Ultraviolet Imprinting Research Articles

Design and fabrication of a porous prism film for display backlight applications

This study demonstrates a fabrication method of a porous brightness enhancement film (pBEF) that offers brightness enhancement, light diffusion, color shift reduction, and improved thermal stability. During the ultraviolet imprinting and solvent evaporation processes, the nano/submicron-sized air pores are generated within the polymer prism structure, and micropatterns spontaneously form on the prism surface. The inner pores ranging from 30 to 450 nm can effectively scatter light to mitigate color shift, which is caused by multiple internal reflections within the prism structure. The micropatterns have multiple rings formed one around another with 5–15-µm diameter on the prism surface improve visual quality. Moreover, the obtained functions are achieved in a single film solution, obviating the need for using multiple materials, and the fabrication process is relatively simple and fast as it is conducted under ambient conditions. When the pBEF is integrated into a liquid-crystal display backlight, it provides the brightness enhancement performance and comparable viewing angle distribution of a regular BEF combined with an additional diffuser (two films) and increases brightness by ∼8% compared to a bead prism (particle-based BEF). Additionally, it reduces the redshift (Δxy) from 0.1605 to 0.1415. Furthermore, the pBEF exhibits a lower coefficient of thermal expansion than the regular BEF.

Fabrication of high-performance lens arrays for micro-concentrator photovoltaics using ultraviolet imprinting

Micro-concentrator photovoltaics (micro-CPV) is a cutting-edge CPV approach aimed at increasing the efficiency and reducing the cost and carbon footprint of solar electricity by downscaling concentrator solar cells and optics. The reduced size of micro-CPV provides several advantages over conventional CPV, including shorter optical paths and lower temperature and resistive losses in the cell, resulting in higher electrical efficiencies. This may increase the energy yield per area compared to conventional CPV or silicon modules. Cost reduction is achieved through material savings and the use of continuous manufacturing methods enabled by the tiny size of cells and optics, such as roll-to-roll (R2R) and roll-to-plate (R2P) ultraviolet (UV) imprinting for optics production. However, adapting these processes to large-area arrays of Fresnel micro-lenses with no wasted areas and high efficiency remains a challenge. In this study, we present a comprehensive methodology for the development of micro-CPV optics with full area coverage—from design and mastering to up-scaling, tooling, and replication. The methodology involves designing a non-rotationally symmetric elementary insert tailored to ultraviolet imprinting. Crucially, multiple inserts are originated via precision machining and recombined to form a single array master mold without wasted areas. The master is then replicated into a flexible working stamp for UV imprinting of Fresnel lens arrays, utilizing different UV curable materials. The functional characterization of the lenses demonstrates an optical efficiency of 80% at 178X under collimated white light, representing the highest effective concentration achieved using UV-imprinted Fresnel lenses. Furthermore, initial reliability tests confirm the absence of degradation during thermal cycling or outdoor exposure. This methodology paves the way for continuous high-throughput manufacturing of micro-lens arrays using R2R or R2P methods, presenting a significant step forward in micro-CPV.

High-performance transparent electromagnetic interference shielding film based on metal meshes

Transparent electromagnetic interference (EMI) shielding films have gained considerable attention for the commercialization of the 5G wireless technology based on electromagnetic waves in the GHz range. In this study, transparent EMI shielding films with embedded metal meshes on a 100 μm thick polyethylene terephthalate film for EMI shielding were fabricated using ultraviolet imprinting and Ag paste filling techniques. The various EMI shielding film types were fabricated by varying the width, aperture size, and height of the mesh electrode to evaluate the efficiency of the EMI shielding according to the incident electromagnetic wave frequency and morphology of the mesh metallic electrodes. The results indicate that the EMI shielding efficiency (SE) increased with a decrease in the aperture size of the metal mesh electrodes and an increase in their height. The average EMI SE values of the fabricated film in the 0.5–18 GHz range reached 48.3 dB and 59.6 dB at a light transmittance of approximately 90% and 77%, respectively. The fabricated EMI shielding films can be used for various applications, such as communications, aerospace, medical equipment, and military.

Portable UV nanoimprint lithography system using hydraulic pressure

Nanoimprint lithography (NIL) is the next-generation alternative to conventional photolithography involving an inexpensive and high throughput process. However, conventional NIL equipment requires a bulky air compressor which possesses a large area. This study aims to overcome this limitation by introducing a novel hydraulic ultra-violet (UV) imprint system. Hydraulic UV imprint system can be used to conduct the UV NIL process by using a simple, low-cost materials without needing complex equipment. To form various patterned layers, UV NIL was conducted with various master stamps.(diameter: micro-cones:3 μm, nanopillars:200 nm, nanoholes: 200 nm) The performance of the imprint system was verified by using scanning electron microscopy analysis of samples fabricated by new system. The results showed that the performance of the new imprint system was comparable to that of a conventional air-pressure UV imprint system.

Manufacture of Binary Nanofeatured Polymeric Films Using Nanosphere Lithography and Ultraviolet Roller Imprinting.

This paper describes the manufacture of binary nanostructured films utilizing nanosphere lithography and ultraviolet (UV) roller imprinting. To manufacture the binary nanofeatured template, polystyrene nanocolloids of two distinct dimensions (900 and 300 nm) were primarily self-assembly spun coated on a silicon substrate. A roller imprinting facility equipped with polydimethylsiloxane molds and ultraviolet radiation was employed. During the imprinting procedure, the roller was steered by a motor and compressed the ultraviolet-curable polymeric layer against the glass substrate, where the nanofeatured layer was cured by the UV light source. Binary nanofeatured films were thus obtained. The influence of distinct processing variables on the imprinting of nanofeatured films was investigated. The empirical data suggested that with appropriate processing conditions, binary nanofeatured plastic films can be satisfactorily manufactured. It also demonstrated that roller imprinting combined with ultraviolet radiation can offer an easy yet effective method to prepare binary nanofeatured films, with a miniatured processing time and enhanced part quality.

Transparent Electromagnetic Shielding Film Utilizing Imprinting-Based Micro Patterning Technology.

Utilization of methods involving component integration has accelerated, owing to the growth of the smart mobile industry. However, this integration leads to interference issues between the components, thereby elucidating the importance of the electromagnetic interference (EMI) shielding technology to solve such issues. EMI shielding technology has been previously implemented via the reflection or absorption of electromagnetic waves by using conductive materials. Nevertheless, to tackle the recent changes in the industry, a transparent and flexible EMI shielding technology is necessitated. In this study, a transparent and flexible EMI shielding material was fabricated by filling a conductive binder in a film comprising an intaglio pattern; this was achieved by using the ultraviolet (UV) imprinting technology to realize mass production. Subsequently, changes in the aperture ratio and shielding characteristics were analyzed according to the structure of the pattern. Based on this analysis, a square pattern was designed and a film with an intaglio pattern was developed through a UV imprinting process. Furthermore, it was confirmed that the transmittance, conductivity, and EMI shielding rate of the film were altered while changing the coating thickness of the conductive particles in the intaglio pattern. The final film prepared in this study exhibited characteristics that satisfied the required EMI shielding performance for electric and electronic applications, while achieving flexible structural stability and transparency.

Study on Preparation of Microlens Array Hydrophobic Flim by Ultraviolet Curing Micro-imprint

In this paper, the formation process of microstructure array hydrophobic film prepared by ultraviolet curing imprinting was studied. The UV-curing micro-imprinting method was used to replace the traditional hot stamping method to prepare the micro-structured hydrophobic film. The effects of illumination time and intensity on the surface hydrophobicity were investigated.Results shown that the micro-structure hydrophobic film prepared by UV-curing micro-imprint has high replication precision and good hydrophobic effect. With the increase of illumination time, the water contact angle of the product increased and reached to 125° at 30s illumination time. With the increase of the illumination intensity, the water contact angle of the micro-imprinted product was also gradually increased.

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 Line & Space Structure and Wiring on 3D Surface Features with Imprinting Process

In recent years, electronic devices made by polymers with wiring materials are widely used, such as FPC (Flexible Printed Circuit) in a PC (Personal Computer), and smartphones. Fine Pitch Wiring technologies on 3D objects are needed for coming IoT(Internet of Things) trends. Therefore, in this research, we conducted a fundamental study of the low-cost replication process with etching less wiring method in order to replicate microstructure on the curved polymer surfaces by using the hybrid nanoimprint technology. The first imprinting step was carried out to produce the intermediate mold on COP (Cyclic Olefin Polymer) surface by thermal nanoimprint process. The aluminum mold with a concavo-convex structure called L/S (Line & Space; concave width of 500 µm, concave depth of 100 µm) was used. The obtained intermediate structure was coated with UV (Ultraviolet) curable resin and it was applied to the 3D substrate (curved surface region: lens shape) having planar region and curved surface region as shown in Fig. 1. In this step, UV imprinting was applied with the pressure of 5 MPa and UV irradiation time of 300 seconds to transfer the L/S structure to the 3D substrate. Thereafter, the silver nanoink was dropped and filled in the concave portion of the L/S structure using a dispenser and sintered at 160 oC for 1 hour. SEM (Scanning Electron Microscope) image revealed that the 3D structure can be precisely transferred on the curved surface by using this hybrid nanoimprint technology (See Fig. 2). In Fig. 3, the transfer rates in the planar region (flat) and curved region were 100 and 60%, respectively. It is considered that the liquid UV curable resin may flow downward from the upper part to the lower part along the inclination of the substrate during the crimping and curing steps, resulting in the decrease in filling amount from the intermediate mold structure. The L/S patterned wired with silver nanoink is shown in Fig. 4. This result proposes that the hybrid nanoimprint technology is considered to be an effective method for the replication of wiring on the curved surface. Figure 1

Fabrication of a light-intensity-enhancement component by using computer-controlled ultraviolet curing and air-pressing imprinting

In this study, the ultraviolet (UV) curing and air pressure imprinting methods are proposed for the fabrication of a light-intensity-enhancement component. The air-pressing process provides a uniform embossing pressure, and the UV curing module enables the process to be performed at room temperature and low pressure. Because the UV resin is sensitive to the process parameters, such as the curing time and pressing pressure, the liquid resin must be filled at a precise pressure. To control the precision, the UV embossing facility comprised a resin-dispensing system, air-pressing system, and UV curing system. These systems were controlled by the Arduino system. In the Arduino system, the computer-controlled input can eliminate artificial errors and each forming step can be programed into one script to achieve automation. In this study, V-groove microstructures were formed. The V-groove pattern was replicated with a width of 47 μm and height of 22 μm on polymethyl methacrylate substrate for use as a light guiding panel. The proposed methodology enables automatic control of the UV microscale imprinting process.

Fabrication of a large-area, flexible micro-pyramid PET film SERS substrate and its application in the detection of dye in herbal tea

A simple method, based on a roll-to-roll ultraviolet micro-pyramid imprinting technique and a nanoparticle self-assembling process in aqueous solution, to fabricate a large-area, flexible surface-enhanced Raman scattering (SERS) polyethylene glycol terephthalate substrate is proposed. The SERS substrate is demonstrated to be of high sensitivity. The detection concentration of Rhodamine 6G (R6G) measured by a portable Raman spectrometer is down to 10−9 mol l−1. The relative standard deviation values of different spots and different substrates are less than 13%. In addition, the feasibility for rapid detection of dye in herbal tea based on this SERS substrate and a portable Raman spectrometer is investigated. Three industrial dyes are employed to simulate the dyeing process. It is presented that R6G of g ml−1, malachite green of 10−6 g ml−1 and Auramine O of 10−6 g ml−1 in herbal tea could be detected rapidly. The experimental results imply that this method could be potentially applied in the field of dyed herbal tea detection.

Filling angle and its effect on filling process in roll-to-roll ultraviolet imprint lithography

Bubble defects are a serious problem in the roll-to-roll ultraviolet imprint lithography process. To investigate the air bubble trapping in microcavities, a numerical simulation method is proposed based on a sliding mesh and open-channel model. Such considerations in the model make it possible to include both the rotation of the roller and the flow of resin, which is different from the conventional methods. Through analysis with our model, a parameter, termed filling angle here, is found to be a significant factor in the imprinting process. With this parameter, the relationships between resin accumulation and bubble defects are studied. The differences in resin filling at different rotation velocities are also discussed. The experimental observations are in good agreement with the theoretical calculations. The results provide a deep understanding of the filling process, which can provide good theoretical guidance to eliminating bubble defects in industrial applications.

Investigation of reflective performance for micro-pyramid arrays by roll-to-roll UV imprinting process

In recent years, devices with reflective performance have been broadly applied in many aspects of our lives such as flexible displays, e-paper, greenhouse temperature control, warning signs and so on. There are many reflective structures, and micro-pyramid, due to its merits of outstanding reflective performance (theoretically up to 66.7%) is an excellent choice and can be effectively fabricated via roll-to-roll (R2R) Ultra Violet (UV) imprinting process. However, possessing the properties of high depth-to-width ratio and sharp edges, micro-pyramid arrays are hard to make without some defects such as volume shrinkage and bubble defects during R2R process. This paper aims to explore the relationship between these defects and reflective performance. At first, the optical thin film with micro-pyramid arrays was fabricated, then the investigation of the relationship was carried out and a novel assessment criterion (equivalent radium,Re) is proposed to roughly characterize the negative relationship between structural defects and reflective performance. Meanwhile, the effects of substrate thickness and different processing methods on reflective performance have also been investigated. The relationship between reflective performance and substrate thickness exhibits periodicity, thus providing the instructions about the large-scale production of reflective films.

Embedded Ag Grid Electrodes as Current Collector for Ultraflexible Transparent Solid-State Supercapacitor.

Flexible transparent solid-state supercapacitors have attracted immerse attention for the power supply of next-generation flexible "see-through" or "invisible" electronics. For fabrication of such devices, high-performance flexible transparent current collectors are highly desired. In this paper, the utilization of embedded Ag grid transparent conductive electrodes (TCEs) fabricated by a facile soft ultraviolet imprinting lithography method combined with scrap techniques, as the current collector for flexible transparent solid-state supercapacitors, is demonstrated. The embedded Ag grid TCEs exhibit not only excellent optoelectronic properties (RS ∼ 2.0 Ω sq-1 and T ∼ 89.74%) but also robust mechanical properties, which could meet the conductivity, transparency, and flexibility needs of current collectors for flexible transparent supercapacitors. The obtained supercapacitor exhibits large specific capacitance, long cycling life, high optical transparency (T ∼ 80.58% at 550 nm), high flexibility, and high stability. Owing to the embedded Ag grid TCE structure, the device shows a slight capacitance loss of 2.6% even after 1000 cycles of repetitive bending for a bending radius of up to 2.0 mm. This paves the way for developing high-performance current collectors and thus flexible transparent energy storage devices, and their general applicability opens up opportunities for flexible transparent electronics.

Effect of resin accumulation on filling process in roll-to-roll UV imprint lithography

A numerical model based on the sliding mesh technique is proposed to study the effect of the accumulation of ultraviolet (UV) resin on the imprint quality for roll-to-roll (R2R) UV imprint lithography. In addition, an open-channel model is proposed to investigate the characteristics of UV resin accumulation during the filling process. The velocity vectors in the filling area when UV resin accumulation occurs were calculated, and the simulation results reveal that UV resin accumulation is beneficial for the escape of air bubbles during the filling process, thus improving the filling quality. Moreover, the UV resin accumulation is related to the imprinting speed, which determines the extent of completion of the filling process. Good agreement was observed between the simulation and experimental results. The presented method provides insights on UV resin accumulation during the filling process and is beneficial for parameter optimization in the R2R imprinting process.

Metal‐mesh based transparent electrodes using roll‐to‐sheet ultraviolet soft imprinting

Transparent conductive electrodes (TCEs) possessing a combination of high optical transmission and good electrical conductivity find applications in numerous optoelectronic devices. A low cost fabrication of a fine metal-mesh structure on rigid (glass) and flexible (polyethylene terephthalate – PET) substrates as a promising and feasible approach for fulfilling large size TCE requirements is proposed. A roll-to-sheet ultraviolet imprinting protocol to transfer microtrench structures using a flexible Polydimethylsiloxane stamp is utilised. The conductive silver ink is filled into the microtrench structures by controlling the processing parameters which include pressure, scraping angle and speed. The metal-mesh based glass and PET substrates show a transmission of about 90%, while the electrical resistance is as low as 6 Ω/□. Thus, this method can be utilised as an economically viable alternative to doped indium oxide in TCE applications.

Study on bubble defects in roll-to-roll UV imprinting process for micropyramid arrays II: Numerical study

Roll-to-roll (R2R) ultraviolet (UV) imprinting process is a very competitive technique for the large-area fabrication of micropyramid structures on polymer substrates. However, bubble defect is a serious problem that significantly deteriorates the performance of the optical films. The elimination of bubble defects in microstructures is pivotal to the successful application of optical films in various fields. Based on previous experimental results, a numerical model for the R2R UV imprinting of micropyramid structure was established to analyze the generation mechanism of the bubble defect in the micropyramid structure. The flow behavior of the resist in the microcavities was observed and discussed. The relationship between processing parameters and bubble defects in the R2R UV imprinting process was investigated, and the possible reasons for the difference in the filling of the micropyramid structures and the generation of bubble defect were presented and discussed. Also, the material inlet direction in accordance with the pyramid structure placement was considered and its influence on the filling process of the technique was analyzed. It was found that inlet direction has a significant impact on the filling condition of the pyramid structure. It could cause the difference between full and partial filling, and the difference of the shape and the location of bubble defects if they exist. These findings provide insights into the flow behavior of the resist material on microscale and shed light on what is causing the differences in the final filling condition of the micropyramid structures, which can be of good theoretical guidance to the industrial application of the technique.

Diamond turning of high-precision roll-to-roll imprinting molds for fabricating subwavelength gratings

Diamond turning of high-precision molds is a vital process for the roll-to-roll-based ultraviolet resin imprinting process in fabricating subwavelength gratings. The effects of the grating shape and grating period on diffraction efficiencies and diffraction angles were simulated. Experiments were then conducted to examine the effects of shape design, grating period, and cutting speed on machinability of the mold. According to the optical measurement results, the performance of the subwavelength gratings matched the design well at various incident angles. The results confirm that diamond turning of high-precision molds is a feasible approach for ensuring the continual mass production of subwavelength gratings.

The effect of mold materials on the overlay accuracy of a roll-to-roll imprinting system using UV LED illumination within a transparent mold

Although several studies on the roll-to-roll (R2R) imprinting process have reported achieving flexible electronics, improving the alignment accuracy in the overlay process of R2R imprinting is recognized as the biggest problem for the commercialization of this technology. For an overlay technique with high alignment accuracy, it is essential to develop a roll mold with high positional accuracy. In this study, a method for fabricating a roll mold with high positional accuracy is proposed by wrapping a thin glass substrate flexible mold around the transparent roll base, because it can provide higher mechanical strength and thermal stability than a conventional polymer substrate. To confirm the usability of the proposed process, the prepared roll mold was used to fabricate a test pattern of thin-film transistor backplane for a rollable display. The positional and overlay accuracy of the roll mold with the proposed thin glass substrate flexible mold were compared with the roll mold with a conventional polymer substrate flexible mold. Large-area transparent flexible molds with a size of 470 × 370 mm were fabricated by an ultraviolet (UV) imprinting process on thin glass and polyethylene terephthalate substrates, and these flexible molds were wrapped around a roll base of 125 mm radius through a precision alignment process. After an anti-adhesion treatment and the wrapping process, the roll mold with the polymer substrate showed a ~180 μm positional error, whereas the thin glass substrate showed a ~30 μm positional error. After the overlay process using the R2R imprinting system with the alignment system, an average overlay error of ~3 μm was obtained when the thin glass flexible wrapped roll mold was used, whereas a ~22 μm overlay error was obtained when the polymer substrate flexible wrapped roll mold was used.

Study on bubble defects in roll-to-roll UV imprinting process for micropyramid arrays. I. Experiments

Bubble defect is a major problem in roll-to-roll (R2R) ultraviolet (UV) imprinting of micropyramid arrays. It renders the structure incomplete and significantly downgrade the optical performance of light reflection ability. This study aims to investigate the influence of processing parameters on the generation of bubble defects in micropyramid arrays fabricated by R2R UV imprinting, and eliminate the existence of bubbles in the fabricated structures by choosing the right processing parameter values. Experiments were carried out on R2R UV imprinting equipment with polyethylene terephthalate substrate and a radical curing system UV resist. Three processing parameters in terms of mold temperature, web speed, and roller pressure were selected, and their impact on the generation of bubble defects in micropyramid arrays was systematically studied. Box-Behnken design method was adopted as the response surface method to design the experiments. The experiment results were measured by KEYENCE VK-X 200 laser confocal scanning microscopy. Two methods were proposed to assess the seriousness of bubble defects in the fabricated structures in the study: the percentage of bubble affected micropyramid arrays was used to assess the defect distribution among the fabricated micropyramid arrays, and the bubble diameter was used to assess the defect seriousness of a single micropyramid. It was found from the study that the processing parameters play important roles in affecting the bubble defect generation in the micropyramid arrays, and possible explanations for their influence were provided in the paper. Also, it was found that it is possible to manufacture micropyramid arrays free of bubble defects using R2R UV imprinting technique through selecting processing parameters within the process window: the web speed between 0.5 and 0.7 m/min, the pressure between 4 and 5 kg/cm2, and the mold temperature between 57 and 65 °C.