Nanoimprint Process Research Articles

A Rapid Fabrication Method of Large-Area MLAs with Variable Curvature for Retroreflectors Based on Thermal Reflow.

Retroreflectors are an important optical component, but current retroreflector structures and manufacturing processes are relatively complex. This paper proposes a rapid, low-cost, large-area method for fabricating retroreflectors based on microlens arrays. Tunable microlens arrays with adjustable curvature, fill factor, and sizes were prepared using photolithography and thermal reflow techniques. Subsequently, a two-step nanoimprinting process was used to create a flexible reverse mold and transfer the structure onto the desired substrate. The microlens arrays, with a diameter of 30 μm, a period of 33 μm, a curvature radius ranging from 15.5 to 18.8 μm, and a fill factor ranging from 75.1% to 88.8%, were fabricated this way. In addition, the method also fabricated microlens arrays with diameters ranging from 10 to 80 μm. Retroreflectors were made by sputtering a layer of silver on the MLAs as a reflecting layer, and tests showed that the microlens-based retroreflector exhibited superior retroreflective performance with a wide-angle response of ±75°. Microlens-based retroreflectors have the advantages of simple operation and controllable profiles. The fabrication method in this paper is suitable for large-scale production, providing a new approach to retroreflector design.

Investigation of speed and temperature effects in mechanical nano-patterning of GaAs via molecular dynamics simulation

Speed and temperature are two key parameters governing GaAs mechanical nano-patterning process. For the first time, this study evaluates the effects of pressing speed and nanoimprinting temperature on the deformation behavior and mechanical properties of GaAs in the mechanical nano-patterning process. Simulation results reveal that high pressing speed and high nanoimprinting temperature facilitate the nano-pattern formation on GaAs. For the pressing speed effect, the maximum force, residual stress, and cubic diamond atomic friction decrease with the elevated pressing speed, while the surface pile-up is minimally influenced by the pressing speed during the nanoimprinting process. Moreover, the morphological accuracy of the nano-patterns is enhanced with increasing pressing speed. For the temperature effect, simulation results reveal that amorphization and plastic activity exhibit a positive correlation with increasing nanoimprinting temperature, whereas the maximum force and residual stress demonstrate a roughly inverse relationship with nanoimprinting temperature. Additionally, the elevated temperature also exerts a substantial influence on the dislocation density, morphological accuracy, and surface pile-up. This study contributes to a comprehensive understanding of the effects of these two key factors on the mechanical properties and deformation behavior of GaAs in mechanical nano-patterning.

Facile Fabrication of Superhydrophobic and Superoleophilic Polyurethane Foil with Micropillar and Microporous Structures for Efficient Oil/Water Separation

Oil spill cleanup in water remains a critical challenge due to the harmful secondary pollution from conventional methods such as burning or chemical degradation. Herein, we present a facile method to fabricate a superhydrophobic and superoleophilic polyurethane (PU) foil for efficient and environmentally friendly oil/water separation. More specifically, micropillar arrays were embedded onto the foil surface through a nanoimprinting process. Microporous structures were generated at the foil cross-section by a supercritical carbon dioxide (CO2) saturation method. The dimensions of pillar and pore structures were optimized with the aim of boosting selective wetting (i.e., water repellency and oil attraction) properties. As a result, the developed PU foil shows an oil absorption efficiency nearly 4 times higher than a pristine reference. Moreover, the structured PU foil stably retains the absorbed oil for over a week, demonstrating an absorption capacity of nearly 400%, which is also much superior than the unstructured sample. Our concept of combining both topographical micropillars and cross-sectional micropores onto PU foil provides a novel approach for achieving efficient and environmental friendly oil/water separation.

Boosting the Efficiency of High-Resolution Quantum Dot Light-Emitting Devices Based on Localized Surface Plasmon Resonance.

With pixel miniaturization, the performance of high-resolution quantum dot light-emitting diodes (QLEDs) usually degrades. Considering the dimension of ultrasmall pixels, herein, a barrier architecture based on localized surface plasmon resonance (LSPR) that promotes the radiative recombination of neighboring quantum dots is rationally designed to improve the device performance. Au nanoparticles (NPs) are embedded in an insulating polymer to form a honeycomb-patterned barrier layer via the nanoimprint process. Each pixel fabricated in the void area (average diameter of 1.5 μm) of the barrier layer is surrounded by a number of LSPR-NPs to enhance the luminescence. The resultant green QLEDs with a resolution of 9027 pixels per inch show a maximum external quantum efficiency of 11.1%, a 42.8% enhancement compared to the control device. Additionally, the lifetime of high-resolution QLEDs is obviously improved by the LSPR effect.

Effects of temperature, punch angle, and loading velocity on mechanical properties of nanoimprinted FeNiCoCrCux high-entropy alloys

In this work, we use molecular dynamics simulations to study the effects of temperature, loading velocity, the taper angle of the punch on mechanical characteristics and deformation behavior of FeNiCoCrCu high-entropy alloys (HEAs) in nanoimprinting process. The impact of these factors is evaluated through loading force, atomic shear strain, structural change, dislocation evolution and elastic recovery rate. It points out that the primary deformation mechanism during imprinting of FeNiCoCrCu HEAs materials is the occurrence and propagation of dislocation and stacking faults. Increasing the temperature reduces the loading force, and the nanopatterning ability is better. When the imprinting speed increases, the stacking fault and dislocation increase, and the loading force improves. The results show that the higher the imprinting speed, the smaller the elastic recovery ratio. That proves the pattern forming at high imprinting speed will be better. As the mold angle increases, the depth of the forming groove increases. With the change of mold angles, the stress changes not only its value but also its distribution. As the taper angle of the punch increases, the high-stress region increases. The lower the concentration of Cu in the compound, the higher the forming ability and the pattern is sharper and clearer. While, the total dislocation and the residual stress in the substrate after withdrawing the punch from the substrate will gradually decrease on the order of FeNiCoCrCu0.2, FeNiCoCrCu1.3, FeNiCoCrCu0.7, FeNiCoCrCu0.4, FeNiCoCrCu1.0.

Experimental and simulation investigation for imprinting and buckling of V-groove/wrinkles hierarchical array

With the in-depth development and continuous innovation of micro and nano manufacturing technology, flexible strain sensors are more and more widely used in various fields, including soft robots, smart clothing and so on, and high-performance sensors are more in demand. An efficient tactic for enhancing the performance of flexible strain sensors is to design and create hierarchical structures, and its efficient and controllable manufacturing is a key problem. The controllable manufacturing of V-groove/wrinkles hierarchical structure is realized by nano-imprint and prestretch-release process, which provides an effective method for large-area controllable preparation of hierarchical structure, and provides a foundation for the construction of high-performance flexible strain sensor. Single-factor experiments were carried out on the slope of the primary structure, pre-strain and substrate thickness, and response surface analysis was carried out by Box–Behnken experiments. The results of the structure’s formation are significantly influenced by the slope and substrate thickness, and the optimal process parameters are obtained by response surface analysis. Then the impact of each process parameter was investigated using a process simulation model. Finally, the forming conditions of hierarchical structure are analyzed. This study can provide guidance for the efficient and controllable manufacturing of micro/nano hierarchical structures.

Assessment of Nano-Imprinting Process in CuZr Amorphous Films Through Combination of Machine Learning and Molecular Dynamics

Assessment of Nano-Imprinting Process in CuZr Amorphous Films Through Combination of Machine Learning and Molecular Dynamics

Narrow Line Width Ni–Cu–Sn Front Contact Metallization Patterns for Low‐Cost High‐Efficiency Crystalline Silicon Solar Cells using Nano‐Imprint Lithography

An in‐house‐developed nanoimprint lithography system is designed and developed to demonstrate a commercially viable method for creating Ni–Cu–Sn‐based narrow linewidth finger patterns capable of effectively reducing shadow loss in the front contact metallization on industrial silicon solar cells. Finite element analysis simulation studies estimate that an optimal force of 138 MPa is required to generate damage‐free patterns during nanoimprinting process. The poly (methyl methacrylate) residual layer is removed using reactive ion etching, which enhances the adhesion strength of Ni–Cu–Sn metallization. A low ohmic contact layer at the Si/Ni interface develops after sintering at 420 °C and formation of NiSi layer is confirmed through X‐ray photoelectron spectroscopy analysis as a function of Ar+ ion etching time. An average cell efficiency of 18.10% is achieved for silicon solar cells with micropatterned Ni/Cu/Sn‐based narrow linewidth front contact grid design, which can exhibit ≈1% enhancement in cell efficiency compared to commercial Ag screen‐printed solar cells. The Ni/Cu/Sn‐based front contact metallization can improve the cell performance by ≈6% compared to the commercial Ag screen‐printed metallization by reducing shadow loss and thereby indicating its suitability toward deploying for cost‐effective industrial solar cell manufacturing.

Engineering Large-Area Antidust Surfaces by Harnessing Interparticle Forces.

Dust accumulation is detrimental to optical elements, electronic devices, and mechanical systems and is a significant problem in space missions and renewable energy deployment. In this paper, we report the demonstration of antidust nanostructured surfaces that can remove close to 98% of lunar particles solely via gravity. The dust mitigation is driven by a novel mechanism, whereby particle removal is facilitated by the formation of particle aggregates due to interparticle forces, allowing the particles to be removed in the presence of other particles. The structures are fabricated using a highly scalable nanocoining and nanoimprint process, where nanostructures with precise geometry and surface properties are patterned on polycarbonate substrates. The dust mitigation properties of the nanostructures have been characterized using optical metrology, electron microscopy, and image processing algorithms to demonstrate that the surfaces can be engineered to remove nearly all of the particles above 2 μm in size in the presence of Earth's gravity. Compared to the 35.0% area coverage on a smooth polycarbonate surface, the particle coverage on nanostructures with 500 nm period is significantly reduced to 2.4%, an improvement of 93%. This work enhances the understanding of the particulate adhesion on textured surfaces and demonstrates a scalable, effective solution to antidust surfaces that can be broadly applied to windows, solar panels, and electronics.

Large-area, omnidirectional metasurface exhibiting unpolarized broadband absorption

Despite the ongoing efforts to design and fabricate large-area metasurface absorbers using cost-effective fabrication processes and low-cost materials, significant challenges persist. In this study, we present an anodized aluminum oxide template-assisted metasurface absorber with an area of 4 cm2, which incorporates glue cylinder arrays, a low-cost nickel layer, and protective glue layers. This device achieved an average absorptivity of approximately 91% (90% in the experiment) in the 200–900 nm wavelength range. The absorption level remained above 80% even at incident angles up to 45°, with calculated and experimental values of 92% and 82%, respectively. Moreover, the absorption is not sensitive to the thickness of the residual glue layers, making it ideal for roll-to-roll nanoimprinting processes. Therefore, the proposed metasurface absorber offers an alternative method for achieving broadband absorption in an effective manner and has great potential for related applications.

Triple and Quadruple Surface Pattern Memories in Nanoimprinted Polymer Blends

Trigger-responsive surfaces with multiple surface properties have wide-ranging application potential from surfaces with trigger-responsive fluid flow to cell culture to optical effects; such surfaces can be achieved through surface morphological changes. Although multiple shape-memory effects are successful in bulk polymers, there is limited programing and recovery of multiple surface memories due to the challenges in fabricating multiple surface topographies with good controllability. Here, we report the synergy between the polymer blend formulation and the thermal nanoimprinting process to achieve multiple microtopography memories. A series of immiscible blends consisting of poly(caprolactone) (PCL) and polyethylene (PE) with distinct thermal transitions governed by distinct crystallization events were augmented with improved elasticity through preferential cross-linking in the polymer blend. The effect of preferential cross-linking by dicumyl peroxide on the elastic property of the PCL/PE has been found to be nonlinearly dependent on the blend composition. This approach enabled triple and quadruple surface pattern fixity and recovery in nanoimprinted PCL/PE blends. Specifically, we demonstrated the recovery of a micropillar structure (diameter: 20 μm and height: 10 μm) from a hierarchical micrograting topography (width: 2 μm and height: 2 μm) when exposed to a thermal stimulus at 60 °C for 180 s. Furthermore, we also demonstrated the recovery of a deformed micrograting followed by a secondary recovery of the micropillar structure.

Curvature-Adjustable Polymeric Nanolens Fabrication Using UV-Controlled Nanoimprint Lithography.

Nanolenses are gaining importance in nanotechnology, but their challenging fabrication is thwarting their wider adoption. Of particular challenge is facile control of the lens' curvature. In this work, we demonstrate a new nanoimprinting technique capable of realizing polymeric nanolenses in which the nanolens' curvature is optically controlled by the ultraviolet (UV) dose at the pre-curing step. Our results reveal a regime in which the nanolens' height changes linearly with the UV dose. Computational modeling further uncovers that the polymer undergoes highly nonlinear dynamics during the UV-controlled nanoimprinting process. Both the technique and the process model will greatly advance nanoscale science and manufacturing technology.

Wire-grid polarizer sheet with low reflectance in the visible and near-infrared regions fabricated by a nanoimprinting and electroless plating process.

In near-infrared systems for optical sensing and imaging technologies, an improved signal-to-noise ratio and more advanced object detection and analysis using polarizers are required. Such polarizers are limited, and broadband wire-grid polarizers are potential candidates. However, their high reflectivity and high cost limit their application. Herein, we fabricated a low-reflectivity wire-grid polarizer sheet that can be used in visible and near-infrared regions by a simple process using only nanoimprinting and nickel electroless plating; further, metal removal steps such as chemical mechanical polishing were not required. The results obtained by optimizing the structural shape of the mold and precisely controlling the thickness of the electroless plating confirmed that the polarizer could be used for visible and near-infrared light. Because the sheets can be manufactured using general-purpose equipment upon mold preparation, they can be employed in wide-ranging applications with small capital investment.

Nanoscale patterning of metallic surfaces with laser patterned tools using a nanoimprinting approach

In this study, we demonstrate that metallic substrates with a nanocrystalline grain size can be structured down to the micro- and nanometre range during a room temperature nanoimprinting process. WC-Co hard metal dies are patterned by Direct Laser Interference Patterning (DLIP), generating an array of separated asperities with a height of 4.7 µm and a spacing of 11.3 µm. Additionally, Laser Induced Periodic Surface Structures (LIPSS) with a spacing of 500 nm are formed as a hierarchical substructure. The patterned dies can be used to deform metallic substrates due to their high hardness. For structure replication, the WC-Co tool is pressed onto a nanocrystalline CuZn30 model alloy at room temperature. The pattern transfer process from the hard metallic tool to the nanocrystalline alloy is discussed in detail: At low contact pressures, the asperities on the tools act as single contact points, leading to the formation of separated dimples with a depth of up to 0.8 µm in the nanocrystalline alloy. At high contact pressures, the tool pattern, including the LIPSS is almost completely transferred to the substrate. The formed asperities protrude out of the surface, with a height to width aspect ratio of 0.4. It is thereby demonstrated, that plastic flow processes during the imprinting of nanocrystalline alloys allow for the replication of the DLIP tool pattern, even down to nano-scaled LIPSS. The imprinted array of surface asperities exhibits hydrophobic properties, due to a combined topographical and chemical influence on the wetting behaviour.

Fabrication of Nanostructures on a Large-Area Substrate with a Minimized Stitch Error Using the Step-and-Repeat Nanoimprint Process.

Nanoimprint lithography (NIL) is suitable for achieving high uniformity and mass production. However, in conventional NIL, a stamp suitable for the substrate size is required to increase the substrate size. To address this issue, we fabricated nanostructures on a large-area substrate using step-and-repeat NIL after making a small stamp. A stamp was produced using glass, and a nano-pillar pattern with a diameter of 600 nm, an interval of 400 nm, and a height of 270 nm was used during the experiment. The area of the pattern on the stamp was 10 mm × 10 mm, and the step-and-repeat process was performed 25 times to transfer the nanostructures to a 4-inch substrate. In addition, stitch gaps were created between the patterns, which could decrease the performance upon future application. To minimize this stitch gap, a high-precision glass scale was attached to the stamp feeder to precisely control the position and to minimize the step difference. Moreover, an experiment was conducted to minimize the stitch gap by adjusting the movement interval of the stamp, and the stitch spacing was minimized by moving the stamp position by 9.97 mm. This approach will facilitate the manufacturing of large-area substrates and other structures in the future.

Molecular simulation study on mechanical properties and elastic recovery of nanoimprinted CuAgAu metallic glasses

By using molecular dynamics simulations, the nanoimprinting process was performed to evaluate the mechanical properties and elastic recovery of the CuAgAu metallic glasses. The influences of cooling rate, stamp velocity, temperature, and percentage of the Cu-Ag-Au elements were considered in detail. By changing the imprint conditions, the mechanical responses of the nanocrystalline CuAgAu metallic glasses were mentioned and analyzed. Some important mechanical factors such as imprint force, shear strain, shear transformation zones, the fraction of atoms with high shear strain, and elastic recovery ratio were significantly influenced by changing the simulated conditions and deeply discussed.

Smart Systems for Material and Process Designing in Direct Nanoimprint Lithography Using Hybrid Deep Learning.

A hybrid smart process and material design system for nanoimprinting is proposed, which is combined with a learning system based on experimental and numerical simulation results. Instead of carrying out extensive learning experiments for various conditions, the simulation learning results are partially complimented when the results can theoretically be predicted by numerical simulation. In other words, the data that are lacking in experimental learning are complimented by simulation-based learning results. Therefore, the prediction of nanoimprint results without experimental learning could be realized under various conditions, even for unknown materials. In this study, material and process designs are demonstrated for a low-temperature nanoimprint process using glycerol-containing polyvinyl alcohol. The experimental results under limited conditions were learned to investigate the optimum glycerol concentrations and process temperatures. Simulation-based learning was used to predict the dependence on press pressure and shape parameters. The prediction results for unknown glycerol concentrations agreed well with the follow-up experiments.

Omnidirectional, thin metasurface exhibiting selective absorption for un-polarized broadband incidence.

Thin devices with large areas have strong and omnidirectional absorption over a wide bandwidth and are in demand for applications such as energy harvesting, structural color, and vehicle LiDAR (laser detection and ranging). Despite persistent efforts in the design and fabrication of such devices, the simultaneous realization of all these desired properties remains a challenge. In this study, a 190-nm-thick metasurface with an area of 3 cm2, incorporating dielectric cylinder arrays, a chromium layer, a silicon nitride (SiNx) layer, and an aluminum layer is theoretically and experimentally demonstrated. The developed device achieves an average absorptivity of ∼99% (97% in the experiment) in the entire visible spectrum 400-700 nm. Moreover, it exhibits strong absorption over a wide range of incident angles (∼91% and 90% at 60° in the calculation and experiment, respectively). Importantly, the feasibility of applying the developed metasurface absorber to solar thermophotovoltaics and vehicle LiDAR (laser detection and ranging) has been explored. Moreover, the photoresist can be replaced by other glues and easily scaled up to a large area using the roll-to-roll nanoimprinting process. With the excellent spectral properties and performance, this device is promising for large-area applications.

Effects of mold shape mold vibration and substrate composition in FeNiCrCoMn high entropy alloys nanoimprint

High entropy alloys (HEAs) have received attraction due to their advantage of strength, hardness, fatigue, anticorrosion, and thermal stability. This article examines the impacts of mold shape, composition, and vibration-assisted on the nanoimprinting process of FeNiCrCoMn HEAs by using molecular dynamics simulation. Changing the shape of the mold could improve the imprinting process. The results reveal that increasing the mold angle results in a higher atomic strain rate and a more robust structure transformation of the imprinted shape. In general, a mold with a corner radius performs better than one with a sharp corner in terms of deformation. In detail, as the deformation process becomes smoother as the corner radius increases, the atomic strain, maximum force, and residual stress decrease. The elastic rate tends to increase as the corner radius increases. The FeNiCrCoMn composition has a significant impact on the imprinting process. The 20%Fe substrate has the lowest highest rate of elastic deformation and lowest rates of residual stress, and structural change when compared to other compositions having 30–60% Fe. This composition has a high resistance to deformation than other compositions. Generally, vibration-assisted imprinting may result in a better imprinted shape. Increasing the vibration amplitude results in a larger imprinted shape height, strain, stress, force, and amorphous structure due to the greater plastic deformation. This study sheds more light on the benefits of suitable mold angle, mold fillet radius, alloys composition, and vibration-assisted in the imprinting process.

Comparison of UV-curable materials for high-resolution polymer nanoimprint stamps

We present a systematic comparison of four resins for nanoimprint intermediate polymer stamps, the standard IPS material, OrmoStamp and two new polymers GMN-PS90 (foil) and GMN-PS40 (liquid). The new polymers are based on acrylated silane and offer sufficiently high Young's modulus and low shrinkage to provide good mechanical stability during the imprint process of well-separated structures. No anti-sticking treatment requires for those new polymers and they can be used at room temperature, which results in better stability of the nanoimprint process. The nanoimprint experiments using four resins showed a superior performance of the GMN-PS90 material in both single-layer and double-layer imprint process with resolution after lift-off about 30 nm. • Comparison of four resins for intermediate nanoimprint stamps • The GMN-PS90 polymer showed the best performance for the Au lift-off applications • Pattern transfer as small as 30 nm demonstrated for the GMN-PS90 resin