Conventional Nanoimprint Lithography Research Articles

Realization of high aspect ratio metalenses by facile nanoimprint lithography using water-soluble stamps

Nanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization.

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.

Uniform Pressing Mechanism in Large-Area Roll-to-Roll Nanoimprint Lithography Process

We aimed to increase the processing area of the roll-to-roll (R2R) nanoimprint lithography (NIL) process for high productivity, using a long roller. It is common for a long roller to have bending deformation, geometric errors and misalignment. This causes the non-uniformity of contact pressure between the rollers, which leads to defects such as non-uniform patterning. The non-uniformity of the contact pressure of the conventional R2R NIL system was investigated through finite element (FE) analysis and experiments in the conventional system. To solve the problem, a new large-area R2R NIL uniform pressing system with five multi-backup rollers was proposed and manufactured instead of the conventional system. As a preliminary experiment, the possibility of uniform contact pressure was confirmed by using only the pressure at both ends and one backup roller in the center. A more even contact pressure was achieved by using all five backup rollers and applying an appropriate pushing force to each backup roller. Machine learning techniques were applied to find the optimal combination of the pushing forces. In the conventional pressing process, it was confirmed that pressure deviation of the contact area occurred at a level of 44%; when the improved system was applied, pressure deviation dropped to 5%.

Portable UV nanoimprint lithography system using hydraulic pressure

Nanoimprint lithography (NIL) is the next-generation alternative to conventional photolithography involving an inexpensive and high throughput process. However, conventional NIL equipment requires a bulky air compressor which possesses a large area. This study aims to overcome this limitation by introducing a novel hydraulic ultra-violet (UV) imprint system. Hydraulic UV imprint system can be used to conduct the UV NIL process by using a simple, low-cost materials without needing complex equipment. To form various patterned layers, UV NIL was conducted with various master stamps.(diameter: micro-cones:3 μm, nanopillars:200 nm, nanoholes: 200 nm) The performance of the imprint system was verified by using scanning electron microscopy analysis of samples fabricated by new system. The results showed that the performance of the new imprint system was comparable to that of a conventional air-pressure UV imprint system.

Two-Dimensional Nanograting Fabrication by Multistep Nanoimprint Lithography and Ion Beam Etching

The application of nanopatterned electrode materials is a promising method to improve the performance of thin-film optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics. Light coupling to active layers is enhanced by employing nanopatterns specifically tailored to the device structure. A range of different nanopatterns is typically evaluated during the development process. Fabrication of each of these nanopatterns using electron-beam lithography is time- and cost-intensive, particularly for larger-scale devices, due to the serial nature of electron beam writing. Here, we present a method to generate nanopatterns of varying depth with different nanostructure designs from a single one-dimensional grating template structure with fixed grating depth. We employ multiple subsequent steps of UV nanoimprint lithography, curing, and ion beam etching to fabricate greyscale two-dimensional nanopatterns. In this work, we present variable greyscale nanopatterning of the widely used electrode material indium tin oxide. We demonstrate the fabrication of periodic pillar-like nanostructures with different period lengths and heights in the two grating directions. The patterned films can be used either for immediate device fabrication or pattern reproduction by conventional nanoimprint lithography. Pattern reproduction is particularly interesting for the large-scale, cost-efficient fabrication of flexible optoelectronic devices.

Nanoimprint lithography for high-throughput fabrication of metasurfaces.

Metasurfaces are composed of periodic sub-wavelength nanostructures and exhibit optical properties that are not found in nature. They have been widely investigated for optical applications such as holograms, wavefront shaping, and structural color printing, however, electron-beam lithography is not suitable to produce large-area metasurfaces because of the high fabrication cost and low productivity. Although alternative optical technologies, such as holographic lithography and plasmonic lithography, can overcome these drawbacks, such methods are still constrained by the optical diffraction limit. To break through this fundamental problem, mechanical nanopatterning processes have been actively studied in many fields, with nanoimprint lithography (NIL) coming to the forefront. Since NIL replicates the nanopattern of the mold regardless of the diffraction limit, NIL can achieve sufficiently high productivity and patterning resolution, giving rise to an explosive development in the fabrication of metasurfaces. In this review, we focus on various NIL technologies for the manufacturing of metasurfaces. First, we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces. We also discuss recent applications of NIL in the realization of metasurfaces. Finally, we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.

Study on vibration-assisted thermal nanoimprint lithography

In thermal nanoimprint lithography, polymethyl methacrylate (PMMA) with good thermoplasticity is often used as transferring media. However, due to the special properties of PMMA, the accuracy of microstructure transferred to the surface will be seriously affected. To address this problem, the micro-vibration by piezo-driven is introduced in conventional thermal nanoimprint lithography. Because of the micro-vibration will change the contact stress during the embossing process and reduce the contact time. This will improve the filling rate of PMMA during the embossing process and transfer the pattern of stamp to PMMA with excellent feature fidelity. Compared with the traditional nanoimprint lithography, the filling rate of PMMA is increased by 92% after the introduction of vibration by piezo-driven. Then, a new motion model is built to verify the rationality of introducing vibration. Besides, the effect of frequency and amplitude with filling rate is analyzed by finite element method to obtain the best frequency and amplitude for experiments. Finally, the transferred pattern is observed by experiment.

Fabrication of 3D microstructures using grayscale lithography

Abstract Following the demand for three-dimensional (3D) micromachined structures, additive and subtractive processes were developed for fabrication of real 3D shapes in metals, alloys and monocrystalline Si (c-Si). As a primary structuring step for well-defined 3D structuring of the photoresist, grayscale lithography by laser direct writing was used. For additive fabrication of 3D microstructures, structured photoresist was used as molds. They were sputtered and subsequently electroplated by a metal (Cu) and an alloy (NiCo). The derived electroplated structures were demolded from the photoresist using an organic stripper. These metal structures are satisfactory replicas of the photoresist pattern. For subtractive pattern transfer of 3D structures into c-Si, reactive ion etching (RIE) was used to transfer the 3D photoresist structure into c-Si with 1:1 pattern transferability. The process parameters of RIE were optimized to obtain a selectivity of 1 and an anisotropy factor close to 1. Whereas conventional X-ray lithography (LIGA) and nanoimprint lithography result in 2.5D patterns, these techniques allow the fabrication of almost any arbitrary 3D shapes with high accuracy. In many cases, 3D structures (‘free forms’) are required, e.g. for molding of optical components such as spheres (or aspheres), channels for lab-on-a-chip and pillars for biological applications. Moreover, 3D structures on Si could be used as optical gratings and sensors.

Large area patterning of residue-free metal oxide nanostructures by liquid transfer imprint lithography

One-dimensional (1D) and three-dimensional (3D) residue-free metal oxide patterns are directly fabricated over large areas using liquid transfer imprint lithography (LTIL) with an ultraviolet-curable metal oxide precursor resist. A 1D line or pillar array of metal oxides nano-patterns without a residual layer is formed by LTIL and annealing processes. A 3D layer-by-layer nanomesh structure is successfully constructed by repeating the LTIL method without a complex etching process. In addition, it is possible to form a hierarchical structure in which zinc oxide nanowires are selectively grown on a desired zinc oxide (ZnO) seed pattern formed by LTIL via a hydrothermal method. Unlike the pattern fabricated by the conventional nanoimprint lithography method, in the case of the pattern formed by LTIL the residues accumulated between the patterns during the patterning procedure can be removed, and thus it is possible to easily form various types of nanostructures.

Pre-stress-assisted nanoimprint lithography for fabricating high-density diffraction gratings

This study presents a pre-stress-assisted nanoimprint lithography (NIL) method for fabricating high-density flexible diffraction gratings. Compared to conventional NIL methods, this method offers the advantage of producing gratings with smaller periods than that of the master mould. First, a polydimethylsiloxane (PDMS) film is patterned by NIL, and is adhered to a flat PDMS substrate under pre-stress tension. Then, patterned PDMS gratings with small periods are obtained when the pre-stress tension is released. The grating periods can be controlled by the degree of the pre-stress tension. In the experiment, flexible diffraction gratings with periods of 3–4 µm are obtained, and the largest reduction ratio of the grating period is approximately 25%. Experimental results primarily verify the effectiveness of the proposed method for fabricating high-density diffraction gratings.Highlights(1) A pre-stress assisted NIL method for fabricating high-density diffraction gratings is proposed.(2) The periods of the diffraction gratings can be controlled by adjusting the degree of the pre-stress tension during the fabrication process.(3) A biggest grating-period reduction ratio of about 25% is obtained in the experiment.

Novel Nano-Materials and Nano-Fabrication Techniques for Flexible Electronic Systems.

Recent progress in fabricating flexible electronics has been significantly developed because of the increased interest in flexible electronics, which can be applied to enormous fields, not only conventional in electronic devices, but also in bio/eco-electronic devices. Flexible electronics can be applied to a wide range of fields, such as flexible displays, flexible power storages, flexible solar cells, wearable electronics, and healthcare monitoring devices. Recently, flexible electronics have been attached to the skin and have even been implanted into the human body for monitoring biosignals and for treatment purposes. To improve the electrical and mechanical properties of flexible electronics, nanoscale fabrications using novel nanomaterials are required. Advancements in nanoscale fabrication methods allow the construction of active materials that can be combined with ultrathin soft substrates to form flexible electronics with high performances and reliability. In this review, a wide range of flexible electronic applications via nanoscale fabrication methods, classified as either top-down or bottom-up approaches, including conventional photolithography, soft lithography, nanoimprint lithography, growth, assembly, and chemical vapor deposition (CVD), are introduced, with specific fabrication processes and results. Here, our aim is to introduce recent progress on the various fabrication methods for flexible electronics, based on novel nanomaterials, using application examples of fundamental device components for electronics and applications in healthcare systems.

Surface preparation and patterning by nano imprint lithography for the selective area growth of GaAs nanowires on Si(111)

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates.

Fabrication of calibration plate for total internal reflection fluorescence microscopy using roll-type liquid transfer imprint lithography

A strong demand exists for the visualization of the fluid flow in microchannels, such as those in labs-on-a-chip. Multilayer nanoparticle image velocimetry (MnPIV) is a promising technique for this purpose, wherein the x- and y-positions of a nanoparticle are measured using a camera and its z-position is estimated via its fluorescence intensity. To accurately estimate the z-position, however, MnPIV requires a fluorescence intensity calibration process. Water is typically used in the microfluidic devices and a nanoscale level with a refractive index matching method is conventionally used for the calibration of MnPIV. Therefore, a calibration plate with nanoscale levels whose refractive index is matched to water is necessary to calibrate the z-position of fluorescent particles in water. In this study, a fabrication technique of the calibration plate was performed using ultraviolet (UV) nanoimprint with a UV-curable resin, whose refractive index is matched to water and which possesses a high viscosity of 2900cps. Although the residual layer thickness (RLT) of the level must be less than 100nm to perform MnPIV, it is challenging to reduce the RLT with such a high viscosity of the resin using conventional UV nanoimprint lithography. To reduce the RLT, therefore, in this study roll-type liquid-transfer imprint lithography (LTIL) was used with the high-viscosity UV-curable resin, where a two-fold repetition of the roll-type LTIL process demonstrated a calibration pattern RLT of 40nm or less. As a result, calibration of the MnPIV system was achieved with the calibration plate developed herein and fabricated by UV nanoimprint lithography.

액적 충돌 거동에 대한 표면 패턴의 영향에 관한 연구

본 논문에서는 나노임프린트 리소그래피 기술을 이용하여 다양한 발수 표면을 제작하였으며, 발수기능의 안정성을 향상시키기 위해 액적의 충돌 거동에 대하여 평탄한 단위면적에 대한 구조물의 면적비인 표면 변수의 영향에 대한 연구를 수행하였다. 액적의 충돌 거동을 연구하기 위해 액적 충돌 시험을 통하여 충돌한 액적이 되튐에서 분산으로 천이 되는 임계높이를 측정하였다. 높은 표면 변수에서는 낮은 임계높이가 관찰되었으나, 낮은 표면변수에서는 임계높이가 증가하는 경향을 관찰하였다. 그러나, 표면변수가 더욱 감소할 경우 임계높이가 다시 감소하는 경향을 보였다. 관찰된 결과는 높은 임계높이를 위해서는 최적의 표면 형상 설계가 요구됨을 제시하고 있다. In this paper, the hydrophobic rough surfaces were prepared by employing a conventional nano-imprint lithography technique, and the effects of surface parameter, ratio of the top surface to the flat unit cell, on the impingement behaviors of liquid droplet were investigated to improve robustness of hydrophobic functionality. The critical height defined for the transition from rebound to fragmentation is measured by droplet impingement test in order to study dynamic behavior of an impinged droplet. It showed the critical height decreased with high surface parameter while it increased with low surface parameter. However, the critical height decreased again as surface parameter decreased further. Observed results suggest that the optimized surface pattern should be designed for the increased critical height.

Pressure free nanoimprinting lithography using ladder-type HSQ material for LSPR biosensor chip

We present an ingenious pressure free NIL (Nanoimprint lithography) method by simply employing a very thin PDMS mold along with a Spin-on-Glass HSQ material. Fabricating nano-patterns (i.e. nano-gratings and 2 D nano-arrays) via NIL is advantageous for LSPR based biosensors for producing large working area with high-throughput. Unlike conventional NIL and other fabrication methods, no complicated equipment is required in this work; only a simple heating plate (at 100°C) was utilized for curing time reduction. Nano-pillar structure with definition down to 50–100nm has been successfully fabricated using this method. After Au-deposition, mushroom like Au-capped nano-pillar chip has been successfully validated as a LSPR bio-sensor chip for IgA detection. In addition, this method provides promising features for higher dimensional structures fabrication which is considered as a beneficial tool for LSPR based biosensor sensitivity enhancement.

Flexible pressure sensor with a “V-type” array microelectrode on a grating PDMS substrate

Purpose This paper aims to provide a fabrication and measurement of a highly stretchable pressure sensor with a “V-type” array microelectrode on a grating PDMS substrate. Design/methodology/approach First, the “V-type” array structure on the silicon wafer was fabricated by the MEMS technology, and the fabrication process included ultra-violet lithography and silicon etching. The “V-type” array structure on the master mold was then replicated into polycarbonate, which served as an intermediate, negative mold, using a conventional nanoimprint lithography technique. The negative mold was subsequently used in the PDMS molding process to produce PDMS “V-type” array structures with the same structures as the master mold. An Ag film was coated on the PDMS “V-type” array structure surface by the magnetron sputtering process to obtain PDMS “V-type” array microelectrodes. Finally, a PDMS prepolymer was prepared using a Sylgard184 curing agent with a weight ratio of a 20:1 and applied to the cavity at the middle of the two-layer PDMS “V-type” array microelectrode template to complete hot-press bonding, and a pressure sensor was realized. Findings The experimental results showed that the PDMS “V-type” array microelectrode has high stretchability of 65 per cent, temperature stability of 0.0248, humidity stability of 0.000204, bending stability and cycle stability. Capacitive pressure sensors with a “V-type” array microelectrode exhibit ideal initial capacitance (111.45 pF), good pressure sensitivity of 0.1143 MPa-1 (0-0.35 Mpa), fast response and relaxation times (<200 ms), high bending stability, high temperature/humidity stability and high cycle stability. Originality/value The PDMS “V-type” array structure microelectrode can be used to fabricate pressure sensors and is highly flexible, crack-free and durable.

Polymer flow and filling behavior for electroosmosis-driven nanoimprint lithography

Electroosmosis-driven nanoimprint lithography (NIL) is an emerging nanopatterning approach which has shown great potential and unique strength in large-area nanoimprinting, fabricating high-aspect-ratio micro/nanostructures, patterning fragile substrates and non-flat surface. However, compared to the conventional pressure-driven NIL and the electrocapillary force-driven UV-imprinting, there is a big difference for such novel electroosmotic flow driven imprinting process. This paper investigated the mechanism, influencing factors and laws of polymer flow and filling behavior for the electroosmosis-driven nanoimprinting. Based on the principle of electroosmosis-driven micro flows, a theoretical model estimated body force was established. And the model of filling velocity and the model of filling time were derived. Furthermore, the dynamic filling process of electroosmosis-driven NIL, influence and laws of polymer flow and filling behavior regarding the imprint process parameters, the feature patterns and the properties of polymer were revealed by numerical simulation using COMSOL Multiphysics simulation software. These findings are valuable in providing a theoretical basis for electroosmosis-driven nanoimprinting, optimizing imprinting process and further enhancing the performance of the new patterning method.

Surface patterning of protein matrix basement by thermal imprinting

In order to hourly monitor the variation of specific cell growth, a grid mesh for the recording of adhesion positions on the surface of glass slides and dishes is printed. However, since the focal length of a lens, used for making observation, changes with the film thickness when using a protein matrix basement membrane, the surface of the basement membrane and the bottom glass plate cannot simultaneously be observed. In one way to address this problem, the author considered patterning fine structures directly on the surface of the protein matrix basement membrane by employing a thermal imprinting technology. As a mold, a 100-μm-wide grid and images of area-numbers were processed using laser lithography and reactive-ion-etching on a 1-inch Si wafer. For a gelatinous protein, we chose a mixture basement membrane BioCoat Matrigel, which was coated on a 35-mm-diameter dish. A protein thin layer was gelled on a dish according to the manufacturer's specs; the dish was then placed on the bottom loading stage of our thermal nanoimprint system. After that, a Si mold was pressed against a Matrigel matrix basement by a servo motor of the thermal nanoimprint system for 30---90 s at a press force of 1 kN. And then the mold was then released from the Matrigel without cooling when a press time reached to a set value. When an imprint temperature was 75 °C, an imprinted pattern could barely be observed. On the other hand, if the mold was heated up to 100 °C or more, the imprinted grid mesh and the printed number on the Matrigel could be clearly observed. However, a heating at 125 °C seemed to have discolored white a part of the protein. Therefore, under our experimental conditions, it can be said that the optimal heating temperature exists around 100 °C. Although in the past, in conventional thermal nanoimprint lithography, a thermoplastic bearing a glass transition temperature has been targeted, the work presented here sets an example of direct patterning on biological materials such as proteins.

Fabrication of ferroelectric polymer nanostructures on flexible substrates by soft-mold reverse nanoimprint lithography

Conventional nanoimprint lithography with expensive rigid molds is used to pattern ferroelectric polymer nanostructures on hard substrate for use in, e.g., organic electronics. The main innovation here is the use of inexpensive soft polycarbonate molds derived from recordable DVDs and reverse nanoimprint lithography at low pressure, which is compatible with flexible substrates. This approach was implemented to produce regular stripe arrays with a spacing of 700 nm from vinylidene fluoride co trifluoroethylene ferroelectric copolymer on flexible polyethylene terephthalate substrates. The nanostructures have very stable and switchable piezoelectric response and good crystallinity, and are highly promising for use in organic electronics enhanced or complemented by the unique properties of the ferroelectric polymer, such as bistable polarization, piezoelectric response, pyroelectric response, or electrocaloric function. The soft-mold reverse nanoimprint lithography also leaves little or no residual layer, affording good isolation of the nanostructures. This approach reduces the cost and facilitates large-area, high-throughput production of isolated functional polymer nanostructures on flexible substrates for the increasing application of ferroelectric polymers in flexible electronics.

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

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