UV-curing Nanoimprint Lithography Research Articles

Nanopatterning of Polymer/Gallium Oxide Thin Films by UV-Curing Nanoimprint Lithography for Liquid Crystal Alignment

Nanopatterning of Polymer/Gallium Oxide Thin Films by UV-Curing Nanoimprint Lithography for Liquid Crystal Alignment

Direct metallic nanostructures transfer by flexible contact UV-curable nano-imprint lithography

We propose a novel flexible contact ultraviolet (UV)-curable nano-imprint lithography process which can directly transfer metallic nanostructures by UV curing bonding force. The flexible template evaporated with metal acts as a mask, allowing only the structural region to penetrate UV light and transfer surface metallic nanostructures. By this process, aluminum metal grating of different feature sizes had been transferred to abtain single-layer or even multi-layer metallic nanostructures. The minimum width and spacing of the wire grating were 50 nm and sub-20 nm, respectively. Aluminum grating polarizer had also been fabricated and tested demonstrating the practicability of the process. © 2019 The Japan Society of Applied Physics

Improvement of color conversion and efficiency droop in hybrid light-emitting diodes utilizing an efficient non-radiative resonant energy transfer

Blue InGaN/GaN nanohole light-emitting diodes have been fabricated by soft UV-curing nanoimprint lithography, filling with CdSe/ZnS core/shell nanocrystals (NCs) as color conversion mediums. The excitonic recombination dynamics of hybrid nanohole light-emitting diodes were investigated by time-resolved photoluminescence, observing a significant reduction in the decay lifetime of excitons as a result of an efficient non-radiative resonant energy transfer, which leads to the improvement of color conversion and efficiency droop in these hybrid nanohole light-emitting diodes compared to hybrid nanocrystals/standard planar light-emitting diodes. The color-conversion efficiency and effective quantum yield of hybrid nanohole light-emitting diodes were nearly twice as much as those of hybrid standard light-emitting diodes. A model on the excitonic recombination process was proposed to explore this situation, explaining the advantages of non-radiative resonant energy transfer that avoiding energy loss associated with the intermediate light emission and conversion steps and transferring energy non-radiatively and resonantly to NCs with a higher quantum yield. The efficiency droop of hybrid nanohole light-emitting diodes was validly suppressed compared to the bare ones, even better than that of hybrid standard light-emitting diodes. It mainly results from the extraction of excess carrier concentrations in InGaN/GaN multiple quantum wells via the rapid non-radiative resonant energy transfer process under the higher injection condition, revealing a great potential to realize efficient white light emitters in the future.

Advances in Nanoimprint Lithography.

Nanoimprint lithography (NIL), a molding process, can replicate features <10 nm over large areas with long-range order. We describe the early development and fundamental principles underlying the two most commonly used types of NIL, thermal and UV, and contrast them with conventional photolithography methods used in the semiconductor industry. We then describe current advances toward full commercial industrialization of UV-curable NIL (UV-NIL) technology for integrated circuit production. We conclude with brief overviews of some emerging areas of research, from photonics to biotechnology, in which the ability of NIL to fabricate structures of arbitrary geometry is providing new paths for development. As with previous innovations, the increasing availability of tools and techniques from the semiconductor industry is poised to provide a path to bring these innovations from the lab to everyday life.

Significant improvements in InGaN/GaN nano-photoelectrodes for hydrogen generation by structure and polarization optimization.

The photoelectrodes based on III-nitride semiconductors with high energy conversion efficiency especially for those self-driven ones are greatly desirable for hydrogen generation. In this study, highly ordered InGaN/GaN multiple-quantum-well nanorod-based photoelectrodes have been fabricated by a soft UV-curing nano-imprint lithography and a top-down etching technique, which improve the incident photon conversion efficiency (IPCE) from 16% (planar structure) to 42% (@ wavelength = 400 nm). More significantly, the turn-on voltage is reduced low to −0.6 V, which indicates the possibility of achieving self-driven. Furthermore, SiO2/Si3N4 dielectric distributed Bragg reflectors are employed to further improve the IPCE up to 60%. And the photocurrent (@ 1.1 V) is enhanced from 0.37 mA/cm2 (original planar structure) to 1.5 mA/cm2. These improvements may accelerate the possible applications for hydrogen generation with high energy-efficiency.

Optical polarization characteristics of c-plane InGaN/GaN asymmetric nanostructures

Highly ordered c-plane InGaN/GaN elliptic nanorod (NR) and nano-grating (NG) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The polarized photoluminescence emission from these elliptic NR and NG arrays has been investigated both theoretically and experimentally. Considerable in-plane optical anisotropy, with a polarization ratio of 15% and 71% and a peak shift of 5.2 meV and 28.1 meV, was discovered from these c-plane InGaN/GaN elliptic NR and NG arrays, respectively. The k·p perturbation theory was adopted to explore this situation, simulating the transitions from conduction subbands to valence subbands and their corresponding optical momentum matrix elements at/around Γ point under the in-plane asymmetric strain. The good agreements of observed and simulated results demonstrate that the in-plane strain asymmetry is the essential cause of the optical polarization in this case, revealing the great potential to utilize strain effect to control the polarization of InGaN/GaN nanostructures.

Great enhancement in the excitonic recombination and light extraction of highly ordered InGaN/GaN elliptic nanorod arrays on a wafer scale

A series of highly ordered c-plane InGaN/GaN elliptic nanorod (NR) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The photoluminescence (PL) integral intensities of NR samples show a remarkable enhancement by a factor of up to two orders of magnitude compared with their corresponding as-grown samples at room temperature. The radiative recombination in NR samples is found to be greatly enhanced due to not only the suppressed non-radiative recombination but also the strain relaxation and optical waveguide effects. It is demonstrated that elliptic NR arrays improve the light extraction greatly and have polarized emission, both of which possibly result from the broken structure symmetry. Green NR light-emitting diodes have been finally realized, with good current–voltage performance and uniform luminescence.

Applications of nanoimprint lithography/hot embossing: a review

This review concentrates on the applications of nanoimprint lithography (NIL) and hot embossing for the fabrications of nanolectronic devices, nanophotonic metamaterials and other nanostructures. Technical challenges and solutions in NIL such as nanofabrication of templates, removal of residual resist, pattern displacement in thermal NIL arising from thermal expansion are first discussed. In the nanofabrication of templates, dry etch in plasma for the formation of multi-step structures and ultra-sharp tip arrays in silicon, nanophotonic chiral structures with high aspect ratio in SiC are demonstrated. A bilayer technique for nondestructive removal of residual resist in thermal NIL is described. This process is successfully applied for the fabrication of T-shape gates and functional high electron mobility transistors. However, pattern displacement intrinsically existing in thermal NIL/hot embossing owing to different thermal expansions in the template and substrate, respectively, limits its further development and scale-up. Low temperature even room temperature NIL (RTNIL) was then proposed on HSQ, trying to eliminate the pattern distortion by avoiding a thermal loop in the imprint. But, considerable pressure needed in RTNIL turned the major attentions to the development of UV-curing NIL in UV-curable monomers at low temperature. A big variety of applications by low-temperature UV-curing NIL in SU-8 are described, including high-aspect-ratio phase gratings, tagging technology by nanobarcode for DNA sequencing, nanofluidic channels, nanophotonic metamaterials and biosensors. Hot embossing, as a parallel technique to NIL, was also developed, and its applications on ferroelectric polymers as well as metals are reviewed. Therefore, it is necessary to emphasize that this review is mainly attempted to review the applications of NIL/embossing instead of NIL technique advances.

High resolution soft mold for UV-curing nanoimprint lithography using an oxygen insensitive degradable material

Soft nanoimprint lithography has been developed to overcome the disadvantages of conventional nanoimprint lithography based on rigid molds. Hybrid nanoimprint-soft lithography mold is an efficient strategy to improve the resolution of soft nanoimprint because a rigid UV-curable material is used as the structural layer. In this paper, the authors design a novel UV-curable material for hybrid soft mold fabrication, which is degradable under mild acidic conditions and insensitive to oxygen during photopolymerization. The material comprises an acid-degradable cross-linker, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxaspiro[4.2.4.2] tetradecane, and an acyrlated polysiloxane, poly[(mercaptopropyl)methylsiloxane]. Oxygen sensitivity of acrylate groups during UV curing is avoided due to the cross-linking mechanism based on thiol-ene chemistry. The cured material can be decomposed into linear chains through the cleavage of acid-labile ketal links and dissolved in organic solvent when heated in an acidic solution. The material possesses necessary properties for mold fabrication and imprinting, including low shrinkage (about 5%), high modulus (1.4 GPa), high thermal stability, high UV transparency and stability in normal environment. Furthermore, since the cured material is degradable in acidic media, the stained master can be renewed in a mild condition. Various nanoscaled patterns are fabricated on planar and curved substrates via UV-nanoimprint when the same material is used as a patterning layer.

A degradable polycyclic cross-linker for UV-curing nanoimprint lithography

In this paper, we design and synthesize a degradable polycyclic diacrylate cross-linker, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxaspiro[4.2.4.2] tetradecane (DAMTT), from easily available starting materials by bis-ketalization of 1,4-cyclohexanedione with glycerol, followed by esterification with acryloyl choride. The chemical structure of the degradable cross-linker is characterized by FTIR, 1H NMR and GC-MS. A UV-NIL resist based on DAMTT has a very low shrinkage (about 2.5%) and a high Young's modulus of 1.89 GPa after UV-curing, and is soluble in acidic media. Patterns with sub-10 nm resolution are faithfully imprinted into the DAMTT film. The degradable cross-linker is incorporated with a multifunctional acrylated siloxane to increase O2 RIE resistance as an etch barrier for high aspect ratio pattern transfer. The cross-linked film of the UV-curable resist remains soluble in acidic media even though its formulation contains acrylated siloxane up to 50% (w/w). Moreover, in contrast to most UV-NIL resists which are insoluble in solvents after curing, the DAMTT resist can independently achieve a lift-off process without the assistance of a thermoplastic polymer layer. That makes it possible to fabricate metal nanostructures on highly curved surfaces via a hybrid UV-curing nanoimprint technique. For example, metal gratings are successfully fabricated on the cylindrical surface of an optical fiber via a lift-off process.

Sub-50 nm UV-curing nanoimprint based on fluoropolymer, CYTOP, mold

In addition to the advantages of conventional fluoropolymers (i.e., chemical resistance, optical transparency, especially low surface energy), CYTOP, a commercially available cyclic fluoropolymer, can be dissolved in a special fluorinated solvent, thus it can be coated on the substrate by a spin-coating or solution casting method. In this paper, we introduced CYTOP as a rapid prototyping mold material for UV-curing nanoimprint lithography. The CYTOP mold is fabricated by a thermal nanoimprint technique on a quartz substrate or a direct solution casting on a master mold. Nanostructures with different geometries and down to sub-50 nm feature size are faithfully duplicated by CYTOP molds through a UV-curing imprint process. The CYTOP mold preserves its lower surface energy property after 20 repeated imprint cycles and there is no peeling off problem or contamination and damage on CYTOP mold observed. However, it is found that the high aspect ratio nanostructures on the CYTOP mold are tilted or deformed after separation with the master mold.

Double transfer UV-curing nanoimprint lithography

A challenge in the fabrication of nanostructures into non-planar substrates is to form a thin, uniform resist film on non-planar surfaces. This is critical to the fabrication of nanostructures via a lithographic technique due to the subsequent pattern transfer process. Here we report a new double transfer UV-curing nanoimprint technique that can create a nanopatterned thin film with a uniform residual layer not only on flat substrates but also on highly curved surfaces. Surface relief gratings with pitches down to 200 nm are successfully imprinted on the cylindrical surface of optical fibers, and further transferred into a SiO2 matrix using reactive ion etching (RIE), demonstrating that our technique is applicable for fabricating high-resolution nanostructures on non-planar substrates.

Large-scale fabrication and luminescence properties of GaN nanostructures by a soft UV-curing nanoimprint lithography

GaN nanorods with a period of 400 nm and diameter of 200 nm, and nano-gratings with a period of 400 nm and gap width of 100 nm are fabricated on wafers by a soft UV-curing nanoimprint lithography. These nanostructures show high periodicity and good morphology. The photoluminescence (PL) spectra exhibit that the integral PL intensity of GaN nanorods is enhanced as much as 2.5 times, compared to that of as-grown GaN films. According to finite-difference time-domain simulations and cathodoluminescence mappings, it is concluded that the enhancement for nanorods is due to the improvements of both spontaneous emission rate and light extraction efficiency caused by periodic GaN structures on the surface. By identifying the Raman shift of E1(TO) and modes of GaN films with nano-gratings and nanorods, the normal-plane strain εzz is determined. The PL emission energy is found to be proportional to the εzz, whose linear proportionality factor is calculated to be −27 meV GPa−1.

Replication of surface nano-structure of the wing of dragonfly (Pantala Flavescens) using nano-molding and UV nanoimprint lithography

The hydrophobicity of a dragonfly’s wing originates from the naturally occurring nano-structure on its surface. The nano-structure on a dragonfly’s wing consists of an array of nano-sized pillars, 100 nm in diameter. We re-create this hydrophobicity on various substrates, such as Si, glass, curved acrylic polymer, and flexible PET film, by replicating the nano-structure using UV curable nano-imprint lithography (NIL) and PDMS molding. The success of the nano-structure duplication was confirmed using scanning electron microscopy (SEM). The hydrophobicity was measured by water-based contact angle measurements. The water contact angle of the replica made of UV cured polymer was 135° ± 2°, which was slightly lower than that of the original dragonfly’s wing (145° ± 2°), but much higher than that of the UV cured polymer surface without any nano-sized pillars (80°). The hydrophobicity was further improved by applying a coating of Teflon-like material.

Fabrication of highly ordered sub-20 nm silicon nanopillars by block copolymer lithography combined with resist design

The control of order and orientation of the self-assembly of cylinder-forming poly(styrene-b-dimethylsiloxane) block copolymer is demonstrated. Copolymer thin films are spun-cast onto topographically patterned (well-defined rectangular cross-section channels) polyhedral-silsesquioxane-type resist templates and annealed in solvent vapor. The templates used here are fabricated by UV-curing nanoimprint lithography and the surface properties of the resist are tuned by the ligands coordinated to the resist's silsesquioxane cages. Depending on the resist's composition and on the surface chemistry at the base of the trench (resist or silicon), various morphologies and orientations of the polydimethylsiloxane cylinders are observed without the use of a brush layer. Some surfaces are demonstrated to be neutral for the copolymer, without any wetting layer and, under favorable conditions, highly ordered features are observed over substrate areas of about 1 cm2 (scalable to larger surfaces). Also, the possibility of using solvents widely accepted in industry for polymer spin-coating and annealing is proved. Due to the high plasma etch resistance of the polydimethylsiloxane block, self-assembled patterns can be transferred to the silicon substrate producing silicon features with aspect ratios up to 2. We demonstrate that the methodology developed here could be integrated into conventional fabrication processes and scaled to wafer production.

A dual-curable transfer layer for adhesion enhancement of a multilayer UV-curable nanoimprint resist system

We invented a dual-curable transfer layer to enhance adhesion of the UV-curable nanoimprint resist to the substrate. Based on this transfer layer, we developed bilayer resist and trilayer resist UV-curable nanoimprint lithography processes, which were used for etching and lift-off processes, respectively. The dual-curable transfer layer combined at least two different types of reactive functions based on different polymerization mechanisms. It formed strong chemical bonds with both the underneath material and the nanoimprint resist layer in two curing steps. It helped improve the adhesion of the low surface energy resist film to the substrate substantially, and, more importantly, made high-resolution patterning much more reliable. Moreover, low aspect ratio imprinted patterns were amplified into high aspect ratio patterns through the transfer layer via a selective etching 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.

One-kilobit cross-bar molecular memory circuits at 30-nm half-pitch fabricated by nanoimprint lithography

We have developed a process to fabricate a cross-bar structure using UV-curable nanoimprint lithography with a UV-curable double-layer spin-on resist, metal lift off and Langmuir–Blodgett film deposition. This process allowed us to produce 1-kbit cross-bar memory circuits at 30-nm half-pitch on both top and bottom electrodes. Read, write, erase and cross talking were also investigated.

UV Curing Nanoimprint Lithography for Uniform Layers and Minimized Residual Layers

Due to its ability to produce nanosized patterns on large area substrates with high throughput, UV curing nanoimprinting lithography is a promising technology for nanosized pattern fabrication. However, for it to become a technology for the mass production of nanosized patterns, we must first show that UV curing imprint lithography can produce a uniform layer with minimal residue. In this study, a pressurized chamber imprinting system was used. Isotropic pressure was applied to insure effective delivery of pressing force and uniform pressing. A UV curable resin, consisting of a perfluorinated acrylate and a photo-initiator, was used. The factors determining residual layer thickness in UV curing imprint lithography were investigated. To obtain imprinted patterns with no residual layer, a thin and uniform initial resin layer and pressurized imprinting must be used to rearrange the resin effectively. The high fidelity transfer of various patterns as small as 150 nm with no residual layer was successfully demonstrated by controlling the initial resin thickness at an imprint pressure of 15 atm.