UV Nanoimprint Lithography Research Articles

Fabrication of slanted gratings with high refractive index starting from master nanoimprint mold

Recently, nanoimprinting has attracted a new round of attention in the industry due to the boom in demand for augmented reality/virtual reality (AR/VR), metalens and microlens, and even semiconductors. Slanted gratings have great application prospects in AR/VR displays because of their high efficiency in light coupling. UV-Nanoimprint lithography (UV-NIL) has been identified as one of the most feasible routes for mass manufacture of high refractive index (RI) slanted gratings. This paper presents a fabrication of high RI slanted gratings based on UV-NIL. A comprehensive study on the optical principles of slanted gratings is conducted, followed by simulation-based optimization of the grating parameters. The key element for applying nanoimprint to fabricate slanted gratings is the master mold, which is acquired by a tilted angle etching of metal gratings as an etching mask on silicon wafers with F-based plasma. The influence of experimental parameters, such as the etching power and etching mask thickness on the morphology of the slanted gratings on the master mold are investigated. The working mold was simply duplicated from the master mold by UV-NIL with a low surface energy working mold material. The high RI slanted gratings were achieved by imprinting a UV-curable resin with high RI. Finally, experimental verification was performed to assess the optical performance of the slanted gratings.

Photocurable Thiol-Ene/Nanocellulose Elastomeric Composites for Bioinspired and Fluorine-Free Superhydrophobic Surfaces.

Artificially prepared superhydrophobic surfaces toward a self-cleaning "lotus effect" and anticontamination performance have become critically important in the past few years. However, most approaches to create the required topology with a hierarchical roughness comprise several manufacturing steps of varying practicality. Moreover, the desired low surface energy is in most cases achieved with fluorinated moieties that are currently criticized due to biological and environmental hazards. In this work, rapidly photocuring but weak thiol-ene resins were reinforced with cellulose nanofibrils (CNFs) to replicate lotus leaves via one-step UV nanoimprint lithography. The CNFs were surface-modified using countercation exchange of carboxyl groups and grafting of thiol and methacrylate functionalities. The formulation methodology resulted in free-flowing, shear-thinning composite resins without surfactants or dispersants. The rheological and photo-cross-linking behavior of the resins, the thermal stability, the mechanical performance, and the hydrophobicity of the cured composites were characterized. Notably, the surface modifications increased the as received fibril diameter (1.9 ± 0.6 nm) by 1.6-2.3 nm and raised the fibril-resin compatibility. The resins underwent rapid polymerization and the high thermal stability of thiol-enes was retained. The methacrylated nanofibrils (10 vol %) significantly strengthened the rubbery network, outperforming the neat thiol-ene polymer in terms of hardness (3.4×), reduced modulus (5.8×), and wear resistance (>100×). Moreover, the surface of lotus-texturized composites was superhydrophobic with a water contact angle of 155°, higher than that of the neat polymer (147°), and was self-cleaning. These CNF composite resins are compatible with fast-cure processes such as 3D printing and roll-to-roll processing, are exempt of fluorine or any other hydrophobization treatment, and are extremely wear-resistant.

(Invited) Optical Probes for Resonance-Enhanced Gold Growth on Nanooptical Substrates

We investigate light-stimulated growth of gold metal lines to technically mimic the growth of long-range axonal connections for neuromorphic computing architectures. Photocatalytic gold growth from an aqueous gold chloride precursor solution (HAuCl4) on a photoactive titanium dioxide layer is used. It is known that local gold growth may be obtained by UV illumination through a shadow mask [1]. To obtain a wavelength and angle dependent growth of metal lines – and thus stimulus-dependent resistances – we suggest to employ a nanooptical template. Inspired by the typical number of 2 to 10 connections to a node found, e.g., in the neuronal system of the biological species Hydra, we decided on the implementation of a hexagonal design. The edges are formed by periodic grating nanostructures with 10 to 20 periods and a grating pitch of 170 nm to 210 nm for excitation in the UV. In order to investigate the resonant optical enhancement, we employ optical probes. It is well known that colloidal quantum dots show enhanced fluorescence emission on photonic crystal surfaces due to resonant excitation and resonant outcoupling [2]. In our initial experiments for investigation of the resonant enhancement, we use 4,4’-Bis(2,2-diphenylvinyl)-1,1’-biphenyl (DBVBi) as the emitter material. Its absorption maximum matches the excitation laser at 355 nm and fluorescence in the visible spectrum around 455 nm may be monitored with our camera and spectrometer setup.The nanostructure master was fabricated by electron-beam lithography (Kelvin Nanotechnology Ltd.) according to our design. We transfer the nanostructure pattern onto 1.1 mm thick soda-lime glass substrates by UV-nanoimprint lithography. Subsequently, a 100 nm titanium dioxide layer is deposited. The 150 nm emissive DBVBi layer is then added by thermal evaporation. The samples are characterized optically with a camera setup and magnifying optics on an in-plane rotatable xy-stage. We use a Cobolt Zouk 355 nm 10 mW laser to excite the emissive layer. The output power of the laser is set to 2 mW and its output beam is widened with a collimating lens setup to a diameter of approximately 1.5 mm². The widened beam is essential to obtain a homogeneous excitation. We observe the successful resonant excitation by the higher emission intensity of DPVBi in specific regions of the nanostructure at specific excitation angles. Already for 10 periods of the grating structure resonance effects are observed. A stronger enhancement is obtained for 20 periods. Further, the enhancement depends on the grating pitch as connections with nanograting pitches below 190 nm are no longer visible. They do not meet the criteria for resonant excitation.This method of using optical probes on the nanooptical template works well for initial characterization of the resonance enhancement at different excitation angles. Now the setup is ready for performing gold growth under different excitation conditions. Without the additional emissive layer, the waveguide thickness change needs to be compensated with an increased titanium dioxide layer thickness. For online monitoring of the resonant enhancement during growth optical probes may be employed below the photoactive titanium dioxide layer. It is expected that the resonance enhancement is reduced with the addition of gold due to the increased absorption and thus reduced quality factor as was previously studied in the context of nanostructured OLEDs [3].We acknowledge funding by the DFG (German Research Foundation) with the CRC 1461- Neurotronics (Project-ID 434434223 – SFB 1461).[1] S. Veziroglu et al. (2018). Photocatalytic growth of hierarchical au needle clusters on highly active TiO2 thin film. Advanced Materials Interfaces, 5(15), 1800465.[2] N. Ganesh et al. (2007). Enhanced fluorescence emission from quantum dots on a photonic crystal surface. Nature nanotechnology, 2(8), 515-520.[3] J. Buhl et al. (2023). Resonance‐Based Directional Light Emission from Organic Light‐Emitting Diodes: Comparing Integrated Nanopatterns and Color Conversion Waveguide Gratings. Advanced Photonics Research, 4(2), 2200143. Figure 1. (Left) Fluorescence image of optical probe layer on nanooptical substrate for 355 nm laser excitation. (Middle) Schematic of nanooptical substrate with grating pitch (in nm) and number of periods given. Same colors represent identical grating features. (Right) Field emission scanning electron microscopy (FESEM) image of one node with 6 connections. Figure 1

High-power 940 nm DFB laser diode with large optical cavity.

High-power laser diodes with a stable wavelength and narrow linewidth are crucial for many applications. In this study, we introduce a first-order distributed feedback (DFB) grating into an asymmetric large-cavity laser diode operating around 940 nm. This design maintains high output power and offers a wide temperature locking range. The nearly sinusoidal shape first-order grating is fabricated by ultraviolet (UV) nanoimprint lithography, inductively coupled plasma (ICP) dry etching, and wet polishing. At a heat sink temperature of 25°C, the DFB laser diode, with a 200 µm stripe width and 4 mm cavity length, achieves a maximum output power of 24.8 W and a full width at half maximum (FWHM) of 0.4 nm under continuous-wave (CW) conditions. The maximum slope efficiency is calculated to be 1.04 W/A. At an output power of 10.7 W, the device reaches a peak wall-plug efficiency of 56%. Under quasi-continuous operation at 20 A, the laser output spectrum remains locked to the DFB grating over a temperature range from -10°C to 110°C, with a temperature coefficient of 0.062 nm/°C.

Fabrication of Rose Petal Surface Using Release-Coated UV-Curable Resin via Ultraviolet Nanoimprint Lithography

Fabrication of Rose Petal Surface Using Release-Coated UV-Curable Resin via Ultraviolet Nanoimprint Lithography

Efficient Fabrication of Plasmonic Nanostructures Using Chloroform‐Mediated Heterogeneous Bonding

AbstractPlasmonic nanostructures (PNs) have transformed nanophotonics by offering control over electromagnetic waves. Although innovative structural designs have demonstrated PNs' potential, significant fabrication challenges, including the precise alignment of heterogeneous materials, remain. Using chloroform and pre‐UV exposure (PUVE) to enhance adhesion at the metal‐dielectric interface (MDI) addresses these challenges. PUVE preserves plasmon resonance in the MDI, ensuring high performance without compromising structural integrity. Through an annealing process, PUVE strengthens van der Waals bonds, ensuring chloroform molecules remain on the Ag surface during chloroform residue removal, allowing: i) stable preservation of the Ag thin film on the PNs during stripping; and ii) the formation of chloroform‐mediated heterogeneous bonding (CMHB) using energy supplied by UV‐nanoimprint lithography (UV‐NIL). This approach enables PNs' scalable fabrication with enhanced adhesion properties, while CMHB increases adhesion between the UV‐curable resin and Ag, offering scalability to large‐area applications with UV‐NIL. The structural morphology and chemical properties of the fabricated PNs are analyzed using scanning electron microscopy, atomic force microscopy, energy‐dispersive spectroscopy, and X‐ray photoelectron spectroscopy. These PNs demonstrate potential in CCD filters, enhancing optical signal amplification for real‐time imaging at the 785 nm band. This robust fabrication method advances PNs' performance and scalability, advancing nanophotonics and related applications.

Surface plasmon resonance biosensor with anti-crossing modulation readout

A novel approach to surface plasmon resonance (SPR) biosensors providing simplified label-free monitoring of biomolecular affinity binding events is reported. It is based on the interrogation of anti-crossing surface plasmon modes traveling along opposite interfaces of a thin metal film on the top of a tailored multi-periodic grating structure. It allows for diffraction-based backside excitation of surface plasmons without the need of optical matching of the sensor chip to a prism and it allows avoiding of optical probing through the analyzed liquid sample. In conjunction with low angular dispersion of resonantly excited surface plasmon modes, it provides sensitive and versatile optical interrogation of SPR changes associated with biomolecular binding-induced refractive index variations. Direct readout with a fiber optic probe, as well as multi-channel configuration compatible with regular SPR readers, is implemented with the use of sensor chips prepared by mass production-compatible UV-nanoimprint lithography. The potential of the reported SPR sensor chips is illustrated by its ability to characterize affinity interaction of antibodies specific to cancer biomarker CSPG4 on antifouling mixed thiolated self-assembled monolayer with zwitterionic carboxybetaine and sulfobetaine headgroups.

Progress in Polyhedral Oligomeric Silsesquioxane (POSS) Photoresists: A Comprehensive Review across Lithographic Systems.

This paper offers a comprehensive overview of the polyhedral oligomeric silsesquioxane (POSS) and POSS-based composites within the realm of photoresist resin. The study involves a systematic exploration and discussion of the contributions made by POSS across various lithographic systems, with specific emphasis on critical parameters such as film formation, sensitivity, resolution, solubility, and edge roughness. These lithographic systems encompass X-ray lithography (XRL), deep ultraviolet nanoimprint lithography (DUV-NIL), extreme ultraviolet lithography (EUV), and guided self-assembled lithography (DSA). The principal objective of this paper is to furnish valuable insights into the development and utilization of POSS-based photoresist materials in diverse lithographic contexts.

UV Nanoimprint Lithography—Impact of Coating Techniques on Pattern Quality

In this work, three different coating techniques are compared and their applicability for ultraviolet nanoimprint lithography (UV-NIL) is investigated. As UV-NIL is considered a suitable volume manufacturing production solution for various emerging applications, it is mandatory to consider environmental aspects such as operational energy use and material consumption as well as waste management. In this paper, spin coating, spray coating, and inkjet coating are used to coat both a high refractive index resin (n = 1.9) and a filler-free resin (n = 1.5), respectively. Variable Angle Spectroscopy Ellipsometry (VASE) was used to analyze the influence of different process parameters on the resin thickness as well as to compare the refractive index achieved from each coating technology. Finally, the applicability of the different coating methods for UV-NIL was investigated by imprinting the resin layers with different test structures. For the final imprints, the resolution, the surface roughness, and the pattern fidelity over 25 imprints was assessed using AFM. Finally, a comparison of the resin consumption and the process time was performed for each coating method.

Colourful cholesteric liquid crystal polymer network gratings prepared through nanoimprinting

ABSTRACT Both the colourful cholesteric liquid crystal polymer network (CLCN) patterns and the gratings have attracted much attention for their applications as optical materials. Herein, the photochromic cholesteric liquid crystal (CLC) mixtures were prepared using a photoisomerizable chiral dopant. The structural colour of the CLC mixtures was tunable by changing the chiral dopant concentration and the intensity of the 365-nm irradiation light. The CLCN gratings with a structural colour were prepared using the UV nanoimprinting lithography method. The structural colours of the CLCN gratings originate from both the cholesteric and the grating structures. Moreover, a patterned CLCN grating was prepared using a photochromic CLC mixture, which could be applied for decoration and anti-counterfeiting.

UV-assisted nanoimprint lithography: the impact of the loading effect in silicon on nanoscale patterns of metalens.

This work studies the impact of the silicon (Si) loading effect induced by deep reactive ion etching (DRIE) of silicon master molds on the UV-nanoimprint lithography (NIL) patterning of nanofeatures. The silicon molds were patterned with metasurface features with widths varying from 270 to 60 nm. This effect was studied by focus ion beam scanning electron microscopy (FIB-SEM) and atomic force microscopy (AFM). The Si loading etching effect is characterized by the variation of pattern feature depth concerning feature sizes because smaller features tend to etch more slowly than larger ones due to etchants being more difficult to pass through the smaller hole and byproducts being harder to diffuse out too. Thus, the NIL results obtained from the Si master mold contain different pattern geometries concerning pattern quality and residual photoresist layer thickness. The obtained results are pivotal for NIL for fabricating devices with various geometrical nanostructures as the research field moves towards commercial applications.

Low Volume Shrinkage Alkaline Degradable UV Nanoimprint Lithography Resists Based on Acrylic Anhydride

The shrinkage phenomenon of UV-NIL resist during photocuring is still regarded as an important problem hindering the wide application of UV-NIL technology. We designed four degradable UV-NIL resists with low...

Nanoimprint lithography guiding templates for advanced magnetic media fabrication

Nanoimprint lithography presents unique opportunities for advanced magnetic storage media with ordered bit arrangements such as bit patterned media or heated dot magnetic recording. Providing sub-10 nm resolution and full disk imprinting capability, UV-nanoimprint lithography based on rigid quartz templates bears the entitlement for patterned recording media manufacturing with high throughput at low cost. However, a key challenge is the fabrication of the high-resolution template that can transfer the desired pattern onto the disk with high fidelity and low line edge roughness. In this article, we present fabrication routes and overcome challenges to the fabrication of quartz templates suitable for self-alignment and guiding purposes to be used for template replication toward full disk imprints. Guiding patterns down to 40 nm pitch are prepared using a rotary electron beam lithography tool. We compare three different process approaches to fabricate an etching mask for patterning the quartz. Two methods target chromium patterning, one with traditional lift-off and another by dry etching, both using an e-beam resist mask. The third approach is based on the development of a carbon-based Tri-layer hard mask. The template pattern profile is optimized for imprint-suitable sidewall angles using dry etching in a CF4/O2 chemistry. The templates were characterized using scanning electron microscopy and atomic force microscopy to evaluate the quality of the transferred pattern as well as line edge roughness. Our results show that the Tri-layer process using carbon resulted in the lowest line edge roughness of ≈0.65 nm at the imprinted disk level. In addition, we show that Tri-layer masking allowed for the use of conventional ZEP e-beam resist and fast writing speeds, while gaining high selectivity during quartz patterning.

The fabrication of high-performance diffraction gratings via vibration-assisted UV nanoimprinting lithography

With the aim of improving the performance of diffraction gratings, a piezoelectric-driven vibration-assisted UV nanoimprint method is proposed for use in the fabrication of diffraction gratings. It is found that by introducing one-dimensional lateral vibrations with a low-frequency and a low-amplitude into nanoimprinting, the surface quality of the resultant grating is improved and the filling rate of the photoresist in the mold is enhanced. The influence of the grating properties on the diffraction efficiency was determined via a simulation analysis, and the effects of the vibration frequency and vibration amplitude on the filling rate were studied; via these studies, a series of parameters describing the imposed vibrations that are expected to be suitable for use in the vibration-assisted UV nanoimprinting method is obtained. Furthermore, a novel asymmetric vibration stage is designed. The performance of the vibration-assisted UV nanoimprinting method was verified via scanning electron microscopy. Under the illumination of a light source, the measured diffraction angle is consistent with that predicted using the diffraction equation. The results indicate that vibration-assisted UV nanoimprinting method can accurately fabricate diffraction grating structures with superior performance compared with gratings obtained via other methods.

An overview of photopolymerization and its diverse applications

AbstractPhotopolymerization is an energy‐saving and environmentally friendly technology and has been widely applied and continuously developed in various fields since the mid‐20th century. Today, photopolymerization technology has become ubiquitous in every aspect of our lives. This review initially explains the principle of photopolymerization reactions, introducing the process of photopolymerization and the initiation mechanisms of photoinitiators. Then, this review focuses on the applications of photopolymerization technology in thiol‐based click reactions, photoinduced controlled/living polymerizations, three‐dimensional printing, ultraviolet nanoimprint lithography photoresist and hydrogels to demonstrate its irreplaceable role at present.

Multi-scale analysis approach for SU-8 in ultraviolet nanoimprint lithography demolding process

Ultraviolet nanoimprint lithography (UV-NIL) is a contact UV lithography fabrication method, and the interfacial properties of resist/mold have considerable attribute on the surface morphology of nanopatterns. Considering molecular interactions of interface region, investigation of the UV-NIL demolding process should associate several length scale approaches from the microscopic details to macroscopic properties. A multi-scale polymer resist/mold model was constructed to study demolding process of UV-NIL. The interfacial properties of resist/mold were studied through atomistic scale investigation, especially the relationship with the crosslinking conversion. It was found that with the increasing crosslinking conversion, the interfacial energy decreased but brought more stress to interfacial separation. The traction-separation description of interface was introduced to continuum model, and the stress distribution indicated that the root of nano-grating was subjected to more stress at the moment of mold detachment. The simulations and laboratory experiments suggested that highly crosslinked resist would increase interfacial adhesion, adversely effect on nanopatterns during the demolding process. Appropriately reducing the conversion could limit the interfacial stress and keep desired morphology.

The implementation of thermal and UV nanoimprint lithography for selective area epitaxy

Semiconductor nanowires (NWs) in horizontal configuration could provide a path for scalable NW-based devices. Bottom–up large-scale manufacturing of these nanostructures by selective area epitaxy (SAE) relies on precise nanopatterning of various shapes on the growth masks. Electron beam lithography offers an extraordinary accuracy suited for the purpose. However, this technique is not economically viable for large production as it has a low throughput and requires high investment and operational costs. Nanoimprint lithography (NIL) has the potential to reduce fabrication time and costs significantly while requiring less sophisticated equipment. In this work, we utilize both thermal and UV NIL for patterning substrates for SAE, elucidating the advantages and disadvantages of each lithography technique. We demonstrate the epitaxial growth of Ge and GaAs NWs on these substrates, where we observe high-quality mono-crystalline structures. Even though both processes can produce small uniform structures suitable for SAE, our results show that UV NIL proves to be superior and enables reliable and efficient patterning of sub-100 nm mask features at the wafer scale.

Research on the Filling Process of Ultraviolet Nanoimprint Lithography with High-Resolution Stamps

With the rapid development of the semiconductor industry, the feature size of integrated circuits (ICs) is continuously getting smaller and smaller. Although traditional optical lithography and extreme ultraviolet (EUV) lithography can fabricate products with 5-nm feature size, the production cost is quite high. As a candidate of next-generation lithography, ultraviolet nanoimprint lithography (UV-NIL) can also achieve high-resolution imprints at a lower cost. Based on contact mechanics, the filling mechanism and imprint quality of UV-NIL were examined using high-resolution stamps with different feature sizes of 3 nm, 4 nm, and 5 nm. It was found that UV-NIL could successfully replicate patterns with feature sizes down to 3 nm; however, the imprinting process was more complicated. Among the selected nine stamps, the replications of 4-nm and 5-nm feature sizes had a relatively uniform residual layer and a symmetric contact pressure, whereas the replications of 3-nm feature size had an asymmetric contact pressure, resulting in an asymmetric residual layer. In order to obtain a more uniform residual layer with higher-quality imprinting, the pressure applied to the left and right sides of a stamp should be appropriately changed. The pattern density of a stamp had a certain influence on the contact pressure. As the pattern density increased, the contact pressure tended to be uniform; however, when the pattern density exceeded 0.5, the situation became worse. This research can provide guidance for promoting the development of UV-NIL in the direction of high-resolution pattern replication.

Transfer Durability of Re-release Coated Replica Mold on Ultraviolet Nanoimprint Lithography

Transfer Durability of Re-release Coated Replica Mold on Ultraviolet Nanoimprint Lithography

Biomimetic high water adhesion superhydrophobic surface via UV nanoimprint lithography

Imitating nature and comprehending its workings is a means for human beings to advance the development of functional materials. In this study, UV nanoimprint lithography technology was utilized to successfully replicate the micro-nano hierarchical structure (micro-papillae and nano-wax tube crystals) present on the surface of the natural fresh lotus leaf. A series of UV-curable waterborne polyurethane acrylate (UV-WPUA) surfaces with special wetting and superhydrophobic properties were obtained. The polydimethylsiloxane negative stamp with lotus leaf’s structure was obtained through a stamp replica molding method, and then the UV-WPUA resin covered with the negative stamp was exposed to 172 nm Xe2* excimer and medium-pressure mercury lamps to mimic the micro-nano hierarchical structure of the lotus leaf. The UV-WPUA surfaces displayed high water adhesion superhydrophobic properties, with a water contact angle reaching 160.8° and water adhesive force of 140.0 μN, and exhibiting excellent self-cleaning performance. The results indicate that higher the UV intensity, the more accurately the surface replicates the micro-nano hierarchical structure of the lotus leaf, resulting in larger water contact angle and higher adhesive force. This study paves the way for the advancement of functional surfaces through the simulation of natural materials.