UV Nanoimprint Lithography Process Research Articles

Achievement of Unidirectional Aluminum Tin Oxide/UV-Curable Polymer Hybrid Film via UV Nanoimprinting Lithography for Uniform Liquid Crystal Alignment

A uniform unidirectional nanostructure composed of aluminum tin oxide and ultraviolet (UV)-curable polymer is introduced herein. The nanostructure was produced by UV-nanoimprint lithography (UV-NIL), and the fabricated hybrid film was used as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy and line profile analysis were performed to confirm a well-ordered nanostructure with 760 nm periodicity and 30 nm height. X-ray photoelectron spectroscopy analysis was also conducted to examine the chemical modifications to the hybrid film surface during UV exposure. Optical transmittance investigation of the nanopatterned hybrid film revealed its compatibility for LC device application. Stable, uniform, and homogeneous LC alignment on the hybrid film was confirmed by polarized optical microscopy observance and analysis of LC pretilt angle. The unidirectional structure on the film surface enabled uniform LC orientation along with surface anisotropy property. Hence, we expect that the proposed UV-NIL process can be applied to fabricate high-resolution unidirectional nanostructures with various inorganic/organic hybrid materials and that these nanostructures have high potential for next-generation LC systems.

Tunable diffraction grating in flexible substrate by UV-nanoimprint lithography

The fabrication of flexible advanced devices is a very important issue that requires the hybrid integration of different nanostructured materials. In this work, we report on a new molding technique based on an improved hard UV-nanoimprint lithography process (hard UV-NIL) using a poly(dimethylsiloxane) (PDMS) film. The large-scale integration of a high-quality nanostructured gold pattern in PDMS is made possible with the use of an inorganic sacrificial layer soluble in hydrogen peroxide. A tunable diffraction gold stripe grating has been fabricated by transferring a 1 cm2 stripe grating from a rigid silicon substrate to a flexible PDMS substrate. As a result, we successfully demonstrate diffraction grating optical tunability when mechanical tension is applied.

Enhancement of light extraction of organic light emitting diodes with photonic quasi-crystal structures by UV nanoimprint lithography process

We demonstrate a method to increase the efficiency of blue and green organic light emitting diodes by a factor of up to 53.9 and 28.3 % when the device brightness was 1000 cd/m2. By fabricating the organic light emitting diodes on top of a planarization 12-fold photonic quasi-crystal microstructure, the 12-fold photonic quasi-crystal microstructure was fabricated on the glass substrate by simple way of UV nanoimprint lithography process, and to decrease the roughness of the substrate, we used the high index filling layer to cover and planarize the photonic quasi-crystal structure. The photonic quasi-crystal microstructure released the light trapped in the wave guide mode, and it is found that photonic quasi-crystal microstructure can overcome the total internal reflection; we attribute that the photonic quasi-crystal microstructure could be integrated into organic light emitting diodes to tremendously enhance the emission efficiency.

Nanoimprinted polymer lasers with threshold below 100 W/cm^2 using mixed-order distributed feedback resonators

Organic semiconductor lasers were fabricated by UV-nanoimprint lithography with thresholds as low as 57 W/cm(2) under 4 ns pulsed operation. The nanoimprinted lasers employed mixed-order distributed feedback resonators, with second-order gratings surrounded by first-order gratings, combined with a light-emitting conjugated polymer. They were pumped by InGaN LEDs to produce green-emitting lasers, with thresholds of 208 W/cm(2) (102 nJ/pulse). These hybrid lasers incorporate a scalable UV-nanoimprint lithography process, compatible with high-performance LEDs, therefore we have demonstrated a coherent, compact, low-cost light source.

UV-Nanoimprint Lithography: Structure, Materials and Fabrication of Flexible Molds

Large-area nanopatterning technology has demonstrated high potential which can significantly enhance the performance of a variety of devices and products such as LEDs, solar cells, hard disk drives, laser diodes, wafer-level optics, etc. But various existing patterning technologies cannot well meet industrial-level application requirements in term of high resolution, high throughput, low cost, large patterned areas, and the ability to pattern on non-ideal surfaces or waters. Soft UV-nanoimprint lithography (UV-NIL) by using a flexible mold has been proven to be a cost-effective mass production method for patterning large-area structures up to wafer-level (300 mm) in the micrometer and nanometer scale, fabricating complex 3-D micro/nano structures, especially making large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. In particular, it provides an ideal solution and a powerful tool for mass producing micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning. That opens the way for many applications not previously conceptualized or economically feasible. The flexible mold is the most critical elements for soft UV-NIL. The performance of the flexible mold has a decisive effect on the soft UV-NIL in term of resolution, patterning area, throughput, uniformity of the imprinted patterns, and repeatability of multi-imprinting. The key enabler that can fulfill mass production of micro-and nanostructures over large areas by NIL is the continual advancement of mold techniques (structures, materials and fabrication processes) towards higher resolution over a larger area at a lower cost. This paper provides a comprehensive review on the structural types, materials used and fabrication methods of various flexible molds in soft UV-NIL, surveys major progress in various flexible molds, particularly highlights some concluding remarks and generalizations. Two key issues for flexible molds, deformation mechanism and controlling solution of soft molds as well as fabrication of large area (wafer level) master template, are described in detail. Furthermore, prospects, challenges and future directions for flexible molds are addressed. Finally, some potential or promising solutions for improving the performance of flexible molds and soft UV-NIL process, as well as some important conclusions are presented.

A simple and novel method for the evaluation of adhesion properties between UV curable resin and stamp in UV-nanoimprint lithography (UV-NIL)

In ultraviolet nanoimprint lithography (UV-NIL), adhesion between the UV curable resin and the stamp is one of the most significant factors affecting successful pattern transfer. Therefore, the adhesion properties of UV curable resins used in UV-NIL must be properly understood. In this study, a simple and novel method is proposed for evaluating the adhesion properties of UV curable resin, using a specially-designed adhesion tester. Using this method, the pull-off tests between a stamp and a UV curable resin for UV-NIL was conducted, and the effects of UV-NIL process conditions on adhesion properties were investigated. PAK-01 was used as the UV curable resin for UV-NIL and a counterpart was a fused silica lens as the stamp. The pull-off forces between the PAK-01 and the lens were measured for various hold times, pulling force rates and relative humidities, and the effects of these process conditions on the pull-off force were examined. The pulling force rate and the relative humidity were found to have a strong influence on the pull-off force. The pull-off force increased as the pulling force rate increased, but decreased as the relative humidity increased. In contrast, the effect of hold time on the pull-off force was insignificant.

Fabrication of SiN_x-based photonic crystals on GaN-based LED devices with patterned sapphire substrate by nanoimprint lithography

SiNx-based photonic crystal (PhC) patterns were fabricated on the ITO electrode layer of a GaN-based light-emitting diode (LED) device on a patterned sapphire substrate (PSS) by a UV nanoimprint lithography process in order to improve the light extraction of the device. A three-dimensional finite-difference time-domain simulation confirmed that the light extraction of a GaN LED structure on a PSS is enhanced when SiNx PhC patterns are formed on the ITO top layer. From the I-V characteristics, the electrical properties of patterned LED devices with SiNx-based PhC were not degraded compared to the unpatterned LED device, since plasma etching of the p-GaN or the ITO layers was not involved in the patterning process. Additionally, the patterned LED devices with SiNx-based PhCs showed 19%-increased electroluminescence intensity compared with the unpatterned LED device at 445 nm wavelength when a 20 mA current is driven.

Fabrication of ZnO nano-structures using UV nanoimprint lithography of a ZnO nano-particle dispersion resin

Recently, nanoimprint lithography (NIL) has gained great attention as an effective patterning technology in the fields of light emitting diodes (LEDs), solar cells, and other optical devices, because of its simplicity and cost effectiveness. The aim of this study is the development of an imprint resin containing dispersed zinc oxide (ZnO) nano-particles that is applicable in the UV NIL process. UV NIL uses conventional monomer-based resins, which contain a UV initiator, but restricts the use of imprinted structures in optical devices due to their relatively low refractive index. In order to resolve this problem, an imprint resin containing dispersed ZnO nano-particles was prepared, using which submicron-scale structures were fabricated by the UV NIL process. The haziness of submicron-scale ZnO nano-particle resin structures and the refractive index of the ZnO nano-particle dispersion resin were measured to analyze the optical properties of the ZnO nano-particle dispersion resin and the resulting structures.

Step and repeat UV nanoimprint lithography on pre-spin coated resist film: a promisingroute for fabricating nanodevices

A step and repeat UV nanoimprint lithography process on pre-spin coated resistfilm is demonstrated for patterning a large area with features sizes down tosub-15 nm. The high fidelity between the template and imprinted structures isverified with a difference in their line edge roughness of less than 0.5 nm (3σ deviation value). The imprinted pattern’s residual layer is well controlled to allow directpattern transfer from the resist into functional materials with very high resolution. Theprocess is suitable for fabricating numerous nanodevices.

Evaluation of Curing Characteristics in UV-NIL Resist

Mechanical characteristics of UV curable resist for UV nanoimprint lithography are investigated using UV rheology meter. Modulations of visco-elatic properties and thickness shrinkages in typical resists are evaluated in variation of exposure UV intensity. The mechanical modulation speed of the resist depends on the UV intensity, which affects to throughput of UV nanoimprint lithography process. To handle the characteristics universally, effective conversion time is newly introduced, which fairly expresses the resist modifications.

Corrugated organic light emitting diodes for enhanced light extraction

Corrugated organic light emitting diodes are fabricated on photonic crystal substrates patterned using UV nanoimprint lithography process at room-temperature and low-pressure. Forty-nine percentage enhancement of light extraction was achieved even with the very low corrugation depth of 50 nm. The power efficiency of the corrugated OLEDs was even further improved by 93% compared to the OLED with a planar structure. Finite difference time domain (FDTD) analyses indicated that the Bragg-reflection by the corrugated metallic mirror plays dominant role in the enhancement of extraction efficiency in the corrugated structure and extraction efficiency can be further improved if corrugation depth is increased to 100 nm.

Gold nanoparticles by soft UV nanoimprint lithography coupled to a lift-off process for plasmonic sensing of antibodies

In this paper, we present the results of gold nanoparticles fabrication on large area by soft UV nanoimprint lithography (UV-NIL) and lift-off process for antibodies sensing. For this fabrication, the hard polydimethylsiloxane (H-PDMS) is used as flexible mold material. A simple AMONIL/PMMA bilayer is used to transfer patterns by etching the substrate with a suitable RIE process. The UV-NIL process has enabled to obtain gold nanoparticles with a diameter about 160 nm on a zone of 1 mm 2. For bioplasmonic applications, the sensing of Anti-Biotin was studied.

Residue-free room temperature UV-nanoimprinting of submicron organic thin film transistors

In this study we report on an innovative nanoimprint process for the fabrication of entirely patterned submicron OTFTs in a bottom-gate configuration. The method is based on UV-Nanoimprint Lithography (UV-NIL) combined with a novel imprint resist whose outstanding chemical and physical properties are responsible for the excellent results in structure transfer. In combination with a pretreated stamp the UV-curable resist enables residue-free imprinting thus making etching obsolete. A subsequent lift-off can be done with water. The UV-NIL process implies no extra temperature budget, is time saving due to short curing times, eco-friendly due to a water-based lift-off, simple because it is etch-free and completely r2r compatible. It works perfectly even if ultra-thin organic and hybrid films are used as gate dielectrics. On this basis entirely patterned functional submicron OTFTs with pentacene as the semiconductor are fabricated showing clear saturation, low switch-on voltage (∼3 V) and a sufficiently high on–off ratio (10 3).

ENHANCED LIGHT EXTRACTION FROM COLLOIDAL ZnO QD FILM BY ADDING ON PHOTONIC CRYSTALS USING UV-NANOIMPRINT LITHOGRAPHY

A significant enhancement in light extraction from colloidal ZnO QD films was observed on an addition of a layer of two-dimensional photonic crystal (2D-PC) structure. The PCs with a rectangular-lattice air-hole array pattern with the period of 1.4 μm and the hole-size of 400 nm were fabricated by UV-nanoimprint lithography (UV-NIL). Unlike the commonly utilized electron-beam lithographic technique, the UV-NIL process can make patterns over a large area with high throughput. The resultant colloidal zinc oxide ( ZnO ) QD films with a PC pattern add-on had nearly double the output power, as measured from the top of the film.

Deformation of a LCD Glass-Panel by Alignment Process in Nanoimprint Lithography

Deformation of a LCD glass-panel during the alignment is analyzed for a UV-nanoimprint lithography process. In UV-nanoimprint processes for manufacturing LCD panels, a glass-panel should be aligned with respect to the position of fiducial marks on imprint mold and the target glass. The alignment process will place the nano-patterns on the mold into the designated position in the glass-panel. Usually, viscoelastic resin is laminated between the mold and glass-panel. The resin is solidified by ultra-violet light and is designed to be nano-patterns. Thus, shear force would be induced on the panel during alignment process. The shear force can cause deformation of glass-panel, which ultimately causes misalignment of nano-patterns. Thus, deformation analysis for the glass-panel in the alignment process is essential to enhance the accuracy and to reduce alignment cost in nano-imprint. In this work, deformation of the glass panel from the viscous force is calculated by using 'ALGOR' package. The mechanism of misalignment of nano-patterns due to the deformation is analyzed. Finally, the effect of misalignment is discussed, and the experimental results are also presented.

UV nanoimprint lithography process optimization for electron device manufacturing on nanosized scale

Imprint specific process parameters like the residual layer thickness and the etch resistance of the UV polymers for the substrate etch process have to be optimized to introduce UV nanoimprint lithography (UV NIL) as a high-resolution, low-cost patterning technique for research and industry into electron device manufacturing. Additionally, UV NIL processes have to be compatible with conventional silicon (Si) semiconductor processing. Within this work, the minimization of the residual layer thickness by using a multi-drop ink-jet system, which was integrated into the imprint stepper NPS300 from S-E-T-(formerly SUSS MicroTec), in combination with a low viscous UV polymer from Asahi Glass Company is shown. The etch resistance of different UV polymers against the poly-Si etch process was increased by 50% with an appropriate post-exposure bake. A poly-Si dry etch process was used to pattern the gates of short channel MOSFETs. After optimizing the poly-Si etch, properly working short channel MOSFETs with a minimum gate length of about 90 nm were fabricated demonstrating successfully the compatibility of UV NIL with conventional Si semiconductor processing on nanosized scale.

Filling high aspect-ratio nano-structures by atomic layer deposition and its applications in nano-optic devices and integrations

Atomic layer deposition, a highly uniform and conformal deposition process, was utilized to fill trenches of various high aspect-ratio nano-grating structures. Dielectric (e.g., SiO2), metal (e.g., aluminum), and dielectric/metal (e.g., Au∕SiO2) hybrid nano-gratings with a linewidth down to <50nm and an aspect ratio up to 14:1 (700nm:50nm) were trench-filled with various materials particularly nano-laminate materials such as TiO2∕SiO2 and SiO2∕Al2O3. Various high-performance optical devices such as true-zero-order optical retarders (i.e., wave plates) and nanowire-grid polarizers were realized based on an UV-nanoimprint lithography process and the atomic layer deposition for trench fillings. Thanks to both unique material properties and nano-structure trench filling capability, the atomic layer deposition opens a path for innovative nano-structure based optical devices and integrated optical devices.

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.