Imprint Lithography Research Articles

Multilayer Nanoimprinting to Create Hierarchical Stamp Masters for Nanoimprinting of Optical Micro- and Nanostructures

Nanoimprinting is a well-established replication technology for optical elements, with the capability to replicate highly complex micro- and nanostructures. One of the main challenges, however, is the generation of the master structures necessary for stamp fabrication. We used UV-based Nanoimprint Lithography to prepare hierarchical master structures. To realize structures with two different length scales, conventional nanoimprinting of larger structures and conformal reversal nanoimprinting to print smaller structures on top of the larger structures was performed. Liquid transfer imprint lithography proved to be well suited for this purpose. We used the sample prepared in such a way as a master for further nanoimprinting, where the hierarchical structures can then be imprinted in one single nanoimprinting step. As an example, we presented a diffusor structure with a diffraction-grating structure on top.

Combining multi-step imprinting with the in-plane compression method

In this study, a micro-patterning process is proposed and developed in order to obtain various functional properties on the surface of an object. Nano imprint lithography has recently been the focus of much attention, because of advantages such as high accuracy, low cost, and ease of operation. However, problems can occur during demolding when using this method, such as the destruction of high aspect patterns during the process. It is also, almost impossible to form an over-hanging pattern. In order to overcome these problems, we propose a multi-step imprinting process with an in-plane compression method. A hierarchical multi-scale pattern can be produced using multi-step imprinting, and the aspect ratio of the imprinted pattern can be increased by using the in-plane compression method. A hierarchical multi-scale pattern with a high aspect ratio was produced by combining the two methods. The effectiveness of the proposed method is demonstrated by the results of the experimentation.

The Fabrication of Micro-/Nano-Structural Antirefraction Coatings Based on the PDMS Film for Solar Cells Application

A simple and low-cost methodology to fabricate micro/nanostructure anti-refraction coatings (ARCs) of solar cells is introduced. The nanostructure ARCs are generated by O2 Plasma surface modification technique for a flexible polydimethylsiloxane (PDMS) polymeric film, the microstructure ARCs are obtained by soft imprint lithography, and the micro-nano complex structure PDMS ARCs are fabricated by soft imprint lithography technique combined with surface modification technique. The measured results show that these PDMS ARCs can decrease reflection loss of light from the solar cells and increase the solar cells photovoltaic conversion efficiency.

Solution-processable multi-color printing using UV nanoimprint lithography

Recently, coloring based on nanostructure-light interaction has attracted much attention, because it has many advantages over pigment-based conventional coloring in terms of being non-toxic and highly durable in the environment, and providing high resolution. The asymmetric Fabry–Perot (FP) cavity absorber is the most manufacturable structure among coloring structures because it is simply produced and easily tunable. However, it cannot be applied practically because of the lack of a manufacturing technique that enables simultaneous fabrication of multi-color structures with different heights. Here, the fabrication of colored reflective characters based on various asymmetric FP absorbers with micrometer-scale pixel size are reported. Various cavities with different thicknesses are fabricated in a single step using UV imprint lithography and a simple deposition process. UV/visible spectroscopy is used to characterize the fabricated FP resonator. This absorber demonstrates high absorption, close to 90%, resulting in vivid colors with high resolution of 12700 DPI. It can be potentially used in reflective color displays field, functionalized color decorations, and security color patterns area. It is believed that this study would open up new possibilities for high density color printing in practical industry by introducing cost effective nanoimprint lithography technology.

Aggregation-induced red-shifted emission and fluorescent patterning of poly(aryleneethynylene) with a lateral AIEgen substituent

A poly(aryleneethynylene)-based polymer with a lateral AIEgen showed a large red-shift in emission during aggregation and various high-resolution fluorescent patterns were fabricated by imprinting lithography.

Multi-step proportional miniaturization to sub-micron dimensions using pre-stressed polymer films.

The ability to define patterns and fabricate structures at the nanoscale in a scalable manner is crucial not only in integrated circuit fabrication but also in fabrication of nanofluidic devices as well as in nano and micromechanical systems. Top down fabrication at the nanoscale often involves fabrication of a master using a direct write method and then its replication using a variety of methods such as by hot embossing, nanoimprint lithography, or soft lithography. Nevertheless, fabrication of the master is a time consuming and expensive process. One interesting approach is to define patterns at larger dimensions on pre-stressed films using methods such as xurography or lithography which are scalable and heat them to de-stress and shrink which can reduce the size proportionally. Although attractive, suitable fabrication processes that can perform iterative shrinking of patterns over several cycles and into the nanoscale have not been demonstrated. Here, we demonstrate a fabrication process that is capable of accurately producing patterns and features over several cycles of miniaturization and shrinking to achieve resolution in the order of 100 s of nanometers. In this approach, a pattern transfer method is developed by combining soft imprint lithography followed by reactive ion etching, both of which are scalable processes, to transfer the original patterns into a shrinkable polymer film. The patterned shrinkable film is heated to allow thermal shrinking. As a result, the pattern size was decreased by 60% of the original size in a single cycle. This reduced pattern was used as the master for the next cycle and three cycles of miniaturization was demonstrated. Sub-micron patterns of 750 nm were generated by the multi-step miniaturization method, showing approximately 20× reduction in size of the original patterns. Finally, these patterns are transferred into features on a silicon substrate to demonstrate its application in semiconductor microfabrication or its use as a master template for microsystems applications.

Study of etching of patterned poly(vinylidene fluoride) film using dimethyl sulfoxide solvents

This paper focuses on the optimization of the etching rate of nano-patterns of Poly (vinylidene fluoride) (PVDF) thin films on glass substrates using Dimethyl Sulfoxide (DMSO). Soft imprint lithography is employed to transfer the nano-patterns onto the PVDF thin films systematically under optimized temperature and pressure. Atomic Force Microscopy (AFM) has been used to analyze the etching rate of patterned films.

Study of Krafty Glue patterning thin films using soft lithography

The present work addresses Krafty Glue patterning thin films using soft imprint lithography. The manufacturing of micro structures at low cost it is a convenient method. Krafty Glue thinfilm are made with white Krafty Glue solution by using spin coating method and SONY-CD -700 MB - Al-layer used as a master. The Al-layer has height in nanometer range and width in micrometer range. The replica of patterns are made using soft imprint lithography and successfully transferred the nano patterns of Al- layer on to white glue and PVDF films. The transformation of patterns, morphology and structural studies are done using Atomic Force Microscopy (AFM).

Biomimetic diodon-skin nanothorn polymer antireflection film for solar cell applications

A biomimetic diodon-skin nanothorn (DSNT) epoxy resin antireflection (AR) film was presented to reduce the incident reflection loss of Si solar cell and thus improve power conversion efficiency (PCE). The fabricated epoxy resin DSNT film, designed using finite-difference time-domain method, was added on bare Si substrate using a polydimethylsiloxane (PDMS) stamps with soft imprint lithography. Optical characteristics showed the reflection was reduced in wavelength range of 300–1100 nm from 40.0% of bare Si substrate to 15.8% of epoxy resin DSNT film/Si substrate. In addition, we found the designed DSNT structure can be facilely transferred onto any flat substrates without deformation into unexpected shapes using soft imprint lithography. Moreover, the epoxy resin DSNT film exhibited superior hydrophobic and self-cleaning properties. By introducing the epoxy resin DSNT film on top of Si solar cell, the reflection loss of the Si solar cell was reduced by 7.8% and thus the PCE was increased from 18.99% to 19.88%. Thus, the epoxy resin DSNT film is a good candidate of AR film for photovoltaic application.

Direct cell imprint lithography in superconductive carbon black polymer composites: process optimization, characterization and in vitro toxicity analysis

Cell imprint lithography (CIL) or cell replication plays a vital role in fields like biomimetic smart culture substrates, bone tissue engineering, cell guiding, cell adhesion, tissue engineering, cell microenvironments, tissue microenvironments, cell research, drug delivery, diagnostics, therapeutics and many other applications. Herein we report a new formulation of superconductive carbon black photopolymer composite and its characterization towards a CIL process technique. In this article, we demonstrated an approach of using a carbon nanoparticle-polymer composite (CPC) for patterning cells. It is observed that a 0.3 wt % load of carbon nanoparticles (CNPs) in a carbon polymer mixture (CPM) was optimal for cell-imprint replica fabrication. The electrical resistance of the 3-CPC (0.3 wt %) was reduced by 68% when compared to N-CPC (0 wt %). This method successfully replicated the single cell with sub-organelle scale. The shape of microvesicles, grooves, pores, blebs or microvilli on the cellular surface was patterned clearly. This technique delivers a free-standing cell feature substrate. In vitro evaluation of the polymer demonstrated it as an ideal candidate for biomimetic biomaterial applications. This approach also finds its application in study based on morphology, especially for drug delivery applications and for investigations based on molecular pathways.

Surfactant-free galvanic replacement for synthesis of raspberry-like silver nanostructure pattern with multiple hot-spots as sensitive and reproducible SERS substrates

Using a galvanic replacement (GR) reaction in the absence of surfactant as a stabilizer, we report a robust synthesis of raspberry-like silver nanostructure pattern with multiple “hot-spots”. First, uniform copper nanodisk patterns were generated on Pt layer via nano imprint lithography and Ag+ ions were added for reaction. The Ag+ ions selectively oxidized Cu atoms, because of the reduction potential variance. The uncovered Cu surface promoted contact between Cu atoms and Ag+ ions to synthesize a sufficient quantity of silver nanostructures on copper nanodisks to generate multiple strong “hot-spots”. However, coating the copper nanodisk with surfactant impeded the progress of the GR reaction and failed to synthesize “hot-spots”. Due to metal bonding between the metal nanostructures and Pt layer, the synthesized silver nanostructure pattern preserved mono-dispersity and stability. We demonstrated applications for rapid detection of Rhodamine 6G (R6G) molecules by surface-enhanced Raman scattering spectroscopy (SERS) with low detection limit (10−12 M) and excellent signal uniformity (RSD < 8.3 ± 1.3%). Since surfactants have been considered as important pre-requisites in GR reactions for stability of nanostructures, the findings reported here offer a new opportunity for synthesizing various kinds of metal nanostructures via GR and expanding their applications.

Optimal Design of Narrow Line‐Width Front Contact Grid Pattern for Silicon Solar Cells and Low‐Cost Fabrication of Electroless Nickel Plated Imprint Lithography Hard Stamp

Herein, a systematic investigation on the design and development of a cost‐effective nickel hard stamp suitable for fabrication of a new front‐side metallization pattern to reduce the shadow losses in solar cells is demonstrated. Finite element analysis (FEA)–based simulations indicate an optimal finger width of ≈20 μm with interfinger spacing of 1000 μm which can effectively enhance solar cell efficiency by ≈1% due to reduced shadow loss. The optimal grid design is further patterned by means of nanoimprint lithography (NIL) followed by an electroless deposition method. A cost‐effective, electroless deposited nickel hard template is developed for NIL patterning using UV pattern transfer. To avoid the physical damages during the imprinting process and improve the durability of the NIL stamp, silane‐based antiadhesive coating is used which can withstand up to 18 cycles of imprinting process. The nickel hard stamp exhibits improved hardness of 5.63 GPa and roughness of 8 nm and is used to transfer the narrow‐line width patterns during the imprinting process. The proposed industry‐ready technology overcomes the limitations of the existing screen printing process pertaining to the formation of high‐aspect‐ratio narrow line‐width finger grid patterns.

Low-cost, durable master molds for thermal-NIL, UV-NIL, and injection molding

Mold cost and mold lifetime are essential concerns for mass production of micro/nano-patterned surfaces by nanoimprint lithography or micro/nanoinjection molding. Master molds are typically produced by subtractive processing using wafer-based clean room techniques. For imprint lithography, polymer copies of such molds can often be employed, but the durability of such molds is quite limited. The conditions of high temperature and pressure for injection molding require use of the durable masters created in stainless steel, nickel or other robust materials, but such approaches are challenged by the high cost of patterning these substrates and limited lifetime. Here, we report the fabrication of durable crystalline zirconium dioxide (ZrO2) masters via a simple direct imprint technique. ZrO2 nanoparticles (NPs) were formulated into an ink and imprinted on a variety of substrates using a solvent-assisted patterning technique and subsequently annealed to increase the mechanical durability of the mold. The hardness and modulus values of the ZrO2 coatings reached 11 ± 2 GPa and 120 ± 10 GPa, respectively after annealing. The hard ZrO2 mold was then employed for precision patterning of polymer surfaces by thermal and UV nanoimprinting lithography (NIL) techniques, and by injection molding. High fidelity pattern transfer continued throughout 115 000 injection molding cycles, there was no evidence of delamination, breakage or wear in the ZrO2 mold. Our simple imprint patterning technique using ZrO2 NPs inks enable us to fabricate robust molds with excellent thermal and mechanical properties as easily as imprinting simple polymer replicas. This simple and low-cost approach to mold preparation can enable a large variety of high throughput or large area nano-replication technologies.

Anodic Imprint Lithography: Direct Imprinting of Single Crystalline GaAs with Anodic Stamp.

Anodic imprint lithography patterns the GaAs substrate electrochemically by applying a voltage through a predefined anodic stamp. This newly devised technique performs anodic etching in a stamping manner. Stamps that serve as anodic electrodes are fabricated precisely, and the patterns can be imprinted continuously on GaAs substrates. The anodic current locally oxidizes the GaAs through the metal attached to the stamp, and the GaAs oxides are subsequently removed by an acid in the solution. The process is simplified because the metal catalyst is not left on the substrate and the use of an oxidizing agent is not required. Anodic imprint lithography integrates the lithography and etching steps without the use of a polymer resist. Predefined anodic stamps with fin, pillar, and mesh arrays clearly imprinted trenches, holes, and embossed disk arrays on the GaAs substrates, respectively. Anodic imprints replace photons and electrons in conventional lithography with electrochemical stamping, which can simplify existing techniques that are highly complex for extreme nanopatterning.

Broadband lightweight flat lenses for long-wave infrared imaging

We experimentally demonstrate imaging in the long-wave infrared (LWIR) spectral band (8 μm to 12 μm) using a single polymer flat lens based upon multilevel diffractive optics. The device thickness is only 10 μm, and chromatic aberrations are corrected over the entire LWIR band with one surface. Due to the drastic reduction in device thickness, we are able to utilize polymers with absorption in the LWIR, allowing for inexpensive manufacturing via imprint lithography. The weight of our lens is less than 100 times those of comparable refractive lenses. We fabricated and characterized 2 different flat lenses. Even with about 25% absorption losses, experiments show that our flat polymer lenses obtain good imaging with field of view of 35° and angular resolution less than 0.013°. The flat lenses were characterized with 2 different commercial LWIR image sensors. Finally, we show that, by using lossless, higher-refractive-index materials like silicon, focusing efficiencies in excess of 70% can be achieved over the entire LWIR band. Our results firmly establish the potential for lightweight, ultrathin, broadband lenses for high-quality imaging in the LWIR band.

A Study on the Parallel Patterning of Nickel Oxide thin Film for Liquid Crystal Display

I investigated the homogeneous liquid crystal alignment using parallel patterning on nickel oxide thin film. Nickel oxide thin film was prepared by sol-gel process, which is and cost effective method to form oxide thin film. Since the sol-gel process is solution driven method, the patterning on nickel oxide can be achieved by imprinting lithography. Nickel oxide with parallel pattern was used for liquid crystal alignment layer, which can be a alternative to conventional rubbing process to aligning a liquid crystal molecules. As a result, a high transmittance of 83.9% was confirmed for the NiO film, and a stable horizontal orientation pretilt angle of 0.2° occurred in the imprinted NiO film. These results were judged to be a positive level that can be commercialized in the horizontal alignment type liquid crystal display that is currently used universally. Anisotropic characteristics of nickel oxide induced by a parallel pattern leads to the alignment of liquid crystals.

Numerical Study on the Optimization of Roll-to-Roll Ultraviolet Imprint Lithography

Roll-to-roll ultraviolet (R2R-UV) imprinting is a low-cost and high-throughput method that includes the manufacturing of large-area functional films. However, the quality of the final product is obstructed by the bubble entrapment during the imprinting process. In this study, a multi-phase volume of fluid (VOF) numerical model was used to remove bubble entrapment during the R2R imprinting process, which covered all parameters. This new modified numerical model with open-channel boundary conditions was based on the single zone that contains the direct contact of UV resin with the imprinting mold during the filling process. In addition, this model simulated the UV resin filling into microcavities at the preceding and succeeding ends of the imprinting mold. Different patterns of imprinting mold were considered to enhance the fidelity of R2R-UV imprinting for the comprehensive analysis. The experimental results validated through numerical simulations revealed that the bubble entrapment can be controlled by varying various parameters such as speed of the imprinting system, viscosity, contact angles, and pattern shape. The proposed model may be useful for a continuous bubble-free R2R imprinting process in industrial applications that includes flexible displays and micro/nano-optics.

Fabrication of fluorescent oxygen gas-sensor probe module based on asymmetric 1 × 2 optical waveguides using UV imprint lithography

We proposed and developed an integrated fluorescent oxygen gas-sensor probe module based on asymmetric 1 × 2 optical waveguides that can transmit power from an optical source and the fluorescence signal with high efficiency simultaneously. The fluorescent oxygen gas-sensor comprises an optical source part, optical detector part, and optical sensing probe part. The optical sensing probe comprises asymmetric 1 × 2 optical waveguides coated with an oxygen-sensitive fluorescent film on the end face of the optical waveguide. This optical probe module can transmit the optical power and fluorescence signal with high efficiency because of the same optical path for transmission of the optical source and the fluorescence signal. In addition, this structure facilitates an integrated miniature oxygen-sensor module. We optimally designed the asymmetric 1 × 2 optical waveguides with an asymmetric structure and different two core size. Then, we fabricated the polymeric asymmetric optical waveguides using the UV imprint lithography process suitable for cost-effective mass production. The optical oxygen-sensor probe transmitted the optical source power and the fluorescence signal with 80% and 82% efficiency, respectively. An oxygen-sensitive fluorescent film was coated on the end face of the optical waveguide by using the spray coating method. The oxygen-gas detection sensor constructed using this optical probe module with 1 × 2 asymmetric optical waveguides could measure the concentration with 0.3% resolution for a gas concentration range of 0% to 25%. This optical oxygen-sensor probe module facilitates a compact, simple, and cheap measurement system.

Fabrication of integrated spectrometer module based on optical waveguide platform with planar nano diffraction grating using UV imprint lithography

We proposed a simple and attractive integrated polymeric spectrometer module based on a planar optical waveguide platform that can be easily manufactured at a low cost. The proposed integrated spectrometer module comprises a planar optical waveguide with a concave mirror, a planar nano diffraction grating with a non-uniform pitch, an optical fiber, and a photodiode array. The nano diffraction grating and optical waveguide platform were precisely fabricated using the UV imprint lithography suitable for mass production. To provide uniform diffraction efficiency in the wavelength range of 400–650 nm, the nano diffraction grating had a quasi-three-dimensional V-shaped groove with a blazing angle of 22.5° and non-uniform pitches, maximum of 0.406 μm and minimum of 0.393 μm. In addition, the optical waveguide platform focuses the optical source to the nano diffraction grating using the concave mirror. The alignment guide pattern is used to align the nano diffraction grating and optical fiber. Through the packaging process of nano grating device, photo diode array and fiber on the planar optical waveguide platform, we manufactured the polymeric integrated polymeric spectrometer module. We confirmed that the fabricated planar spectrometer module had a resolution of 5 nm or less for spectral bandwidth of 250 nm, and it can be used widely in portable spectrometer sensors.

Elastohydrodynamics of Roll-to-Roll UV-Cure Imprint Lithography

The inextensible cylindrical shell theory and lubrication theory combine into a model for the elastohydrodynamics of a rolling-imprint modality of nanoimprint lithography (NIL). Foil-bearing theory describes the formation of the lubrication gap due to relative motion between a tensioned substrate and a rigid, cylindrical surface. Reproduction of the results of foil-bearing theory for both stiff and perfectly flexible substrates validates this coupled model and reveals a highly predictable region of uniformity that provides low shear stress conditions ideal for UV-cure. These results show theoretical limitations that are used to construct an operating window for predicting rolling-mode NIL process feasibility.