UV-NIL Process Research Articles

Roll-to-roll nanoimprint lithography of high efficiency Fresnel lenses for micro-concentrator photovoltaics

Roll-to-roll nanoimprint lithography (R2R-NIL) is an enabling technology for the low-cost mass production of high-quality micro- and nano-sized optical elements. Particularly, the fabrication of Fresnel lenses using R2R-NIL is a promising approach to produce optical arrays for micro-concentrator photovoltaic modules. This work investigates the application of a continuous R2R imprinting process based on ultraviolet curing of transparent photopolymer resins (UV-NIL) to fabricate high-efficiency and low-cost Fresnel lenses. The morphological attributes and the related optical performance of the lenses fabricated using roll-to-roll UV-NIL on flexible PET sheets yielded optical efficiency values up to ∼ 69% at a concentration ratio of 178X, whereas a value of ∼ 77% was obtained for the UV-NIL batch processed on a flat rigid substrate. Further improvement of the optical efficiency has been achieved by adding moth-eye inspired antireflective (AR) features on the side opposite to the Fresnel motifs via a double-sided R2R UV-NIL process. The process developed paves the way for cost-effective mass production of high-efficiency Fresnel lenses for micro-concentrator photovoltaics.

High-quality nano structures fabrication on organic/inorganic hybrid thin films by using UV nanoimprint lithography

In this study, UV nanoimprint lithography (NIL) was conducted with a hybrid mixture of tin oxide and a UV-curable polymer to fabricate 1-D nanostructures on a thin film. It was confirmed through chemical and physical surface analyses that a hybrid thin film with high-resolution nanostructures had been successfully fabricated. In particular, nanostructures with an amplitude of 49.354 nm and a wavelength of 0.768 μm, which are the most similar to the mold used, were achieved at 3 min UV exposure time. Afterward, they were applied to a liquid crystal (LC) device as an alignment layer, with which uniform and homogeneous LC alignment were achieved due to the anisotropic characteristics and geometric restrictions caused by fabricated nano structures. In addition, a high thermal budget was obtained due to the thermal endurance of the hybrid thin films with an LC alignment performance of a UV exposure time of 3 min. Furthermore, the electro-optical performance under this condition was superior to LC devices with conventional rubbed polyimide layers From the results, it was determined that using the UV-NIL process on a hybrid mixture of tin oxide and a UV-curable polymer created high-resolution 1-D nano structures on films that can be used this as LC alignment layers with strong potential for high-quality LC applications.

Continuous Fabrication of Highly Conductive and Transparent Ag Mesh Electrodes for Flexible Electronics

Flexible transparent conductive electrodes are essential components for flexible electronics and more functions are combined in such components for miniaturization and design flexibility. The embedded metal mesh, due to the merits of good transparency, conductivity, and flexibility, is regarded as a promising candidate for transparent conductive electrodes. This study reports a continuous fabrication approach of roll-to-roll ultraviolet-nanoimprint lithography (R2R UV-NIL) for the large-area fabrication of embedded Ag mesh electrodes on polyethylene terephthalate (PET) substrates. It has the potential to replace the vacuum-based metal deposition process in flexible transparent electrodes fabrication for large-area, high-throughput, and low-cost production. A R2R UV-NIL system is developed that consists of microchannels patterning stage, doctor blading stage, and Ag residual layer removing stage. A nickel mold for the R2R UV-NIL process is theoretically designed and practically developed. A prototype of embedded metal mesh has been fabricated using water-based, nanosilver paste, and the electrical and optical performance have been further improved by wet etching. The optical transmittance and the sheet resistance are 82.0% and 22.1 Ω/sq, respectively. The electrical and optical performance is excellent with a high ${\sigma _{{\rm{dc}}}}/{\sigma _{{\rm{opt}}}}$ value of 81. It is demonstrated that the R2R UV-NIL process is feasible for the large-area fabrication of embedded Ag mesh for flexible transparent electrodes.

Optimization and continuous fabrication of moth-eye nanostructure array on flexible polyethylene terephthalate substrate towards broadband antireflection.

Reflection loss can cause harmful effects on the performance of optoelectronic devices, such as cell phones, notebooks, displays, solar cells, and light-emitting diode (LED) devices. In order to obtain broadband antireflection (AR) properties, many researchers have utilized surface texture techniques to produce AR subwavelength structures on the interfaces. Among the AR subwavelength structures, the moth-eye nanostructure is one of the most promising structures, with the potential for commercialization in the near future. In this research, to obtain broadband AR performance, the optimization of moth-eye nanostructures was first carried out using the finite difference time domain method within the spectral ranges of 400-800nm, including the optimization of shape, height, pitch, and residual layer thickness. In addition, the continuous production of moth-eye nanostructure array upon a flexible polyethylene terephthalate substrate was demonstrated by using the roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) process and anodic aluminum oxide mold, which provided a solution for the cost-effective fabrication of moth-eye nanostructure array. The AR performance of moth-eye nanostructure array obtained by the R2R UV-NIL process was also investigated experimentally, and good consistence was shown with the simulated results. This research can provide a beneficial direction for the optimization and cost-effective production of the moth-eye nanostructure array.

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.

Fabrication of Moth-Eye Nanostructure Arrays Using Roll-to-Roll UV-Nanoimprint Lithography With an Anodic Aluminum Oxide Mold

Broadband antireflection is essential for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. The moth-eye nanostructure is one of the most promising structures with potential for commercialization in the near future and roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the commercialization due to its high speed, large area, high resolution, and high throughput. In this study, the anodic aluminum oxide (AAO) mold is used to produce the moth-eye nanostructure arrays upon a flexible polyethylene terephthalate substrate combining with R2R UV-NIL process, which provides a solution for continuous production of moth-eye nanostructure arrays cheaply. In addition, the influence of mold parameters and process parameters on the forming quality were investigated, respectively. The influence of process parameters was investigated by using the one-variable-at-a-time method, including feeding speed, imprinting pressure, and mold temperature. The qualitative and quantitative height evaluation for the moth-eye nanostructure arrays were carried out based on the atomic force microscope images. As a result, the feasibility is verified to produce moth-eye nanostructure arrays continuously with AAO mold and R2R UV-NIL technique. A rapid feeding speed of 20 m/min was possible to produce good moth-eye nanostructure arrays with mold 1, which were 300 nm in diameter, 300 nm in height. However, different forming results were observed as the mold parameters decreased including unsatisfactory demolding phenomenon and incomplete filling phenomenon. The incomplete filling phenomenon was improved to a certain extent by changing the process parameters and higher forming height was accomplished with lower feeding speed, higher imprinting pressure, and higher mold temperature. This research is beneficial to provide a direction for the industrial production of the moth-eye nanostructure arrays.

Application of thermal initiator for characteristic improvements of polymeric replica mold for UV nanoimprint lithography.

We report the improvement of the hardness and modulus properties in a silsesquioxane-based soft replica mold by adding thermal initiator, without deteriorating the UV transmittance at the wavelength of 365 nm. It is found that thermal initiator used for this work contributes to improving the hardness and modulus values up to 0.175 and 3.585 GPa, while the UV transmittance value is still above 75%. The optimized soft replica mold built on a flexible plastic substrate allows submicron-scale patterns to be transferred onto a rigid Si substrate by means of UV-NIL process. Consequently, we demonstrate that the developed soft replica mold can be a suitable replacement for typical hard molds, promising further use in mold-based nanolithography for the fabrication of high-resolution nanopatterns over large areas.

Injection Compression Molding of Replica Molds for Nanoimprint Lithography

As a breakthrough in the cost and durability of molds for nanoimprint lithography (NIL), replica molds are fabricated by injection compression molding (ICM). ICM is commonly used for optical disks such as DVDs or Blu-ray disks and is also a practical fabrication method for nanostructures. In this paper, I successfully demonstrated the fabrication of cycloolefin polymer replica molds with structures smaller than 60 nm by ICM. Furthermore, ultraviolet (UV)-NIL using these replica molds was demonstrated. UV-cured resist was replicated over an area of 60 mm diameter. The degree of replication by UV-NIL in the first usage of each replica mold had good repeatability. Because ICM is a high-throughput, low-cost process, the replica mold can be disposed of after a certain time for UV-NIL. This method leads to a high-integrity UV-NIL process of patterned media because multiple large-area replica molds can be fabricated simultaneously.

Nanoimprint Lithography of 20-nm-Pitch Dot Array Pattern Using Tone Reversal Process

The nanoimprint lithography (NIL) of a hexagonal dot array pattern with 20 nm pitch was demonstrated using a tone reversal process. The dot array was formed by the self-assembled polystyrene–poly(dimethylsiloxane) (PS–PDMS) diblock copolymer. The dot pattern was transferred to a hole pattern on the imprint resist layer by a UV-NIL process. The hole pattern was filled with spin-on-glass (SOG). By removing the imprint resist matrix, the SOG dot pattern was formed as a final mask layer. The surface tension of the imprint resist was adjusted to achieve high-quality pattern transfer and demolding. The standard deviation of the diameter and pitch of the dot pattern suffered about 1% drop through the UV-NIL and tone reversal process.

A comparative study based on optical and electrical performance of micro- and nano-textured surfaces for silicon solar cells

In this contribution, we report on the optical and electrical properties of textured silicon surfaces usually incorporated in solar cell fabrication. In order to study the influence of different texturisation processes, nano-texturing was accomplished by soft UV-NIL process while the micro-textured surfaces were prepared using TMAH solution. The nano-textured surfaces were found to be superior in terms of optical performance but the passivation of these structures was solely limited by the fabrication process itself. Any defect or damage caused during the pattern transfer (plasma etch) process acts as a recombination centre which significantly reduces carrier lifetime. By systematically removing the plasma induced damage from the surface, it was possible to improve the effective lifetime from 24.4μs to 1092μs. We report that, despite of the enlarged surface area compared to micro-textured surface, our nano-texture surface possess excellent optical properties and higher lifetime.

Macro-optical inspection method for deterioration evaluation of release-coated mold surfaces for nanoimprint lithography

Nanoimprint lithography (NIL) is a very useful technique for the fabrication of nanopatterns, and it is used in some practical applications. The key issue for mass production is the understanding of the lifetime and deterioration of the release coating on the mold surface, because the adhesion of polymer materials on the mold surface results in defective products. However, a method for inspecting the durability and evaluating the release layer on the mold surface has not yet been established. This paper describes a method for inspecting the deterioration of the release-coated mold surface using the macro-optical inspection method, which is based on the detection method using edge-reflected light. This macroinspection enables the evaluation of the nonuniformity of the surface and local defects on the macroscale (from 0.1 to several millimeters), because the edge-reflected light is very sensitive to undulations in the surface morphology. In order to examine the deterioration of the release-coated mold surface, a silicon mold with 220 nm diameter holes and a fluorine release agent were used. For the release agent treatment, the mold was first dipped into 0.1% release agent for 30 min. Then, the mold was removed from the release agent and baked at 100 °C for 3 min on a hot plate, after which several undulations were observed. After three such treatments, the overcoating of the release agent caused a lot of defects such as water marks, stripes, and coating nonuniformities. Thus, the macroinspection method enables a facile understanding of the differences in such surface undulations. In order to evaluate the lifetime and deterioration, the UV-NIL process was carried out 100 times. The results showed that the macroinspection method reveals the thin-release-layer behavior during the UV-NIL process and the deterioration and undulation of the release layer.

Digital spectrometer-on-chip fabricated by step and repeat nanoimprint lithography on pre-spin coated films

Digital planar holograms (DPH) are successfully fabricated for the first time by nanoimprint lithography (NIL). A state-of-the-art UV-NIL process is developed by combining both advantages of step and repeat technology at low pressure for high-throughput and imprinting of spin-coated resist films for easy pattern transfer. The nanofabrication process is used in order to replicate DPH structures, composed of multi-million lines grating, into silicon dioxide waveguide core. Optical performances of nanospectrometers chips are in agreement with simulations.

Impact of exposure doses on demolding process in UV nanoimprint lithography

The impact of exposure doses on the demolding process in UV nanoimprint lithography (UV NIL) was studied. The PAK-01 resist was examined to investigate the dependence of the conversion ratio and elastic modulus on the adhesion force between the mold and the resist for various UV dosages. Also, adhesion forces between the resist and SiO2/Si mold were measured in various dosages. The results show that the adhesion force is unstable until the elastic modulus of the resist is enlarged enough even if the chemical conversion is completed by UV exposure. On the other hand, the demolding in the actual UV NIL experiment is not successful at lower dosages. Based on these evaluations, a suitable exposure dosage is suggested for successful demolding to realize a high throughput UV NIL process without demolding defects.

Heat transfer analysis during a curing process for UV nanoimprint lithography

A heat transfer analysis during the curing process in UV-Nanoimprint lithography was carried out. To imprint nano/micro patterns into a large-area target glass such as LCD panels, a mold with a poly-urethane-acrylate layer is often used, on which layer the micro/nano patterns are inscribed for the UV-NIL process. After UV resin is coated between the target glass and the flexible mold, the UV resin is cured by exposing UV light on the resin. In the curing process, heat from the phase change of the resin and the radiation by UV lamp would induce a temperature change and thermal distortion of the mold. In this study, we measured the temperature change of the flexible mold, and established an analytic model of the heat transfer. From the result, we derived the thermal properties of the PUA layer, and a thermal resistance layer between the PUA and the cured resin layer.

3D structures for UV-NIL template fabrication with grayscale e-beam lithography

The individual steps in fabrication of templates for UV-NIL processes are described. After spin coating a conductive copolymer (ESPACER 300) on top of the resist, insulating substrates have been structured by use of electron beam lithography at 20 keV beam energy. A three-dimensional (3D) pattern has been created in a low contrast positive tone resist PMMA 35k. By RIE in a CHF 3 – O 2 – process, the pattern has been transferred into the quartz substrate. Finally, the 3D structures have been replicated in a UV-NIL process.

Sub-100nm Hybrid Stamp Fabrication by Hot Embossing

The fabrication of nano-structured materials using nanoimprint lithography has become more prevalent in recent years, due to its cost effectiveness and readiness. However, One of the biggest drawback of this technique is the fabrication of the imprinting stamp, which is expensive and difficult to fabricate. This paper describes a method of replication original Si or quartz made imprinting template into a polymer stamp which has many advantages, such as the simplicity and low cost of the fabrication process and the flexibility of the resulting stamp. Using the hot embossing method, PVC based imprint stamp with sub 100nm patterns can be fabricated. Due to its high UV transmittance, reasonable mechanical hardness and low surface energy, PVC based nanosized template can be used as a stamp for UV-NIL and sub 100nm patterns were successfully transferred by the UV-NIL process with PVC based imprint stamp.

High-throughput step-and-repeat UV-nanoimprint lithography

UV-NIL has shown promise as new disruptive technology to displace photolithography in patterning nano features. This method is attractive because of their high-throughput with easy operation at room temperature and low pressure with low-cost. In this paper, a new UV-NIL process is proposed to apply a large-area stamp to a high-throughput step-and-repeat process at atmospheric conditions, utilizing additive gas pressurization and an elementwise patterned stamp (EPS), which consists of elements separated by channels. With the proposed method, only four imprints were required to press an 8 in. wafer. EPS features measuring 50–80 nm were successfully transferred onto the wafers. The experiments demonstrated that a 5 × 5 in.2. EPS could be used with a step-and-repeat UV-NIL process to imprint 8 in. wafers under atmospheric conditions.

Comparison of multilayer stamp concepts in UV–NIL

Template fabrication is one of the crucial issues in nanoimprint lithography. In this paper three different multilayer stamp concepts are compared that allow the imprinting of high resolution pattern on wafer scale with one imprint step only. Fabrication recipes as well as an analysis of local and global deformation, resolution and area homogeneity in an UV–NIL process are given. Advantages and disadvantages of the concepts are discussed.