Ultraviolet Nanoimprint Lithography Process Research Articles

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.

Investigation on the neighbouring effect of filling micro‐ and nanoscale cavities in ultraviolet nanoimprint lithography

Micro-/nanoscale patterns are popular in many applications, and several techniques to transfer multiscale patterns have been developed. Ultraviolet nanoimprint lithography (UV-NIL) is the most promising method to quickly produce complex structures, and there is a need to understand the resist-flowing and cavity-filling processes because partial cavity filling may cause defects. One crucial element of a stamp is the layout of the multiscale cavities. They can induce some difficulties during the filling process. Five different stamps with various micro-/nanoscale features (one nano feature, one micro feature, one nano feature, and one micro feature, two nano features and one micro feature) were employed in this simulation study. The imprint results show that the neighbouring effect has major issues in the microscale cavity filling and minor issues in the nanoscale. The layout of complex cavities results in poor filling proportions, and longer imprint times and a symmetrical layout were needed to improve the imprint quality. The contact pressure between the stamp and the resist generally declined when the location approached the centre from both sides of the stamp. The overall physical performance of neighbouring micro-/nanoscale cavities was obtained, and this is essential for further understanding and improvements in the UV-NIL process.

Ultra-violet nanoimprint lithography-compatible positive-tone electron beam resist for 3D hybrid nanostructures

A fabrication technique to produce a 3D hybrid structure having different scale complex patterns is studied. A fabrication method is proposed and an ultra-violet nanoimprint lithography (UV-NIL) compatible positive-tone electron beam (EB) resist is prepared. The resist can be shown to have microstructures and nanostructures on the same surface by UV-NIL and positive-tone EB lithography (EBL). The resist can behave as a positive-tone EB resist after the UV-NIL process, exhibits a high UV-NIL and EBL resolution, and a sensitivity sufficient to fabricate 3D hybrid nanopatterns with a high aspect ratio. The fabrication of a micro lens array with nano anti-reflection structure is then performed. The resist can accurately duplicate a microscale concave lens array with a diameter of 3.8 μm. Nano anti-reflection structures are then directly fabricated by EBL on the lens array. The positive-tone dot pattern, as anti-reflection structures with diameters of 220 nm, are fabricated with an EB dose of 1000 μC/cm2. UV-NIL is used to duplicate the fabricated sample. The resist products can successfully be used as the master mold for UV-NIL. A convex micro lens array, with a 200 nm anti-reflection structure, is obtained. These methods using the resist can provide complex 3D hybrid nanostructures. The usefulness of this method and resist are discussed.

Long-term analysis of PV module with large-area patterned anti-reflective film

Due to the PV modules are exposed to the external environment, it is necessary to clean or replace the protective glass upon contamination. In this study, we demonstrate a large-area and flexible anti-reflective film with moth-eye pattern fabricated by using a highly straightforward process: nanoimprint lithography. In addition, the fabricated anti-reflective film exhibits self-cleaning capability owing to the hydrophobic self-assembled-monolayer-coated surface. The fabricated anti-reflective film was attached to the surface of photovoltaic (PV) modules and placed outdoor to conduct a seven-month field test in order to observe the increase in the conversion efficiency of the PV modules as a result of the effect of the anti-reflective film. Two types of patterns, moth-eye pattern and micro-cone pattern, were used as the anti-reflective patterns; these were conveniently fabricated by the ultraviolet-nanoimprint lithography process, which does not use vacuum or high-temperature process. In the seven-month analysis, the moth-eye patterned film was observed to exhibit a transmittance that is 5.0% higher than that of the flat film, whereas the PV module with the moth-eye patterned film on its surface exhibited a conversion efficiency that is 2.85% higher than that of the PV module with the flat film.

Fabrication of flexible silver nanowire conductive films and transmittance improvement based on moth-eye nanostructure array

Transparent conductive electrodes (TCEs) are widely used in optoelectronic devices, such as touch screens, liquid-crystal displays and light-emitting diodes. To date, the material of the most commonly used TCEs was indium-tin oxide (ITO), which had several intrinsic drawbacks that limited its applications in the long term, including relatively high material cost and brittleness. Silver nanowire (AgNW), as one of the alternative materials for ITO TCEs, has already gained much attention all over the world. In this paper, we reported a facile method to greatly enhance the transmittance of the AgNW TCEs without reducing the electrical conductivity based on moth-eye nanostructures, and the moth-eye nanostructures were fabricated by using a roll-to-roll ultraviolet nanoimprint lithography process. Besides, the effects of mechanical pressure and bending on the moth-eye nanostructure layer were also investigated. In the research, the optical transmittance of the flexible AgNW TCEs was enhanced from 81.3% to 86.0% by attaching moth-eye nanostructures onto the other side of the flexible polyethylene terephthalate substrate while the electrical conductivity of the AgNW TCEs was not sacrificed. This research can provide a direction for the cost-effective fabrication of moth-eye nanostructures and the transmittance improvement of the flexible transparent electrodes.

Surface force measurement of ultraviolet nanoimprint lithography materials

Ultraviolet nanoimprint lithography (UV-NIL) has advantages such as room-temperature operation, high through-put, and high resolution. In the UV-NIL process, the mold needs a release coating material to prevent adhesion of the transfer resin. Usually, fluorinated silane coupling agents are used as release coating materials. To evaluate the release property, surface force analyzer equipment was used. This equipment can measure the surface forces between release-coated or noncoated mold material surfaces and UV-cured resin surfaces in the solid state. Lower surface forces were measured when a release coating was used on the mold material surface.

Eco-friendly, water-repellent, light-transparent film derived from psicose using nanoimprint lithography

Plant-based materials have recently been used as eco-friendly, water-repellent, light-transparent films for liquid crystal displays and optical devices. An approach using an eco-friendly light-transparent film derived from psicose has been successfully developed to prepare a regular nanostructure of conical moth-eyes with diameter less than approximately 200nm, water contact angle of 120°, and relatively low imprint pressure of 9 N for 100 imprints. We report and demonstrate the water repellency of the new eco-friendly film with 97% light transparency prepared with a 40wt% ratio of plant-based materials using an ultraviolet nanoimprint lithography process.

Measurements of flow distribution in a thin resin layer during ultraviolet nanoimprint lithography by means of digital holographic particle-tracking velocimetry

There is an urgent need for micro- and nano-scale patterning methods with a high throughput and cost-effective process for the manufacture of devices of the next generation. Ultraviolet nanoimprint lithography (UV-NIL) represents a major breakthrough for next-generation lithography because of its higher resolution and greater simplicity compared with conventional technologies. However, transfer defects such as bubble defects or filling failures in the UV-NIL process have been problematic, because UV-NIL is a contact-type method. Therefore, it is important to elucidate the behavior of UV-curable resin flowing to permit control over defects in the duplicated pattern. In this study, we succeeded in measuring the flow distribution of a thin layer of UV-curable resin in situ during the press process of UV-NIL by means of microscale digital holographic particle-tracking velocimetry, and we clearly showed that a release agent coated onto the nanoimprint mold affects the resin flow. We believe that this study will help to provide a better understanding of the behavior of UV-NIL.

A large deflection and high payload flexure-based parallel manipulator for UV nanoimprint lithography: Part II. Stiffness modeling and performance evaluation

This paper presents a flexure-based parallel manipulator (FPM) that delivers nanometric co-planar alignment and direct-force imprinting capabilities to automate an ultra-violet nanoimprint lithography (UV-NIL) process. The FPM is articulated from a 3-legged Prismatic-Prismatic-Spherical (3PPS) parallel-kinematic configuration to deliver a θx–θy–Z motion. The developed FPM achieves a positioning and orientation resolution of ±10nm and 0.05″, respectively, and a continuous output force of 150N/A throughout a large workspace of 5°×5°×5mm. Part I mainly focuses on a new theoretical model that is used to analyze the stiffness characteristics of the compliant joint modules that formed the FPM, and experimental evaluations of each compliant joint module. Part II presents the stiffness modeling of the FPM, the performance evaluations of the developed prototype, and the preliminary results of the UV-NIL process.

A large deflection and high payload flexure-based parallel manipulator for UV nanoimprint lithography: Part I. Modeling and analyses

This paper presents a flexure-based parallel manipulator (FPM) that delivers nanometric co-planar alignment and direct-force imprinting capabilities to automate an ultra-violet nanoimprint lithography (UV-NIL) process. The FPM is articulated from a novel 3-legged prismatic-prismatic-spherical (3PPS) parallel-kinematic configuration to deliver a θx–θy–Z motion. The developed FPM achieves a positioning and orientation resolution of ±10nm and 0.05″ respectively, and a continuous output force of 150N/Amp throughout a large workspace of 5°×5°×5mm. Part I mainly focuses on a new theoretical model that is used to analyze the stiffness characteristics of the compliant joint modules that formed the FPM, and experimental evaluations of each compliant joint module. Part II presents the stiffness modeling of the FPM, the performance evaluations of the developed prototype, and the preliminary results of the UV-NIL process.

Geometric phase analysis based on the windowed Fourier transform for the deformation field measurement

Abstract The geometric phase analysis (GPA), an important image-based deformation measurement method, has been used at both micro- and nano-scale. However, when a deformed image has apparent distortion, non-ignorable error in the obtained deformation field could occur by using this method. In this paper, the geometric phase analysis based on the windowed Fourier transform (WFT) is proposed to solve the above-mentioned issue, defined as the WFT–GPA method. In WFT–GPA, instead of the Fourier transform (FT), the WFT is utilized to extract the phase field block by block, and therefore more accurate local phase information can be acquired. The simulation tests, which include detailed discussion of influence factors for measurement accuracy such as window size and image noise, are conducted with digital deformed grids. The results verify that the WFT–GPA method not only keeps all advantages of traditional GPA method, but also owns a better accuracy for deformation measurement. Finally, the WFT–GPA method is applied to measure the machining distortion incurred in soft ultraviolet nanoimprint lithography (UV-NIL) process. The successful measurement shows the feasibility of this method and offers a full-field way for characterizing the replication quality of UV-NIL process.

Removal of UV-cured resin using a hybrid cleaning process for nanoimprint lithography

Removal of resin residues from quartz surfaces is one of the most critical issues in ultraviolet (UV) nanoimprint lithography (NIL) processes. Traces of resin residues on a quartz stamp cause pattern defects in the subsequent UV NIL process. This study investigates the effect of UV/O3 pretreatment and ozone dissolved water (DIO3) cleaning with megasonic (MS) on UV-cured resin removal. The bulk resin was oxidized and removed partly by UV/O3 pretreatment. The resin residues which remained on the surface in the form of a thin layer were removed by cleaning with 40ppm DIO3 along with megasonic. Subsequently, cleaning was performed using a 10% KOH solution to remove the adhesion promoter. Contact angle measurements showed that the surface was hydrophilic in nature, which confirms the complete removal of the UV-cured resin and adhesion promoter from the SiO2 substrate.

Lifetime evaluation of release agent for ultraviolet nanoimprint lithography

Ultraviolet-nanoimprint lithography (UV-NIL) is a powerful tool for mass production of nano-scale patterns. In UV-NIL, it is very important to coat the mold surface with a release agent to prevent the adhesion of UV curable resin. However, the release agent has a lifetime and by repeated UV-NIL transfer, the release layer is degraded and finally, UV curable resin adheres to the mold surface. To clarify the lifetime of the release agent, this study measured the contact angles of a silicon (Si) mold surface coated with various release agents during repetition of the UV-NIL process. Three types of fluorinated silane coupling agent were used for the release layer and the durability was measured on a patternless Si surface. The results showed that the C8H4Cl3F13Si release agent has high durability. A patterned Si mold was then coated with this release agent and evaluated during repetition of the UV-NIL process. After 1000 repetitions, the contact angle was still high, but a defect occurred at the 900th repetition. Therefore, the lifetime of this release agent when using a Si mold and UV curable resin is approximately 800times.

Surface adhesion and demolding force dependence on resist composition in ultraviolet nanoimprint lithography

Demolding, the process to separate stamp from molded resist, is most critical to the success of ultraviolet nanoimprint lithography (UV-NIL). In the present study we investigated adhesion and demolding force in UV-NIL for different compositions of a model UV-curable resist system containing a base (either tripropyleneglycol diacrylate with shorter oligomer length or polypropyleneglycol diacrylate with longer oligomer length), a cross-linking agent (trimethylolpropane triacrylate) and a photoinitiator (Irgacure 651). The demolding force was measured using a tensile test machine with homemade fixtures after imprinting the UV resist on a silicon stamp. While decreasing the cross-linking agent content from 49 to 0wt% has little effect on the resist surface energy, it reduces the resist's elastic modulus drastically. The decrease in elastic modulus results in a decreased adhesion force at the resist/stamp interface thereby facilitating the demolding. The decrease in elastic modulus and, therefore, demolding force by lowering the cross-linking agent content was markedly less pronounced in tripropyleneglycol diacrylate-based resists due to its shorter oligomer length. These general findings will be useful in designing new resists for UV-NIL process.

Hybrid-polymer nanostructures forming an anti-reflection film using two-beam interference and ultraviolet nanoimprint lithography

This study presents a new process to fabricate the two-dimensional periodic nanostructures with the specific material of inorganic–organic hybrid polymers (ORMOCER). In this study, the photoresist (PR) originated nanostructures can be fabricated with a two-beam interference method by use of 351 nm-wavelength Argon ion laser light. The replicated mold is performed by Ni electroless plating that Ni mold can be used by ultraviolet nanoimprint lithography (UV-NIL) process imprinting into the hybrid-polymer surface. The hybrid-polymer material is coating on the flexible polyethylene terephthalate (PET). Here, a self-assembled monolayer of fluoroalkyl silane (FAS-SAM) can be effective by means of atmospheric-pressure plasma (AP) chemical vapor modification of Ni mold surface for the duplicated ORMOCER/PET nanostructure film in UV-NIL process.

The replication of three dimensional structures using UV curable nanoimprint lithography

The study investigates the use of ultraviolet nanoimprint lithography (UV-NIL) for patterning three dimensional (3D) structures. Generating the 3D structures is a challenging task especially on an insulating substrate such as quartz. These transparent molds are essential for the UV-NIL process. The 3D profiles were created on the negative tone photoresist, Microresist ma-N2403 using a Raith150 electron beam lithography (EBL) tool in a single step variable dose controlled exposure. The developed 3D resist profiles subsequently were utilized as the 3D masking layer. The 3D patterns were transferred into the quartz mold substrates by a single-step reactive ion etching (RIE). The replication of the 3D mold structure by using the UV-NIL technique requires a two-step imprint process. The master mold profile was replicated onto a Microresist Ormocomp US-S4 resist on the first imprint to become the soft mold. The cured Ormocomp soft mold, subsequently was used as a mold for replicating the 3D pattern structures on the Microresist mr-UVCur06 resist in the second imprint step to create a positive replica of the original mold. A test pattern of a 3D pyramid-shaped array with multilevel features was successfully been replicated using this technique. The replicated 3D pattern on the resist could be utilised as the final 3D masking layer for the pattern transfer onto final substrate using RIE.

Improved release strategy for UV nanoimprint lithography

The adhesion between the fused silica mold and the resist remains a key issue in ultraviolet nanoimprint lithography (UV-NIL), especially in step and repeat processes. In this paper, we present results on antisticking layers (ASLs) of tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (F13-TCS) deposited in vapor phase and of a commercial product, Optool DSX™, from Daikin Chemical, deposited in liquid phase. The antisticking properties and structural morphologies of the formed self-assembled monolayer are investigated using contact angle, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy (XPS) measurements. Obtained surface energies are as low as 10mN∕m for both types of ASL. The stability of these formed layers during the UV-NIL process remains the main important issue. It was tested on an EVG® step and repeat UV-NIL equipment using acrylate-based resists. After only 50 prints, we observed a high increase in the surface energy of the mold, which indicates a drastic degradation of the antisticking (ASL). XPS measurements show that fluorine is removed from the mold surface. This surface modification is attributed to free radicals created during the UV cure of the resist that easily cleaved fluorocarbon chains. This can be confirmed by the fact that the polar component of the surface energy was increased by a larger factor than the dispersive one. To overcome this problem, the acrylate-based resist was modified by the adjunction of a fluorine surfactant. To go further in the mold/resist interaction, we developed an original setup that directly measures the adhesion force. Almost no degradation of the organic ASL was observed in terms of surface energy and fluorine density after more than 50 prints when using a sufficiently high amount of surfactant in the resist.

Nanoimprinting of Thin Polymer Films Using Elementwise Patterned Stamp and Subsequent Application of Pressurized Air

To improve the throughput of ultraviolet nanoimprint lithography (UV-NIL), a cost-effective method of fabricating sub-100-nm-wide line patterns, research studies have been focused on employing a large-area nanostamp. In this paper, we present a new single-step UV-NIL process in which an elementwise patterned stamp (EPS), a 5''×5'' quartz stamp, is first used to imprint resin droplets against a 4'' Si wafer. The EPS consists of 16 elements, each of which is 10×10 mm2, i.e., the effective pattern area is 1,600 mm3. Pressurized air is subsequently applied to the bottom of wafer to improve directly the contact between the wafer and the elements, and resultantly, the uniformity of residual layer thickness. To examine the effects of the pressurized air on the uniformity, we have carried out experiments. Resin droplets with a viscosity of 7 mPa s were dispensed on each element. Results show that the air pressure range of 0.05–0.2 MPa does not significantly affect the average thickness distribution but the uniformity. Line patterns of 70 nm width were successfully imprinted at an air pressure of 0.1 MPa.

EPS(Elementwise Patterned Stamp)를 이용한 UV 나노임프린트 공정에서 웨이퍼 변형에 따른 잔류층 분석

Imprint lithography is a promising method for high-resolution and high-throughput lithography using low-cost equipment. In particular, ultraviolet-nanoimprint lithography (UV-NIL) is applicable to large area imprint easily. We have proposed a new UV-NIL process using an elementwise patterned stamp (EPS), which consists of a number of elements, each of which is separated by channel. Experiments on UV-NIL are performed on an EVG620-NIL using the EPS with 3mm channel width. The replication of uniform sub 70 nm lines using the EPS is demonstrated. We investigate the nonuniformity of residual layer caused by wafer deformation in experiment with varying wafer thickness. Severely deformed wafer works as an obstacle in spreading of dropped resin, which causes nonuniformity of thickness of residual layer. Numerical simulations are conducted to analyze aforementioned phenomenon. Wafer deformation in the process is simulated by using a simplified model, which is a good agreement with experiments.

A step-and-repeat UV-nanoimprint lithography process using an elementwise patterned stamp

Ultraviolet-nanoimprint lithography (UV-NIL) is a promising cost-effective method for defining nanoscale structures at room temperature and low pressure. To apply a large-area stamp to a high throughput step-and-repeat process at atmospheric conditions, we proposed a new UV-NIL process that uses an elementwise patterned stamp (EPS), which consists of elements separated by channels, and additive gas pressurization. The proposed UV-NIL process required just four imprints 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.