Nanoimprint Research Articles

Top‐Down Fabrication of Ordered Nanophotonic Structures for Biomedical Applications

AbstractThe size, distribution, and specific shape of ordered nanophotonic structures are crucial for their biomedical applications. Bottom‐up approaches such as self‐assembly, emulsification, and precipitation are commonly fabricated nanophotonic structures, which often lack control of nanophotonic structures morphologies and monodispersed sizes. On the contrary, top‐down nanofabrication techniques offer the advantages of high fidelity and high controllability and are employed in the fabrication of nanophotonic structures. This review focuses on top‐down nanofabrication techniques to fabricate ordered nanophotonic structures and their biomedical applications. Several top‐down approaches used in the semiconductor industry and other fields requiring micro‐ and nanopatterns are used, including electron beam lithography/ion beam lithography, photolithography, interference lithography, nanoimprint lithography, nanosphere lithography, nanotransfer lithography, and nano‐electrodeposition. Various current and emerging biomedical applications of the ordered nanophotonic structures are also covered: i) surface‐enhanced Raman scattering, ii) plasmonics, including surface plasmon resonance and localized surface plasmon resonance, and iii) fluorescence enhancement. Finally, a future perspective of nanophotonic structures fabricated by top‐down techniques in biomedical applications is also summarized.

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.

Advanced thin films of indium-tin oxide doping with photosensitive polymer via embossing process

We propose a sol-gel thin film formation process involving nanoimprint lithography. First, indium tin oxide was dissolved in 2-methoxyethanol at a ratio of 5:5 and the mixture were mixed with 10 wt% of a UV-curable. Subsequently, a polydimethylsiloxane sheet prepared by covering a silicon wafer with a polydimethylsiloxane mold was attached to a InSnO thin film to duplicate the nanostructure through UV irradiation exposure. The replicated nanostructured thin films formed about morphological and chemical composition changes on the surface, we progressed to x-ray photoelectron spectroscopy and atomic force microscopy analysis. Furthermore, atomic force microscopy image analysis showed superior patterned grooves for a UV exposure time of 3 min. A suitability test involving the measurement of the transmittance was performed for examining the suitability of the thin film for use in display devices.

Cost-Effective and Environmentally Friendly Mass Manufacturing of Optical Metasurfaces Towards Practical Applications and Commercialization

AbstractOptical metasurfaces consisting of two-dimensional nanostructures have rapidly developed over the past two decades thanks to their potential for use as optical components, such as metalenses or metaholograms, with ultra-compact form factors. Despite these rapid developments, major challenges for the commercialization of metasurfaces still remain: namely their mass production and use in real-life devices. A lot of effort has been made to overcome the limitations of electron beam lithography which is commonly used to fabricate metasurfaces. However, a breakthrough in mass production is still required to bring the cost of metasurfaces down into the price range of conventional optics. This review covers deep-ultraviolet lithography, nanoimprint lithography, and self-assembly-based fabrication processes that have the potential for the mass production of both cost-effective and environmentally friendly metasurfaces. We then discuss metalenses and future displays/sensors that are expected to take advantage of these mass-produced metasurfaces. The potential applications of mass-produced optical metasurfaces will open a new realm for their practical applications and commercialization.

Revised mathematical model for assessment of thermal characteristics developed in ultrafast pulse laser assisted nanoimprinting lithography on silicon based substrate surface

Revised mathematical model for assessment of thermal characteristics developed in ultrafast pulse laser assisted nanoimprinting lithography on silicon based substrate surface

Nanoimprinted Hierarchical Micro-/Nanostructured Substrates for the Growth of Cardiomyocyte Fibers

Investigating the behavior of cardiomyocytes is an important part of drug development. We present a structure and a related nanoimprint-based fabrication method, where the cardiomyocytes form isolated fibers, which is beneficial for drug testing, more closely representing the structure of the cardiomyocytes in vivo. We found that channel structures with walls with a rough top surface stimulate cardiomyocytes to form such fibers, as desired. Nanoimprint lithography is used as a fast and cost-efficient method to fabricate our hierarchically structured cell growth substrates.

Large-area ultracompact pixelated aluminum-wire-grid-based metamaterials for Vis-NIR full-Stokes polarization imaging

The study of pixelated metamaterials that integrate both the functions of linear and circular polarization filters is rapidly growing due to the need for full-Stokes polarization imaging. However, there is a lack of large-area, ultracompact pixelated full-Stokes metamaterials with excellent performance, especially circular polarization filters with a high extinction ratio, a broad operating bandwidth, and a low-cost, high-quality, efficient manufacturing process, which limits the practical applications of pixelated full-Stokes metamaterials. In this study, we propose a universal design and fabrication scheme for large-area, ultracompact pixelated aluminum wire-grid-based metamaterials used in Vis-NIR full-Stokes polarization imaging. The aluminum wire-grid was designed as a linear polarization filter with an average linear polarization extinction ratio of 36,000 and a circular polarization filter with an average circular polarization extinction ratio of 110 in Vis-NIR. A large-area, ultracompact 320×320 pixelated aluminum wire-grid-based full-Stokes metamaterial was fabricated using nanoimprint lithography and nano transfer printing with the advantages of low cost and high efficiency. This metamaterial was used to achieve full-Stokes polarization imaging with errors within 8.77%, 12.58%, 14.04%, and 25.96% for Stokes parameters S0, S1, S2, and S3, respectively. The inversion errors of the compensated Stokes parameters can be reduced to 0.21%, 0.21%, 0.42%, and 1.96%, respectively.

Replicated biopolymer pattern on PLLA-Ag basis with an excellent antibacterial response

The design of functional micro or nanostructured surfaces is undergoing extensive research for their intriguing multifunctional properties and for large variety of potential applications in biomedical field (tissue engineering or cell adhesion), electronics, optics or microfluidics. Such nanosized topographies can be easily fabricated by various lithography techniques and can be also further reinforced by synergic effect by combining aforementioned structures along materials with already outstanding antibacterial properties. In this work we fabricated novel micro/nanostructured substrates using soft lithography replication method and subsequent thermal nanoimprint lithography method, creating nanostructured films based on poly (l-lactic acid) (PLLA) fortified by thin silver films deposited by PVD. Main nanoscale patterns were fabricated by replicating surface patterns of optical discs (CDs and DVDs), which proved to be easy, fast and inexpensive method for creating relatively large area patterned surfaces. Their antimicrobial activity was examined in vitro against the bacteria Escherichia coli and Staphylococcus epidermidis strains. The results demonstrated that nanopatterned films actually improved the conditions for bacterial growth compared to pristine PLLA films, the novelty is based on formation of Ag nanoparticles on the surface/and in bulk, while silver nanoparticle enhanced and nanopatterned films exhibited excellent antibacterial activity against both bacterial strains, with circa 80 % efficacy in 4 h and complete bactericidal effect in span of 24 h.

Cost-Effective Nanophotonic Metasurfaces with Spatially Gradient Structures for Ultrasensitive Imaging-Based Refractometric Sensing.

Nanophotonic metasurfaces are widely utilized in various domains, such as biomedical, healthcare, and environmental monitoring, benefiting from their unique advantages of label-free, noninvasive, and real-time response. However, nanophotonic metasurfaces usually rely on sophisticated instruments, and expensive and time-consuming fabrication processes, which severely restricts their practical applications. Herein, a spatially gradient metasurface is integrated with an imaging-based sensing scheme, waiving the requirement of spectrometers and achieving an ultrahigh imaging-based sensitivity of 3321 pixels/refractive index unitsuperior to that characterized using conventional compact spectrometers. The metasurface is fabricated by nanoimprint lithography using a reusable cyclic olefin copolymer template featuring millions of unique nanostructures. Under the illumination of monochromatic light, the transmittance of different nanostructures on the metasurface differs, resulting in grayscale images with varied intensity distributions. Analyzing the intensity change of the metasurface's recorded image can obtain the covering medium's refractive index. Furthermore, through theory and experimentation, the high reliability of the proposed reusable and flexible template has been verified for nanophotonic metasurface fabrication which further reduces the fabrication cost of core sensing elements. Finally, with proper optimization of the metasurface structure and imaging system, this setup is expected to be applied to many emerging areas of point-of-care, real-time, and on-site biosensing.

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.

Top-down fabrication of Ge nanowire arrays by nanoimprint lithography and hole gas accumulation in Ge/Si core–shell nanowires

Germanium (Ge) nanowire arrays are expected to be a promising channel material for high-performance field-effect transistors (FETs) due to their excellent electronic transport properties, silicon (Si) compatibility, and high integration. To produce highly ordered Ge nanowires with low contamination, nanoimprint lithography (NIL) and Bosch etching are used and the size of nanowires is reduced from 220 nm to ∼30 nm by adjusting the H2O2 etching time. Furthermore, Ge/Si core–shell nanowires are prepared by forming p-Si shells on Ge nanowires, and their good crystalline quality and sharp interfaces are confirmed by transmission electron microscope (TEM) observations. This structure is used as a high-mobility channel in HEMT-type devices, and the accumulation of hole gas inside the Ge nanowires, which is the most important, is demonstrated and investigated by Raman scattering.

Engineering inlet structures to enhance DNA capture into nanochannels in a polymer nanofluidic device produced via nanoimprint lithography

Operating nanofluidic biosensors requires threading single molecules to be analyzed from microfluidic networks into nanostructures, mostly nanochannels or nanopores. Different inlet structures have been employed as a means of enhancing the number of the capture events into nanostructures. Here, we systematically investigated the effects of various engineered inlet structures formed at the micro/nanochannel interface on the capture of single λ-DNA molecules into the nanochannels. Different inlet geometries were evaluated and ranked in order of their effectiveness. Adding an inlet structure prior to a nanochannel effectively improved the DNA capture rate by 190–700% relative to that for the abrupt micro/nanochannel interface. The capture of DNA from the microchannel to various inlets was determined mainly by the capture volumes of the inlet structures and the geometrically modified electric field in the inlet structure. However, as the width of the inlet structure increased, the hydrodynamic flow existing in the microchannel negatively influenced the DNA capture by dragging some DNA molecules deep into the inlet structure back to the microchannel. Our results indicate that engineering inlet structures is an effective means of controlling the capture of DNA molecules into nanostructures, which is important for operation of nanofluidic biosensors.

Bi-Layer nanoimprinting lithography for metal-assisted chemical etching with application on silicon mold replication

This paper reports a new type of nanoimprinting method called Bi-layer nanoimprinting lithography (BL-NIL), which can work along with metal-assisted chemical etching (MaCE) for fabricating nanostructures on silicon. In contrast to conventional nanoimprinting techniques, BL-NIL adds an interposing layer between the imprinting resist layer and silicon substrate. After the standard imprinting process, dry etching was used to etch away the residual imprinting layer and part of the interposing layer. Finally, the remaining interposing layer was wet-etched using its remover. This innovative approach can ensure cleanliness at the metal/silicon interface after metal lift-off processes, and therefore guarantees the success of MaCE. By combining BL-NIL and MaCE, expensive silicon molds with sub-micrometer/nanometer-scale feature sizes can be easily replicated and preserved. This is important for the application of nanoimprinting technologies in industrial manufacturing.

In Situ Observation of Micro-Patterned Elastomeric Surfaces: The Formation of the Area of Real Contact and the Influence on Its Friction and Deformation Behaviour.

Structured surfaces, which are the basis of the lotus blossom effect, have great potential to serve/operate as functionalised surfaces, i.e., surfaces with specific and/or adjustable properties. In the present study, the aim is to use micro-structured elastomeric surfaces to specifically influence the friction and deformation behaviours on the basis of the shape and arrangement of the structures. Thiol-acrylate-based photopolymers patterned via nanoimprint lithography were investigated by using an in situ tribological measurement set-up. A clear influence of the different structures on the surface's friction behaviour could be shown, and, furthermore, this could be brought into relation with the real area of contact. This finding provides an important contribution to further development steps, namely, to give the structures switchable properties in order to enable the control of friction properties in a targeted manner.

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.

Research on the influence of stamping materials on resist flow and the residual layer in thermal nanoimprint lithography

AbstractVarious stamp materials can significantly affect the filling quality of nanoimprint lithography (NIL). The effects of different stamp materials on the imprinting process were investigated from the angles of residual layer (RL) thickness, contact pressure, and filling proportion. The selection of various stamp materials affects the thickness and uniformity of the RL. Soft stamps (PDMS, PU) leave a thin but uneven RL distribution, while the RL imprinted by hard stamps (Si, Ni) is thicker but more uniform. The contact pressure using soft stamps is relatively more evenly distributed than hard stamps. The uneven distribution of contact pressure leads to poor cavity‐filling proportion, especially for hard stamps. This study offers guidance for choosing proper nanoimprint stamp materials for different NIL applications.

Anti-reflection effect of high refractive index polyurethane with different light trapping structures on solar cells

The textured surfaces to reduce light reflectivity by using acid-alkali chemical etching and SiNx films are generally necessary for commercial crystalline silicon solar cells. However, this etching process requires a large amount of environmentally harmful acid-alkali solution and has limited options for texture and size. To overcome these disadvantages, a new anti-reflection strategy is proposed in this study, which is using soft nanoimprint lithography to prepare the textured structures on the outside of the SiNx films. The polyurethane with a high refractive index of 1.64 is selected as the texture material, and different templates are selected to prepare it into different light trapping structures, including positive-inverted pyramids, inverted lace cones, and positive-inverted moth-eye nanostructures allowing for easy customization of the textured structures. The finite element simulation and experiments demonstrate that these light trapping structures have a wide spectrum anti-reflection performance in visible and near-infrared bands. With the back surface of the commercial passivated emitter rear contact (PERC) bi-facial solar cells as the imprint substrates, some light trapping structures can reduce the surface weighted average light reflectivity (Rw) at the band of 300–1200 nm from 18.31% to less than 10% and the optimal structures can reduce Rw to 8.71%. This anti-reflection strategy can also be applied to thin-film solar cells and crystalline silicon solar cells of other structures, such as HIT, Topcon, Perovskite/c-Si tandem, and so forth, which shows great development potential.

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.

Racetrack Ring Resonator Integrated with Multimode Interferometer Structure Based on Low-Cost Silica–Titania Platform for Refractive Index Sensing Application

In this work, a racetrack ring resonator (RTRR) integrated with a multimode interferometer (MMI) structure based on a silica–titania (SiO2:TiO2) platform is projected for refractive index sensing application. The typical ring resonator structure requires a gap of ~100 nm to 200 nm between the bus waveguide (WG) and the ring structure which makes it challenging to fabricate a precise device. Thus, the device proposed in this paper can be considered a “gapless” ring resonator structure in which the coupling of light between the ring and bus WG can be achieved via an MMI coupler. A minor change in the refractive index in the vicinity of the MMI structure can trigger a shift in the resonance wavelength of the device. Thus, this simple and fascinating structure can be employed as a refractive index sensor. The device’s sensitivity is ~142.5 nm/RIU in the refractive index range of 1.33 to 1.36 with a figure of merit (FOM) of 78.3. This simple device structure can potentially be fabricated via a low-cost and highly efficient sol–gel process and dip-coating method combined with the nanoimprint lithography (NIL) method.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here,wedemonstrate a nanoimprinted nonlinear barium titanate2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonicdevices.