Nanoimprint Lithography Process Research Articles

Nanoimprinting and backside ultraviolet lithography for fabricating metal nanostructures with higher aspect ratio

This paper introduces an innovative approach to increasing the aspect ratio of metal nanostructures fabricated using nanoimprint lithography (NIL). Although conventional NIL and metal lift-off processes can fabricate metal nanostructures, the achievable aspect ratio is often limited by the inherent constraints of NIL. In this study, we demonstrate that for an ultraviolet (UV) transparent substrate, metal nanostructures patterned via NIL can serve as a photomask. A negative-tone photoresist (PR) layer was then deposited on top of the patterned metal nanostructures. By illuminating the substrate from the backside with UV light and subsequently developing the PR, PR structures complementary and self-aligned to the metal layer were obtained. This enabled a second round of metal deposition and lift-off, thereby increasing the height of the metal structures and enhancing the aspect ratio. Experimentally, we demonstrated that this method can improve the aspect ratio from less than 1.0 to as high as 2.1. This paper also addresses the further developments and potential applications of this technique.

Amorphous to Crystalline Transition in Nanoimprinted Sol-Gel Titanium Oxide Metasurfaces.

Crystalline titanium dioxide (TiO2) (anatase and rutile) possesses a higher refractive index than amorphous TiO2 and near-zero absorption in the visible region, making them an ideal material for visible metasurfaces. However, fabrication limitations hinder their implementation into flat optics. In this work, a wafer-scale manufacturing platform is proposed for crystalline TiO2 metasurfaces. Sol-gel TiO2 is developed as a printable material in which its material phase can be precisely controlled to produce amorphous, anatase, or rutile, depending on the crystallization temperature. Therefore, anatase or rutile metalenses can be fabricated on a wafer scale using thermal nanoimprint lithography and sintering process. The high refractive index of the crystalline TiO2 contributes to the enhanced conversion efficiency of the fabricated metalenses. The fabricated metalenses exhibit diffraction-limited focusing and imaging capabilities, comparable to the theoretically ideal lenses.

Synthesis of micro-crosslinked adamantane-containing matrix resins designed for deep-UV lithography resists and their application in nanoimprint lithography.

A certain type of photoresist used for deep-UV lithography (DUVL) can also be used for other types of photolithography. Thus, to meet the requirements of two or more lithography technologies simultaneously, it is necessary to design a variety of corresponding functional groups in the molecules of materials and obtain the required properties. Herein, we designed four matrix resins based on acrylate for DUVL, employing alkyl sulfide, adamantane, methyladamantane, and hydroxyl as dangling groups and a microcrosslinking network by adding a small amount of crosslinker. These polymers were used in the thermal nanoimprint lithography (NIL) process, and distinct patterns with a resolution of 100 nm were observed. The acrylate copolymers designed for DUVL in this work can be used as thermal NIL resists and to obtain good patterns. It was found that ethylene dimethacrylate (EDMA) and adamantane endowed the matrix resins with good thermal stability and that PMMHM demonstrated the best patterning performance among the four resins. These polymers can be applied in the manufacturing of high-density integrated circuits, nano-transistors, optoelectronic devices and other components in the future.

Scalable Nanoimprint Manufacturing of Functional Multilayer Metasurface Devices

AbstractOptical metasurfaces, consisting of subwavelength‐scale meta‐atom arrays, hold great promise of overcoming the fundamental limitations of conventional optics. Due to their structural complexity, metasurfaces usually require high‐resolution yet slow and expensive fabrication processes. Here, using a metasurface polarimetric imaging device as an example, the photonic structures and the Nanoimprint lithography (NIL) processes are designed, creating two separate NIL molds over a patterning area of > 20 mm2 with designed Moiré alignment markers by electron‐beam writing, and further subsequently integrate silicon and aluminum metasurface structures on a chip. Uniquely, the silicon and aluminum metasurfaces are fabricated by using the nanolithography and 3D pattern‐transfer capabilities of NIL, respectively, achieving nanometer‐scale linewidth uniformity, sub‐200 nm translational overlay accuracy, and <0.017 rotational alignment error while significantly reducing fabrication complexity and surface roughness. The micro‐sized multilayer metasurfaces have high circular polarization extinction ratios as large as ≈20 and ≈80 in blue and red wavelengths. Further, the metasurface chip‐integrated CMOS imager demonstrates high accuracy in broad‐band, full Stokes parameter analysis in the visible wavelength ranges and single‐shot polarimetric imaging. This novel, NIL‐based, multilayered nanomanufacturing approach is applicable to the scalable production of large‐area functional structures for ultra‐compact optic, electronic, and quantum devices.

Development of a broadband photodetector utilizing ZnO nanorods with grating structure fabricated via nanoimprint lithography

Nanoimprint lithography (NIL) has emerged as a powerful technique for integrating nanostructures into semiconductor devices, particularly optoelectronic devices such as solar cells, light-emitting diodes (LEDs), lasers, and photodetectors (PDs). This study presents the successful fabrication of nano-patterned ZnO nanorods (ZnO NRs) using NIL, resulting in a grating structure with a linewidth of approximately 600 nm and a periodicity of around 1080 nm. Morphological characterizations, including scanning electron microscopy (SEM), atomic force microscopy (AFM), and laser confocal microscopy, reveal well-defined grating structures with ZnO NRs deposited on the substrate. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) further confirm the crystallinity of ZnO NRs, both with and without NIL treatment. X-ray photoemission spectroscopy (XPS) shows that the NIL-patterned samples with ZnO NRs exhibit fewer defects, attributed to the O2 plasma treatment during the NIL process. The NIL-defined grating structure of ZnO NRs is then employed in the fabrication of photodetectors, which demonstrate higher sensitivity than the pure ZnO-NRs-based PDs, presumably due to the reduce defects. Moreover, the NIL-defined PDs exhibit broadband photoresponses across the UV and visible light spectrum. This work introduces a novel device design for photodetectors and highlights the potential of NIL in creating broadband ZnO-based PDs, promising applications in future optoelectronics.

Development of Nanopillar Arrays Nanopatterning Without Lift‐Off for Transferable GaN‐Based µLEDs

AbstractThe mass production of µLEDs requires an upscaling approach on 200 mm wafers, which implies the deployment of a technology that achieves zero defectivity without liftoff. In this report, Nanoimprint lithography (NIL) processing is successfully optimized for nanostructuring GaN‐based Silicon‐On‐Insulator (SOI) substrates. The etching of SiO2/GaN/AlN/Si/SiO2 layers using different plasmas is conducted and multi‐layer nanopillars 100–200 mm in diameter are fabricated. This approach generates zero‐defect arrays of pillars, which is particularly advantageous for the growth process. In addition, the SiO2 at the bottom of the pillar allows it to twist during the subsequent GaN regrowth, as this layer becomes soft at the growth temperature >1000 °C. This ability to deform enables a coalescence of pillars into layers with reduced dislocation density. As a result, high‐quality GaN microplatelets and µLEDs are grown via a bottom‐up approach based on pendeoepitaxy using metal–organic vapor phase epitaxy (MOVPE). The fabricated µLEDs have a very smooth surface with a roughness of 0.6 nm which facilitated the implementation of an easy and simple transfer protocol. Adhesive tape and metalmetal bonding, are used to bond the µLEDs onto a metal‐coated silicon substrate. The reported findings offer exciting new insights into the development of high‐performance displays.

Nanoimprint lithography for grayscale pattern replication of MEMS mirrors in a 200 mm wafer

In this work a multilevel nanoimprint lithography (NIL) replication process was demonstrated to produce 1D MEMS mirrors employing vertical asymmetric comb-drive electrostatic actuation, in a 200 mm wafer SOI-based process. In comparison with a direct write laser (DWL) grayscale lithography step (which for the proposed layout requires around 40 h of exposure time per wafer), this NIL method greatly enhances fabrication throughput by reliably reproducing a master's multilevel topography onto the photoresist. This study describes the NIL master fabricated using grayscale lithography, the working stamp, and the replication micromachining processes. An extensive characterization of the morphology and topography of the intermediate working stamp is provided, along with an optimization study of the replica fabrication and the alignment procedure between the replica and the mirror substrate. The MEMS device pattern was effectively replicated, exhibiting electrode gaps of 3.66 μm (grayscale process yields gaps of 3.5 μm). Discrete photoresist levels of 1.53 μm and 2.83 μm were observed, with a misalignment to the preceding metal layer of 5 μm and 10 μm in the x and y directions, respectively. These deviations were found to be within the required alignment margin defined by the layout (20 μm). The 1D MEMS mirror fabricated using this NIL process was successfully characterized using Scanning Laser-Doppler Vibrometry under atmospheric pressure conditions. The results obtained were in accordance with the theoretical design parameters, demonstrating that NIL can be successfully used as a fast, low-cost alternative lithography process to fabricate multilevel MEMS structures.

High‐Throughput Fabrication of Large‐Scale Metaholograms via One‐step Printing

AbstractMetasurfaces, which are designed by arranging meta‐atoms, have attracted attention for their ability to create optical properties that do not exist in nature. However, previous methods for fabricating metasurfaces using electron‐beam lithography (EBL) are expensive and have limited production capacity, inhibiting mass production. In this study, a new manufacturing method is demonstrated using an argon fluoride (ArF) scanner and nanoimprint lithography (NIL) to overcome these limitations. 266 millimeter‐scale metasurfaces are designed and fabricated on an 8‐inch wafer using an ArF scanner. Subsequently, this is used as a master stamp to replicate the metasurfaces in an 8‐inch wafer scale through the NIL process all at once. To demonstrate the effectiveness of this method, a metahologram is designed and manufactured using TiO2 nanoparticle‐embedded‐resin (nano‐PER). The metahologram achieves an 81.2% yield during replication, with a maximum efficiency of 60.7% at 450 nm, 45.1% at 532 nm, and 38.5% at 635 nm, respectively. The results demonstrate the production of practical metaholograms using low‐cost and high‐throughput processes.

Fabrication Technology of Self-Dissolving Sodium Hyaluronate Gels Ultrafine Microneedles for Medical Applications with UV-Curing Gas-Permeable Mold.

Microneedles are of great interest in diverse fields, including cosmetics, drug delivery systems, chromatography, and biological sensing for disease diagnosis. Self-dissolving ultrafine microneedles of pure sodium hyaluronate hydrogels were fabricated using a UV-curing TiO2-SiO2 gas-permeable mold polymerized by sol-gel hydrolysis reactions in nanoimprint lithography processes under refrigeration at 5 °C, where thermal decomposition of microneedle components can be avoided. The moldability, strength, and dissolution behavior of sodium hyaluronate hydrogels with different molecular weights were compared to evaluate the suitability of ultrafine microneedles with a bottom diameter of 40 μm and a height of 80 μm. The appropriate molecular weight range and formulation of pure sodium hyaluronate hydrogels were found to control the dissolution behavior of self-dissolving ultrafine microneedles while maintaining the moldability and strength of the microneedles. This fabrication technology of ultrafine microneedles expands their possibilities as a next-generation technique for bioactive gels for controlling the blood levels of drugs and avoiding pain during administration.

Visible Metalenses with High Focusing Efficiency Fabricated Using Nanoimprint Lithography

AbstractMetasurfaces enable precise control over the properties of light and hold promise for commercial applications. However, fabricating visible metasurfaces suitable for high‐volume production is challenging and requires scalable processes. Nanoimprint lithography is a cost‐effective and high‐throughput technique that can meet this scalability requirement. This work presents a mask‐templating nanoimprint lithography process for fabricating metasurfaces with varying fill factors and negligible wavefront aberrations using composite stamps. As a proof‐of‐concept, a 6 mm diameter metalens formed of silicon nitride nano‐posts with a numerical aperture of 0.2 that operates at 550 nm is demonstrated. The nanoimprinted metalens achieves a peak focusing efficiency of (81 ± 1)%, comparable to the control metalens made with electron beam lithography with a focusing efficiency of (89 ± 1)%. Spatially resolved deflection efficiency and wavefront data, which informs design and process optimization, is also presented. These results highlight nanoimprint lithography as a cost‐effective, scalable method for visible metasurface fabrication that has potential for widespread adoption in consumer electronics and imaging systems.

Multi-biomarker combination detection system for diagnosis and classification of dry eye disease by imaging of a multi-channel metasurface

Dry eye disease (DED) is one of the most common ocular surface diseases, characterized by unstable tear film and ocular inflammation, affecting hundreds of millions of people worldwide. Currently, the clinical diagnosis of DED mainly relies on physical methods such as optical microscopy and ocular surface interferometric imaging, but classifying DED is still difficult. Here, we propose a compact and portable immune detection system based on the direct imaging of a nanophotonic metasurface with gradient geometry, for fast and ultra-sensitive detection of multiple biomarkers (i.e. Matrix metalloproteinase-9 (MMP-9), Lipocalin-1 (LCN-1), Lactoferrin (LTF)) in tears for the diagnosis and classification of DED. This centimeter-scale concentric nanophotonic metasurface, which consists of millions of unique metallic nanostructures, was fabricated through a cost-effective nanoimprint lithography (NIL) process. The immune detection system based on the antibody-modified metasurface shows favorable detection selectivity, an ultra-high sensitivity (3350 pixels/Refractive Index Unit (RIU)) and low limit of detection (LOD) (0.3 ng/mL for MMP-9, 1 ng/mL for LTF, and 0.5 ng/mL for LCN-1). Further clinical sampling and detection results demonstrated that this multi-biomarker detection system enabled accurate determination and symptom classification of DED, manifesting high correlation and consistency with clinical diagnosis results. The advantages such as low sample consumption, one-step detection, simple operation, and simultaneous detection of multiple biomarkers make the platform promising for screening and detecting a broader range of biomarker combinations in clinical practice.

Deformation Mechanism of Aluminum, Copper, and Gold in Nanoimprint Lithography Using Molecular Dynamics Simulation.

Material deformation during nanoimprinting of aluminum (Al), copper (Cu), and gold (Au) was explored through molecular dynamics simulations. A comparative understanding of the deformation behavior of three substrate materials important for design and high-resolution pattern transfer was highlighted. In this study, we analyzed three metrics, including von Mises stresses, lattice deformation, and spring-back for the chosen materials. Of the three materials, the highest average von Mises stress of 7.80 MPa was recorded for copper, while the lowest value of 4.68 MPa was computed for the gold substrate. Relatively higher von Mises stress was observed for all three materials during the mold penetration stages; however, there was a significant reduction during the mold relaxation and retrieval stages. The Polyhedral Template Matching (PTM) method was adopted for studying the lattice dislocation of the materials. Predominantly Body-Centered Cubic (BCC) structures were observed during the deformation process and the materials regained more than 50% of their original Face-Centered Cubic (FCC) structures after mold retrieval. Gold had the lowest vertical spring-back at 6.54%, whereas aluminum had the highest average spring-back at 24.5%. Of the three materials, aluminum had the lowest imprint quality due to its irregular imprint geometry and low indentation depth after the NIL process. The findings of this research lay a foundation for the design and manufacture of Nanoimprint Lithography (NIL) molds for different applications while ensuring that the replicated structures meet the desired specifications and quality standards.

Implementation of various colors in Cu(In,Ga)Se2 thin-film solar cells by diffractive nanostructures

With the increasing use of building-integrated photovoltaic technology, it has become necessary for solar panels to blend in with their surroundings. Diffractive nanostructures can be used to reduce optical losses and create colorful solar cells. To make colors on Cu(In,Ga)Se2 (CIGS) thin-film solar cells, we fabricated diffractive nanostructures by using nanoscale imprinting and transfer lithography methods. And we investigated how the material types and pattern shapes of the nanostructures influence the optical properties of solar cells, such as the short-circuit current density (JSC), coordination of color appearance, and color quality. We used two types of pillars and grating nanostructures of SiO2 and TiO2 layers on CIGS thin-film solar cells to produce various colors. The SiO2 demonstrated an increase in JSC without significant loss of color quality compared to TiO2. By utilizing a hexagonally arrayed pillar pattern, colors are observed at various angles on an axis different from the incident light, unlike colors observed only at a single angle in the grating structure. The nanoimprinting lithography process allowed us to produce high-quality nanostructures on both rigid glass and flexible stainless-steel substrates.

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.

High aspect ratio slanted grating replication process with low residual layer thickness

Slanted subwavelength gra2ngs are used today in various products, especially as TIR couplers in Augmented Reality (AR) waveguides. Nano-Imprint Lithography (NIL) has been successfully used as a produc2on technique for such gra2ngs with periods down to 250nm, slants up to 45 degrees and aspect ra2o up to 2x. In order to propagate larger Field of View (FOV) in AR headsets, the refrac2ve index of the waveguide substrate needs to be large (1.9 to 2.0). NIL technologies typically produce a Residual Layer Thickness (RLT) of 100nm or more, requiring the index of the gra2ng resin to match the index of the substrate to prevent FOV clipping. This brings up many challenges such as TiO2 nanofiller migra2on during NIL process, scaRering and absorp2on in blue, and hard master / soT stamp combined degrada2ons. These challenges triggered a need for non-NIL techniques to mass produce such nanostructures, also at a lower cost. Avoiding mechanical contact between the master and the gra2ng material is desired, as costly masters would not deteriorate with volume produc2on. First experimental results are presented with aspect ra2o of 10x at 45deg slant and RLT below 3nm.

Scaling up the sub-50 nm-resolution roll-to-roll nanoimprint lithography process via large-area tiling of flexible molds and uniform linear UV curing

Scaling up the sub-50 nm-resolution roll-to-roll nanoimprint lithography process via large-area tiling of flexible molds and uniform linear UV curing

Ultrathin metal-mesh Janus membranes with nanostructure-enhanced hydrophobicity for high-efficiency fog harvesting

Fog harvesting has been reckoned to be one of the most facile and economical approaches to alleviating the ever-increasing global water scarcity . Janus membranes, especially metallic ones, outperform other fog collectors in terms of fog harvesting efficiencies , and they are the selection for sustainable water production as passive devices. However, there is a lack of metallic Janus membranes with ultrasmall feature sizes due to the limitation of the laser ablation fabrication method , which restrains parameter optimization for highly efficient fog collectors. In this work, photolithography , electroplating , and nanoimprint lithography processes are employed to enable ultrasmall membrane thickness and nanopatterning on the membrane surface . A 4-μm-thick ultrathin hierarchical metal-mesh Janus membrane with a pitch of 60 μm is fabricated, which achieves a record-high water collection rate of 233 mg cm −2 ·min −1 with nanostructure-enhanced hydrophobicity . The morphology of the membrane is characterized by scanning electron microscopy, while the fog collection process is observed under a high-speed camera and a microscope. Experimental measurement and finite-element numerical modeling unveil that a smaller membrane thickness and a more hydrophobic water-collecting surface contribute to higher water collection rates by accelerating water transport and reducing water re-evaporation. • Photolithography and electroplating replace laser ablation for metallic JM fabrication. • Metallic JMs with unprecedented thickness down to 4 μm are fabricated. • Nanopatterning on JMs changes surface wettability and boosts WCR. • A record-high WCR of 233 mg cm −2 ·min −1 is achieved.

Plasmonic Gold Trimers and Dimers with Air-Filled Nanogaps.

The subwavelength confinement of light energy in the nanogaps formed between adjacent plasmonic nanostructures provides the foundational basis for nanophotonic applications. Within this realm, air-filled nanogaps are of central importance because they present a cavity where application-specific nanoscale objects can reside. When forming such configurations on substrate surfaces, there is an inherent difficulty in that the most technologically relevant nanogap widths require closely spaced nanostructures separated by distances that are inaccessible through standard electron-beam lithography techniques. Herein, we demonstrate an assembly route for the fabrication of aligned plasmonic gold trimers with air-filled vertical nanogaps having widths that are defined with spatial controls that exceed those of lithographic processes. The devised procedure uses a sacrificial oxide layer to define the nanogap, a glancing angle deposition to impose a directionality on trimer formation, and a sacrificial antimony layer whose sublimation regulates the gold assembly process. By further implementing a benchtop nanoimprint lithography process and a glancing angle ion milling procedure as additional controls over the assembly, it is possible to deterministically position trimers in periodic arrays and extend the assembly process to dimer formation. The optical response of the structures, which is characterized using polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, shows properties that are consistent with simulation. This work, hence, forwards the wafer-based processing techniques needed to form air-filled nanogaps and place plasmonic energy at site-specific locations.

Dense high aspect ratio nanostructures for cell chip applications - Fabrication, replication, and cell interactions

Culturing human cells on the surface of a microchip brings living cells in direct contact with artificial microstructured surfaces. This work focuses on the effect of high aspect ratio nanostructures – dense nanoneedle arrays – on the mechanical response and proliferation of fibroblasts. We present a fabrication process for micropatterned chips that feature areas with hierarchical high aspect ratio nanostructures directly adjacent to flat chip areas. The chip was pre-patterned by conventional lithography. We have fabricated “black silicon” arrays of densely packed, sharp, vertical nano-needles for high aspect ratio structures by cryogenic reactive ion etching in an SF 6 /O 2 plasma. An essential requirement for many real-life applications is to make such complex high aspect ratio 3D nanostructures available in larger areas. We have successfully demonstrated the 3D replication of black silicon by a UV- nanoimprint lithography process. This study provides insight into the extent to which such nanoneedle arrays influence the growth of human fibroblasts. We have investigated microstructured samples featuring a combination of (i) smooth surfaces and (ii) nanoneedle surfaces for the mechanical behavior and proliferation of fibroblasts. Our results show that the bonding viability and proliferation of the fibroblast on the high aspect ratio nano-needle surfaces differ significantly from flat surfaces. The nanoneedles only provide a minimum area for cell attachment compared to the neighboring flat, unstructured chip areas. • Development of a fabrication process for a dense nano-needle array. • Reproduction of black silicon using nanoimprint lithography. • Black silicon structures were investigated with regard to NIL replication and cell proliferation.

Large-area homogeneous corrosion process for electrochemical nanoimprint lithography on GaAs wafer by modulating contact pressure

• The contact pressure and its homogeneity was proven to be crucial to the ECNL process. • To avoid mold distortion and product defects, the contact pressure was optimized as 143 kPa. • A dual-cushion omnidirectional structure was designed to improve the uniformity of the contact pressure. • The homogeneity of the imprinted microstructures in large area was improved to 99% or higher finally. Electrochemical nanoimprint lithography (ECNL) has proven to be a prospective manufacturing technology for the fabrications of three-dimensional micro/nano-structures (3D-MNSs) directly on semiconductors. It is an essentially spatially confined corrosion process induced by the contact potential across the metal/semiconductor boundaries, so the contact pressure between the metallized mold and the semiconductor workpiece and its homogeneity play crucial roles for the product quality. For this, we investigated the influence of the contact pressure on the ECNL process by combining experiments with theoretical analyses. The results show that the pressure should be optimized to improve the imprinting efficiency and accuracy, and excessive pressure would lead to mold distortion and product defects. To ensure the homogeneity of the pressure distribution between the metallized mold and the semiconductor workpiece, a dual-cushion omnidirectional structure was designed and, consequently, the imprint uniformity in large area was improved. The finite element simulations are in harmonious accordance with the experimental results, which is proved very instructive for the optimization of ECNL technical parameters.