Liquid Transfer Imprint Lithography Research Articles

System for Fabrication of Large-Area Roll Molds by Step-and-Repeat Liquid Transfer Imprint Lithography.

The effective production of nanopatterned films generally requires a nanopatterned roll mold with a large area. We report on a novel system to fabricate large-area roll molds by recombination of smaller patterned areas in a step-and-repeat imprint lithography process. The process is accomplished in a method similar to liquid transfer imprint lithography (LTIL). The stamp roll with a smaller area takes up the liquid resist by splitting from a donor substrate or a donor roll. The resist is then transferred from a stamp roll to an acceptor roll and stitched together in a longitudinal and, if necessary, in a circumferential direction. During transfer, the nanostructured resist is UV-exposed and crosslinked directly on the acceptor roll. The acceptor roll with the stitched and recombined stamp patterns is ready to be used as a large-area roll mold for roll-based imprinting. A system for this purpose was designed, and its operation was demonstrated taking the example of an acceptor roll of 1 m length and 250 mm diameter, which was covered by 56 patterned areas. Such a system represents an elegant and efficient tool to recombine small patterned areas directly on a large roll mold and opens the way for large-area roll-based processing.

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.

Large area patterning of residue-free metal oxide nanostructures by liquid transfer imprint lithography

One-dimensional (1D) and three-dimensional (3D) residue-free metal oxide patterns are directly fabricated over large areas using liquid transfer imprint lithography (LTIL) with an ultraviolet-curable metal oxide precursor resist. A 1D line or pillar array of metal oxides nano-patterns without a residual layer is formed by LTIL and annealing processes. A 3D layer-by-layer nanomesh structure is successfully constructed by repeating the LTIL method without a complex etching process. In addition, it is possible to form a hierarchical structure in which zinc oxide nanowires are selectively grown on a desired zinc oxide (ZnO) seed pattern formed by LTIL via a hydrothermal method. Unlike the pattern fabricated by the conventional nanoimprint lithography method, in the case of the pattern formed by LTIL the residues accumulated between the patterns during the patterning procedure can be removed, and thus it is possible to easily form various types of nanostructures.

Fabrication of the antireflection structure on aspheric lens surface and lens holder

The properties of optical lenses deteriorate due to reflection in the lens surfaces. Therefore, there is an increasing demand for an antireflection structure (ARS). To solve this problem, lens surfaces are designed based on a moth-eye structure, which is a type of ARS. In our previous study, we have successfully fabricated an ARS on a spherical lens such as a convex lens by using the roll-press method, which consists of a liquid transfer imprint lithography (LTIL) process. LTIL is the most effective process for producing a thin residual layer. The most ideal construction of the residual layer is less than a quarter of the wavelength of the incident light. When the incident light is within the wavelength of visible light (400–700nm), the ideal condition is less than 140nm. In this case, a flexible mold was used in the process of fitting the spherical lens with a mold. However, when the substrate is more complex, such as an aspheric lens or an aspheric lens with groove area, the aforementioned roll-press method is not suitable because the mold and the transfer substrate do not fit well together. As the solution, we prepared the reverse type replica lens, the shape of which is reverse of an aspheric lens. This reverse lens was fabricated the ARS, and used the reverse type replica mold to transfer an aspheric lens surface and holder part. Additionally, the reverse type replica mold was fabricated using the flexible replica mold. Further, the flexible replica mold completely fit with the lens groove area, so it was possible to fabricate the ARS on the reverse type replica lens. In this study, the process of fabricating the ARS on the aspheric lens surface and the holder part was investigated. We successfully developed a process of fabricating the ARS with a uniform height of more than 200nm and a thin residual layer of less than 80nm in all parts. Furthermore, this process achieved a low reflectance rate of around 0.2% at the wavelength of visible light. Thus, this process would suitable for fabricating ARS on more complicated substrates, such as double-sided aspheric lenses.

Fabrication of calibration plate for total internal reflection fluorescence microscopy using roll-type liquid transfer imprint lithography

A strong demand exists for the visualization of the fluid flow in microchannels, such as those in labs-on-a-chip. Multilayer nanoparticle image velocimetry (MnPIV) is a promising technique for this purpose, wherein the x- and y-positions of a nanoparticle are measured using a camera and its z-position is estimated via its fluorescence intensity. To accurately estimate the z-position, however, MnPIV requires a fluorescence intensity calibration process. Water is typically used in the microfluidic devices and a nanoscale level with a refractive index matching method is conventionally used for the calibration of MnPIV. Therefore, a calibration plate with nanoscale levels whose refractive index is matched to water is necessary to calibrate the z-position of fluorescent particles in water. In this study, a fabrication technique of the calibration plate was performed using ultraviolet (UV) nanoimprint with a UV-curable resin, whose refractive index is matched to water and which possesses a high viscosity of 2900cps. Although the residual layer thickness (RLT) of the level must be less than 100nm to perform MnPIV, it is challenging to reduce the RLT with such a high viscosity of the resin using conventional UV nanoimprint lithography. To reduce the RLT, therefore, in this study roll-type liquid-transfer imprint lithography (LTIL) was used with the high-viscosity UV-curable resin, where a two-fold repetition of the roll-type LTIL process demonstrated a calibration pattern RLT of 40nm or less. As a result, calibration of the MnPIV system was achieved with the calibration plate developed herein and fabricated by UV nanoimprint lithography.

Growth of ZnO nanowires on multi-layered polymer structures fabricated by UV liquid transfer imprint lithography

Among various growth processes, hydrothermal synthesis has been widely employed to produce ZnO nanowires because of its simplicity and cost effectiveness. However, the successful hydrothermal synthesis of ZnO nanowires on multi-layered polymer structures has been limited due to growth inhibitors in the polymer. For example, alpha-hydroxy ketone resolved in a UV-curable polyurethane acrylate (PUA) resin forms carboxyl groups (COOH) consisting of hydroxyl groups (OH) on the α-position of a carbonyl group (CO) during the UV-curing process. The carboxyl groups suppress the nucleation and growth of ZnO nanowires. In this paper, multi-layered polymer structures were fabricated by liquid transfer imprint lithography (LTIL), and the enriched carboxyl groups on the UV-cured PUA structures were pre-treated with hexamethylenetetramine (HMTA) solution to reduce the inhibitive influence of the carboxyl groups. The HMTA pre-treatment enabled ZnO nanowires to grow hydrothermally on multi-layered PUA structures despite the initial presence of carboxyl groups.

Metallic Antireflection Structures Made from Silver Ink by a Liquid Transfer Imprint Lithography Technique

Metallic Antireflection Structures Made from Silver Ink by a Liquid Transfer Imprint Lithography Technique

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.

Stacking of nanoscale metal dot array using liquid transfer imprint lithography with roll press

The stacking of metal nanodot patterns is useful for plasmonic devices and metamaterials. Metal nanodot patterns are conventionally fabricated using a combination of lithography and a metal lift-off process, which is time consuming and complex. On the other hand, there is demand for an expanding the patterning area of the metal nanodot. Therefore, the fabrication method with high throughput is required. For more efficient fabrication, we have developed a novel stacking technique that combines ultraviolet nanoimprint lithography (UV-NIL), contact printing, and liquid transfer imprint lithography (LTIL). UV-NIL was used to obtain a replica mold with a nanoscale hole pattern, and the metal layers were deposited onto the mold by using a vacuum evaporator. Contact printing removed a metal layer on the mold surface, leaving a metal nanodot array in the holes of the replica mold. This metal layer was transferred by UV-NIL, and LTIL with a roll press was employed to reduce the thickness of the UV-curable resin; the excess resin was split from the mold in the liquid phase. The replica mold with the metal dots and an intermediate layer was then transferred to a polyester film. In this study, this process was repeated to obtain five stacked layers. The stack consisting of gold nanodots displayed a red plasmonic color, whereas the stack of silver nanodots displayed a blue plasmonic color. Scanning electron microscopy was used to confirm the stacked structures through tilted angle and cross-sectional view images. The advantages of this process are that nanodot patterns can be fabricated using a mold-based process and that the thickness of the stacking layer can be controlled by LTIL. This process can therefore be used for high-throughput fabrication of stacking metal devices such as plasmonic memory.

Removal of residual layer by liquid transfer imprint lithography using roll-to-roll UV-NIL

Nanoimprint lithography (NIL) is a very useful tool for the fabrication of nanoscale pattern devices. In particular, roll-to-roll (RtR) ultraviolet (UV) NIL offers a high-throughput transfer of nanoscale structures. However, RtR UV-NIL is a film based process, and so the residual layer tends to be thick. To control the residual layer thickness, we have attempted to implement RtR liquid transfer imprint lithography (LTIL). LTIL can remove excess resin by splitting the mold and substrate in the liquid phase. In this case, excess resin remains on the substrate, and the mold with a thinner resin is transferred to another target substrate and cured. This LTIL process is also possible during RtR motion by controlling the nip pressure and UV exposure timing. That is, the excess resin is split off from the feed film during one rotation, and UV exposure to cure the resin is carried out in the next rotation. We prepared a roll mold with adhered replica molds for this process, and we show that the residual thickness with LTIL is thinner than that without LTIL. In addition, heating and two rounds of LTIL are effective for obtaining a thin residual layer. A roll mold temperature of 70°C, feed speed of 1.8m/min, and two rounds of LTIL produced no-residual-layer transfer of nanoscale structures onto the film. RtR LTIL will contribute to the continuous, high-speed transfer of nanoscale patterns that have a thin residual layer.

1305 Fabrication of Silicon Mold for Eight-step Diffractive Optical Element using Liquid Transfer Imprint Lithography

1305 Fabrication of Silicon Mold for Eight-step Diffractive Optical Element using Liquid Transfer Imprint Lithography

Fabrication of hybrid structures using UV roll-typed liquid transfer imprint lithography for large areas

Recently, nanoscale and micro-scale hybrid structures have become the focus of significant research interest due to their potential for use in anti-reflection films, anti-fingerprint films, solar cells, nanofluidic and microfluidic channels, and some functional optical films. Moreover, the nanopatterning lithography systems using roll-typed stamps have become increasingly appealing technologies for undertaking mass production and for continuous fabrication of hybrid patterns on large area substrates. In this study, we propose a roll-typed liquid transfer imprint lithography (R-LTIL) system and process for fabrication of nanoscale and microscale hybrid patterns on flat and micro lens substrates. Using the proposed system, nanoscale patterns that are 350nm hole and 250nm in height are flawlessly transferred to 6in. Si wafer and microscale lens substrates.

Liquid transfer imprint lithography: A new route to residual layer thickness control

In this study, an extension of the soft UV nanoimprint process is presented with improved control of the residual layer thickness and significant reduction of the nanoimprint proximity effect. The process is based on the consecutive halving of the liquid resist layer by a liquid transfer process. In the initial stage, this liquid transfer process uses a thick initial resist layer to ensure complete filling of the stamp cavities. The thick residual layer is then thinned down to about half by peeling off the applied imprint stamp, a process that can be repeated until the desired residual layer thickness is achieved. The information carrying layer remaining on the stamp can be transferred conformally to any substrate even with nonplanar surfaces. The fabrication of silicon photonic waveguides and photovoltaic antireflection textures are two applications where the advantage of this process becomes particularly clear.