patterning of high etch-resistant epoxy novolac resins using gas permeable templates in ultraviolet nanoimprint lithography

This study aimed to expand the resolution limits of epoxy novolac resins that have enhanced etch resistance as one of the desirable resist properties for next-generation devices. Epoxy novolac resins have high etch resistance. However, because epoxy novolac resins are either solid or semisolid at room temperature, and because the use of volatile solvents in resist can be a cause of pattern failure in nanoimprint lithography, epoxy novolac resins have been of limited utility as resist. Excellent sub-70 nm resolution patterning can be achieved by diluting 15 wt % acetone in an ultraviolet nanoimprint lithography using gas-permeable templates.

Multilength Scale Patterning of Functional Layers by Roll-to-Roll Ultraviolet-Light-Assisted Nanoimprint Lithography.

Top-down fabrication of nanostructures with high throughput is still a challenge. We demonstrate the fast (>10 m/min) and continuous fabrication of multilength scale structures by roll-to-roll UV-nanoimprint lithography on a 250 mm wide web. The large-area nanopatterning is enabled by a multicomponent UV-curable resist system (JRcure) with viscous, mechanical, and surface properties that are tunable over a wide range to either allow for usage as polymer stamp material or as imprint resist. The adjustable elasticity and surface chemistry of the resist system enable multistep self-replication of structured resist layers. Decisive for defect-free UV-nanoimprinting in roll-to-roll is the minimization of the surface energies of stamp and resist, and the stepwise reduction of the stiffness from one layer to the next is essential for optimizing the reproduction fidelity especially for nanoscale features. Accordingly, we demonstrate the continuous replication of 3D nanostructures and the high-throughput fabrication of multilength scale resist structures resulting in flexible polyethylenetherephtalate film rolls with superhydrophobic properties. Moreover, a water-soluble UV-imprint resist (JRlift) is introduced that enables residue-free nanoimprinting in roll-to-roll. Thereby we could demonstrate high-throughput fabrication of metallic patterns with only 200 nm line width.

Fabrication and Characterization of High-Optical-Quality-Factor Hybrid Polymer Microring Resonators Operating at Very Near Infrared Wavelengths

In this paper, we present a new fabrication method for large-area hybrid polymer microring resonators using a rib waveguide configuration with a minimum residual layer down to 40 nm by ultraviolet (UV) nanoimprint lithography. In a first step, a negative photoresist (SU8-2) patterned by photolithography on an SiO 2/silicon wafer was used as a master mold. Flexible soft molds using perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding are explored in the imprinting process flow. High-quality devices with $Q$ -factors up to 39 000 and finesses up to $F \sim 14$ were demonstrated for very-near infrared wavelengths (around 900 nm). Finally, we report that the resonances can be thermooptically tuned by 0.05 nm/°C at 896 nm, thus demonstrating that, at shorter wavelengths, the fabricated microring resonators are less sensitive to temperature change, compared with longer wavelengths. The results of this paper demonstrate the capability of our method to fabricate optical devices with high performance for operation in a single mode at wavelengths where cheap light sources are available and water absorption is much lower than at far-infrared wavelengths. Because of this, it is expected that the fabricated structures have high potential for biosensing applications.

Combined Chemical Groups and Topographical Nanopattern on the Poly(ε-Caprolactone) Surface for Regulating Human Foreskin Fibroblasts Behavior

Surface chemistry and substrate topography could contribute significantly to providing a biochemical and topographical cues for governing the fate of cells on the cell-material interface. However, the synergies between these two properties have not been exploited extensively for biomaterial design. Herein, we achieved spatial-controlled patterning of chemical groups on the poly(ε-caprolactone) (PCL) surface by elegant UV-nanoimprint lithography (UN-NIL). The introduction of chemical groups on the PCL surface was developed by our newly 6-benzyloxycarbonylmethyl-ε-caprolactone (BCL) monomer, which not only solved the lack of functional groups along the PCL chain but also retained the original favorable properties of PCL materials. The synergetic effect of the chemical groups and nanopatterns on the human foreskin fibroblasts (HFFs) behaviors was evaluated in detail. The results revealed that the patterned functional PCL surfaces could induce enhanced cell adhesion and proliferation, further trigger changes in HFFs morphology, orientation and collagen secretion. Taken together, this study provided a method for straightforward fabrication of reactive PCL surfaces with topographic patterns by one-step process, and they would facilitate PCL as potential candidate for cell cultivation and tissue engineering.

Enzyme-coupled nanoplasmonic biosensing of cancer markers in human serum

As the use of biosensors for early in-vitro diagnosis of malignant diseases has expanded, issues associated with ultra-sensitivity and wide dynamic range have become paramount. In this study, we designed a sub-zeptomolar sensor for detecting the alpha-feto protein (AFP) that utilizes localized surface plasmon resonance (LSPR) assisted by bio-catalytic reaction and a self-controlled detection scheme. A gold nanodot array (GNA), serving as a plasmonic material, was fabricated using an improved UV nanoimprint lithography (NIL) method that employs a sacrificial layer. In the new approach, LSPR observation is highly stable because it employs a back reflection mode, which avoids passing the signal through the sample solution. An enzyme-precipitation reaction was conducted on the AFP antigen-antibody complex on the surface of the gold nanodot (nano-ELISA) using a procedure that was described previously. To extend the AFP detection limit below femtomolar concentrations, a scheme involving self-controlled detection was employed to lower the limit. In this method, the sample including the target simultaneously plays the role of both sample and negative control. Using this scheme, AFP can be detected at concentrations as low as 14 aM (0.7 zeptomole in 50μL serum) and with the wide dynamic range of 10fgmL−1–10ngmL−1. Importantly, the new method provides a platform for studying and monitoring interactions between biochemically active substances that are present in very low concentrations. Finally, the general strategy used to design the detection method can be easily adapted to the ultra-sensitive detection of other biomarkers and pathogens.

Fabrication of antireflection structures on the surface of optical lenses by using a liquid transfer imprint technique

Antireflection (AR) films are useful for preventing the reflection of light from flat-panel displays, solar-cell panels, optical lenses, and similar structures. We previously reported the successful high-throughput fabrication of an AR structure (ARS) with less than 0.1% reflectivity by means of ultraviolet nanoimprint lithography (UV-NIL). However, this process tended to produce a thick residual layer that reduced transmittance and produced interfacial reflections at surfaces of lenses coated with the ARS. To reduce interfacial reflections, the thickness of the residual layer must be less than 100nm or so (roughly a quarter of the wavelength of visible light). Liquid transfer imprint lithography (LTIL) is an effective technique for reducing the thickness of the residual layer. In the LTIL process, residual resin is eliminated by separating the mold and substrate in the liquid phase. This process is also effective for curved surfaces, and it has been reported an ARS with a thin residual layer can be obtained by LTIL. However, the method that was used required a vacuum process and was time consuming. To obtain a thin residual layer at atmospheric pressure, we devised an LTIL process involving a roll-press method that does not require the presence of a vacuum. The technique permitted the fabrication of a high-performance ARS with improved reflectance and transmittance thickness, in which the thickness of the residual layer was less than a quarter of the wavelength of visible light. Furthermore, the technique could be used to produce a homogeneous ARS with a residual layer less than 100nm thick on a curved surface, such as that of a lens, in a reproducible and stable manner. The process is therefore suitable for fabricating ARSs that show a high performance with a thin residual layer on curved surfaces such as convex lenses.

Fabrication of flexible oriented magnetic thin films with large in-plane uniaxial anisotropy by roll-to-roll nanoimprint lithography

Here, we report wafer scale fabrication of densely packed Fe nanostripe-based magnetic thin films on a flexible substrate and their magnetic anisotropy properties. We find that Fe nanostripes exhibit large in-plane uniaxial anisotropy and nearly square hysteresis loops with energy products (BHmax) exceeding 3MGOe at room temperature. High density Fe nanostripes were fabricated on 70nm flexible polyethylene terephthalate (PET) gratings, which were made by a roll-to-roll (R2R) UV nanoimprint lithography technique. We observed large in-plane uniaxial anisotropies along the long dimension of nanostripes that can be attributed to the shape. Temperature dependent hysteresis measurements confirm that the magnetization reversal is driven by non-coherent rotation reversal processes.