Imprint Mold Research Articles

Facile Fabrication of Hierarchical Structured Anodic Aluminum Oxide Molds for Large-Scale Production of Superhydrophobic Polymer Films.

Anodized aluminum oxide (AAO) molds were used for the production of large-area and inexpensive superhydrophobic polymer films. A controlled anodization methodology was developed for the fabrication of hierarchical micro-nanoporous (HMN) AAO imprint molds (HMN-AAO), where phosphoric acid was used as both an electrolyte and a widening agent. Heat generated upon repetitive high-voltage (195 V) anodization steps is effectively dissipated by establishing a cooling channel. On the HMN-AAO, within the hemispherical micropores, arrays of hexagonal nanopores are formed. The diameter and depth of the micro- and nanopores are 18/8 and 0.3/1.25 µm, respectively. The gradual removal of micropatterns during etching in both the vertical and horizontal directions is crucial for fabricating HMN-AAO with a high aspect ratio. HMN-AAO rendered polycarbonate (PC) and polymethyl methacrylate (PMMA) films with respective water contact angles (WCAs) of 153° and 151°, respectively. The increase in the WCA is 80% for PC (85°) and 89% for PMMA (80°). On the PC and PMMA films, mechanically robust arrays of nanopillars are observed within the hemispherical micropillars. The micro-nanopillars on these polymer films are mechanically robust and durable. Regular nanoporous AAO molds resulted in only a hydrophobic polymer film (WCA = 113-118°). Collectively, the phosphoric acid-based controlled anodization strategy can be effectively utilized for the manufacturing of HMN-AAO molds and roll-to-roll production of durable superhydrophobic surfaces.

Nanoimprint-induced strain engineering of two-dimensional materials

The high stretchability of two-dimensional (2D) materials has facilitated the possibility of using external strain to manipulate their properties. Hence, strain engineering has emerged as a promising technique for tailoring the performance of 2D materials by controlling the applied elastic strain field. Although various types of strain engineering methods have been proposed, deterministic and controllable generation of the strain in 2D materials remains a challenging task. Here, we report a nanoimprint-induced strain engineering (NISE) strategy for introducing controllable periodic strain profiles on 2D materials. A three-dimensional (3D) tunable strain is generated in a molybdenum disulfide (MoS2) sheet by pressing and conforming to the topography of an imprint mold. Different strain profiles generated in MoS2 are demonstrated and verified by Raman and photoluminescence (PL) spectroscopy. The strain modulation capability of NISE is investigated by changing the imprint pressure and the patterns of the imprint molds, which enables precise control of the strain magnitudes and distributions in MoS2. Furthermore, a finite element model is developed to simulate the NISE process and reveal the straining behavior of MoS2. This deterministic and effective strain engineering technique can be easily extended to other materials and is also compatible with common semiconductor fabrication processes; therefore, it provides prospects for advances in broad nanoelectronic and optoelectronic devices.

CMOS-compatible electrochemical nanoimprint: High throughput fabrication of ordered microstructures on semiconductor wafer by using a glassy carbon mold

Metal assisted chemical etching (MACE), as an emerging wet chemical etching method for fabricating semiconductor micro/nano-structures (MNS), has attracted wide attentions because of the advantages of simple processes, high accuracy and no damages. However, the conventionally used noble metal catalysts (such as Au, Pt,) trapped in the micro/nano-structures are difficult to be removed or reused, not only harming the electronic performances of the CMOS devices but also increasing the fabrication cost. Here, we propose an electrochemical nanoimprint strategy by using a glassy carbon (GC) imprint mold. To overcome the poor electrocatalytic activity of GC, the GC imprint mold was connected to a Pt sheet in the cathodic chamber. To avoid the contaminations of metal and metallic cations in the fabricated MNS, the cathodic chamber was separated from the anodic one by a proton exchange membrane. By optimizing the technical parameters, including the contact pressure between GC imprint mold and GaAs wafer, the concentration of electron acceptor in the cathodic chamber, the imprinting time, etc., a remarkable corrosion rate of about 130 nm/min was achieved, comparable with that of platinum electrocatalyst. Furthermore, by using a GC imprint mold in the spatially separated corrosion cell, the contaminations caused by both the metal catalysts and metallic ions can be eliminated. Thus, this work provides a potential approach to the high throughput fabrication of CMOS-compatible semiconductor microdevices.

Hydrodynamics and solid mechanics structural analysis of mold deformation in nanoimprint lithography

A hybrid model has been developed to study the dynamic filling process of the resist and to investigate the subsequent capillary force induced structural deformation of the imprint mold during nanoimprint lithography (NIL). The dynamic behavior of resist filling with varied physical parameters was investigated by a hydrodynamic model. The capillary force induced deformation of mold structures was modeled using beam bending mechanics for various mold structures. Theoretically calculated results were cross-validated with finite-element simulations using fluid–structure interaction and two-phase flow methods. Based on the theoretical analysis, a general parameter of critical aspect ratio is proposed as the guideline for the design of NIL mold, especially for the NIL mold with a high aspect ratio and ultra-high resolution. The theoretical investigation helps to deepen the understanding of the dynamic mechanism of resist filling and structural deformation, and enables the optimization for high-fidelity NIL.

Dielectric Metalens by Multilayer Nanoimprint Lithography and Solution Phase Epitaxy

Metasurfaces have ushered in a huge development for their superior ability in manipulating light properties including phase, amplitude, and polarization, which show great potential as alternatives for the refractive optical devices. Recently, many applications of metasurface including metalens have been proposed and investigated, aiming at substituting their refractive counterparts. However, the commonly used fabrication approaches employ electron‐beam lithography (EBL) followed by dry etching or atomic layer deposition (ALD) of dielectric materials, which are expensive and inefficient. Besides, dry etching of dielectric materials at sub‐100 nm scale with a high aspect ratio is challenging. Herein, a new approach for dielectric metalens fabrication is presented, which combines multilayer nanoimprint lithography and solution phase epitaxy. High aspect ratio ZnO nanopillars with a height‐to‐diameter ratio of over 7:1 are demonstrated. By using the multilayer nanoimprint lithography, increased aspect ratio nanostructures from shallow imprinting molds are obtained. The highly anisotropic growth characteristic enables nanopillars to grow at a height that exceeds the resist thickness. With this ability, ZnO metalenses are fabricated where the height of nanopillar reaches 1.1 μm, achieving a focusing efficiency of 50%. The process is cost‐effective with a high throughput, which can be widely used for many optical applications.

Quasi-seamless stitching for large-area micropatterned surfaces enabled by Fourier spectral analysis of moiré patterns

The main challenge in preparing a flexible mold stamp using roll-to-roll nanoimprint lithography is to simultaneously increase the imprintable area with a minimized perceptible seam. However, the current methods for stitching multiple small molds to fabricate large-area molds and functional surfaces typically rely on the alignment mark, which inevitably produces a clear alignment mark and stitched seam. In this study, we propose a mark-less alignment by the pattern itself method inspired by moiré technique, which uses the Fourier spectral analysis of moiré patterns formed by superposed identical patterns for alignment. This method is capable of fabricating scalable functional surfaces and imprint molds with quasi-seamless and alignment mark-free patterning. By harnessing the rotational invariance property in the Fourier transform, our approach is confirmed to be a simple and efficient method for extracting the rotational and translational offsets in overlapped periodic or nonperiodic patterns with a minimized stitched region, thereby allowing for the large-area and quasi-seamless fabrication of imprinting molds and functional surfaces, such as liquid-repellent film and micro-optical sheets, that surpass the conventional alignment and stitching limits and potentially expand their application in producing large-area metasurfaces.

Rapid fabrication of highly integrated and high numerical aperture chalcogenide glass microlens arrays

The fast growing of infrared imaging system for non-contact thermography, night surveillance and night driving assistance demands highly integrated optical lenses with miniaturization feature for performance improvement. Herein, we fabricated refractive type of microlens arrays (MLAs) with high numerical aperture (NA) and high integration on commercial chalcogenide glass (As2Se3) by combining photoresist thermal reflow and soft mold imprinting process. PDMS-type concave MLAs (PDMS-MLAs) soft mold is prepared by transfer printing process on photoresist (PR) MLAs fabricated by standard UV exposure and thermal reflow. The microlens on PDMS-MLAs could be further transferred to the surface of As2Se3 glass through the hot imprinting technique, forming chalcogenide glass-based MLAs (ChG-MLAs). Morphology detection reveals that microlens in the MLAs has an average aperture and sag height of approximately 13.4 and 1.7 μm. Moreover, the optical performance investigation demonstrated that the ChG-MLAs with 700 × 700 array size (10.5 × 10.5 mm) showed good structure uniformity and focusing capability, and through the FDTD simulation, it further evidenced that focal lengths of 6.3–8.1 µm with high numerical aperture of 0.64–0.73 (wavelength range: 3–5 μm) was achieved.

Abrasion resistance of femtosecond pulsed laser processed micro molds of laser shock imprinting

Laser shock imprinting is highly efficient to manufacture functional thin films. However, the manufacturing process and the service life of the mold were the main issues that restrict its large-scale application. In this study, high-quality and high-precision periodic patterns were manufactured on aluminum foils of thickness 4 μm by laser shock imprinting with femtosecond pulsed laser etched Si and SiC molds. It proved that the micro molds with different patterns fabricated by variant femtosecond pulsed laser were able to tailor the optical function performance of the films. The effects of shock imprinting times on the abrasion resistance of different material mold were studied. The SiC mold with higher hardness has better wear resistance. The reflection rate attenuation of the film based on the SiC mold was obviously smaller than that on the Si mold after 100 times of imprinting. The difference performance caused by different molds was analyzed by elastoplastic mechanics of finite element method. The higher hardness of SiC suffer less wear and plastic deformation in the imprinting process, so it can mitigate the mold microstructure size degradation, which fabricates more accurate surface microstructures on the aluminum foils. The SiC is more suitable for making laser shock imprinting mold for future large-scale industrial application.

Theoretical and experimental study of metallic glass die-imprinting for manufacturing large-size micro/nano structures

Preparing large-size metallic glassy micro/nano devices/structures with high uniformity and precision is a very important and challenge topic in up-down nanotechnologies and their applications. It shows that the newly developed die-imprinting technique of metallic glasses (MGs) exhibits better uniformity and precision than the commonly used free-imprinting, despite of that the related mechanism and processing kinetics have not been fully understood. However, understanding the factors affecting the processing uniformity and processing kinetics is very important for controlling the processing precision of the MG nano/micro structures. In the present study, the processing uniformity as well as the processing kinetics of MGs in die-imprinting was systematically investigated. A universal processing kinetics formula of MGs’ die-imprinting has been proposed by quantitative evaluating the effect of the oxide layer for the first time. The processing kinetics formula has been proved applicable both in micro and nano scale by experimental results. It has also been found that the die-imprinting possesses excellent and stable processing uniformity in a large area, which is an order of magnitude better than that of the free-imprinting. Especially, the prepared MG with micro/nano structures could be employed as imprinting mold to fabricate micro/nano structures on plastics or other materials in larger scale. The present work provides both experimental and theoretical fundaments for the controllable die-imprinting of MGs to fabricate micro/nano structure or devices, and can greatly promote the further applications of MGs.

Отсекающий УФ-светофильтр для предотвращения отрицательного наклона стенок мастер-штампа мягкой литографии

The general-purpose polymers formation issues are solved using micro- and nanoelectronic technologies of new generation, for example a nanoimprint lithography. One of its subspecies, soft lithography, includes topology formation using soft master die fabricated by hard mold imprint. Therefore, engineering study of possibility of hard molds self-dependent fabrication for the purposes of optoelectronic data bus formation for new generation printed circuit boards using general-purpose polymer materials is a priority. In this work, to rationalize the purchasing cost of an expensive hard mold of soft lithography, an original technological process for a hard mold fabrication based on the SU-8 photoresist has been developed and implemented. During the performing of the proposed technological process, the reason for the negative slope (T-topping) formation of soft lithography hard mold walls made of SU-8 photoresist was determined. A series of cut-off UV filters for optical wavelengths less than 350 nm has been developed and fabricated to eliminate T-topping. Based on the UV radiation intensity experimental measurements data from the i-line mercury lamp of the automated alignment and exposure system EVG620 NT, the UV radiation intensity attenuation dependences on the functional layer thickness of the developed optical UV filter for 365 and 400 nm wavelengths are plotted. The developed UV filters application effectiveness has been proven due to T-topping elimination during the technological process of producing the soft lithography hard mold test topology. The use of soft lithography will make it possible in the future to create a new generation printed circuit boards with a built-in optoelectronic data bus in the form of a polymer planar optical waveguides array and optical input / output elements.

Process Optimization for Manufacturing Functional Nanosurfaces by Roll-to-Roll Nanoimprint Lithography.

Roll-to-roll nanoimprint lithography (RTR-NIL) is a low-cost and continuous fabrication process for large-area functional films. However, the partial ultraviolet (UV) resin filling obstructs the ongoing production process. This study incorporates UV resin filling process into the nanopillars and nanopores by using RTR-NIL. A multiphase numerical model with a sliding mesh method is proposed in this study to show the actual phenomena of imprint mold rotation and feeding of UV resin on the polyethylene terephthalate (PET) substrate. The implementation of UV resin filling under environmental conditions was performed by utilizing the open-channel (OC) boundary conditions. The numerical model was solved by using the explicit volume of fluid (VOF) scheme to compute the filling on each node of the computational domain. The effects of different processing parameters were investigated through the proposed numerical model such as imprinting speed (IS), contact angles (CAs), viscosity, initial thickness of the PET, and supporting roll diameter. A good agreement was found between numerical simulations and experimental results. The proposed numerical model gives better insights of the filling process for the mass production of functional surfaces with nanopillars and nanopores patterns for different applications on an industrial scale.

Nanopatterned Polymer Molds Using Anodized Aluminum Templates for Anti-Reflective Coatings.

This work introduces a facile geometry-controlled method for the fabrication of embossed and engraved polymeric moth-eye-inspired nanostructures in imprinting molds using anodic aluminum oxide (AAO) templates, resulting in a novel anti-reflective transparent coating. The moth-eye nanostructures are prepared directly on the surface of a flexible polyethylene terephthalate (PET) substrate. As a prerequisite procedure, a UV-curable polyurethane acrylate resin is spun on the PET. The shape of the moth-eye nanostructures can then be adjusted by controlling the size and shape of the nanopores in the AAO templates. Both embossed and concaved polymer moth-eye nanostructures were successfully mounted on a PET substrate. Embossed polymer replica molds were prepared using the AAO master templates in combination with an imprinting process. As revealed by field-emission electron microscope (FE-SEM) images, conical nanopatterns in the AAO template with a diameter of ~90 nm and a depth of ~100 nm, create a homogeneous embossed morphology in the polymer moth-eye nanostructure. The polymeric molds with the depths of 300 and 500 nm revealed the amalgamated structures in their apexes. In addition, a dip-imprinting process of the polymeric layers was implemented to yield a concaved mold by assembly on the surface of the 100 nm embossed polymer mold substrate. Considering that the embossed structures may be crumbled due to their protuberant shapes, the concaved geometries can have an advantage of stability in a certain application concerning physical degradation along with a higher transmission by ~2%, despite somewhat nonuniform structure. The experimental and theoretical results of this study indicate that this polymer layer has the potential for use in anti-reflective coating applications in transparent films.

Monolithic digital patterning of polyimide by laser-induced pyrolytic jetting

Based on its outstanding mechanical, thermal, and chemical properties, a Polyimide (PI) is useful in a wide range of applications. Its usage in biomedicine is drawing great attention owing to the recent confirmation of the biocompatibility of various PIs. However, the conventional patterning of a PI, based on photolithographic methods, which is expensive and time-consuming, hampers the rapid advancement of research-oriented fields that require frequent design changes. To resolve this problem, we introduce the method of the monolithic digital patterning of PI up to the quasi-three-dimensional (3D) structures at the microscale resolution by laser-induced jetting of highly porous Laser-induced graphene (LIG) from the PI matrix. Pyrolytic jetting of the LIG is dependent not only on the laser-induced temperature but also on its temporal and spatial gradients. However, the surfaces of the remaining PI can be exceptionally smooth at the optimum laser condition, comparable to the pristine surface at the microscopic level, as confirmed by Raman spectroscopy and Atomic force microscopy (AFM) measurements. On-demand microfluidic channels and multilevel imprinting molds are created in a single step using the proposed method as a proof-of-concept, substantiating its potential application in relevant research.

Interfacial Interactions during Demolding in Nanoimprint Lithography.

Nanoimprint lithography (NIL) is a useful technique for the fabrication of nano/micro-structured materials. This article reviews NIL in the field of demolding processes and is divided into four parts. The first part introduces the NIL technologies for pattern replication with polymer resists (e.g., thermal and UV-NIL). The second part reviews the process simulation during resist filling and demolding. The third and fourth parts discuss in detail the difficulties in demolding, particularly interfacial forces between mold (template) and resist, during NIL which limit its capability for practical commercial applications. The origins of large demolding forces (adhesion and friction forces), such as differences in the thermal expansion coefficients (CTEs) between the template and the imprinted resist, or volumetric shrinkage of the UV-curable polymer during curing, are also illustrated accordingly. The plausible solutions for easing interfacial interactions and optimizing demolding procedures, including exploring new resist materials, employing imprint mold surface modifications (e.g., ALD-assisted conformal layer covering imprint mold), and finetuning NIL process conditions, are presented. These approaches effectively reduce the interfacial demolding forces and thus lead to a lower defect rate of pattern transfer. The objective of this review is to provide insights to alleviate difficulties in demolding and to meet the stringent requirements regarding defect control for industrial manufacturing while at the same time maximizing the throughput of the nanoimprint technique.

Durable Soft Mold for Imprinting of High-Adhesive Resin

A variety of polymer resins have been used to fabricate micro/nano structures via imprint lithography. In addition, with an interest in productivity, there is an increasing demand for the study of the process of easily demolding a cured resin from a mold for continuous fabrication of micro/nanostructures applying imprint lithography to the roll-type equipment. Among these polymer resins, Norland optical adhesive (NOA) in particular is widely used to fabricate micro/nano structure-based functional surfaces because of its shape memory characteristics, biocompatibility, and great optical characteristics. However, the cured NOA is originally used as an epoxy-based adhesive with its high adhesion. NOA has many advantages as a UV-curable adhesive, but as a resin in the imprint process, such adhesion brings a limitation. This high adhesion of cured NOA causes defects in the mold during the demolding process, so it is difficult to apply it to the continuous fabrication process. Here, we present a durable polyurethane acrylate (PUA) soft mold capable of clean demolding of an epoxy-based polymer resin having high adhesion by depositing metal on a surface. Au and Ni were deposited to a thickness of 100 Å by using an E-beam evaporator. To verify the surface characteristics, each metal-deposited soft mold was compared with the previously used soft mold by measuring the contact angle and calculating surface energy. To test a performance of our soft mold, we imprinted nanoline pattern with NOA as a resin using metal-deposited soft mold in roll to roll (R2R) process for more than 240 replications for 90 min of operation time. It is expected that this study can be used for mass production of pattern with epoxy-based patterns required in many fields.

Preparation of Ni micropillar arrays with high aspect ratios using anodic porous alumina template and their application to molds for imprinting.

Anodic porous alumina templates with controlled microscale geometrical structures were prepared by a process combining mask formation and subsequent selective etching of the alumina layer. In this process, the anisotropic etching of anodic porous alumina allows the preparation of anodic porous alumina with microhole array patterns having high aspect ratios. The electrodeposition of Ni using the obtained alumina templates generated an array of Ni micropillars with high aspect ratios. The height of Ni micropillars could also be controlled by adjusting the thickness of the anodic porous alumina. The obtained Ni micropillar array with a high aspect ratio was applied as a mold for imprinting. The ordered microstructures of TiO2 with high aspect ratios were prepared by imprinting using the Ni mold.

Demolding force dependence on mold surface modifications in UV nanoimprint lithography

Nanoimprint lithography (NIL) has attracted broad interests in nano/micropatterning of photonic structures, however, one of the major challenges in NIL is the defect rate due to a large demolding force which causes difficult demolding. In this work, adhesion and friction stresses were obtained from the overall demolding force through measurements of the total demolding force and the geometries of the mold patterns. The friction stress on the sidewalls is generally larger than the adhesion stress between the mold and the resist. In addition, a conformal Al2O3 layer grown via atomic layer deposition was introduced to cover the imprint mold and reduce the surface roughness on the sidewalls. Together with the coating of an anti-adhesion layer for easy mold releasing, both adhesion and friction stresses were reduced, leading to a lower total demolding force compared to the conventional mold with merely a hydrophobic surfactant layer or mold without any surface modification. The results obtained from this work will help the design and optimization of the NIL molds to mitigate the defect rate in UV-NIL.

Resist-free nanoimprinting on optical fibers for plasmonic optrodes

Nanostructure patterning on optical fibers enables miniaturized optrodes for photonic and plasmonic applications. Here we report a direct nanoimprint technique to produce high-quality nanostructure arrays on optical fiber endfaces. It has only one single step: imprinting optical fiber tips against a mold with nanostructures at the elevated temperature. This new method abandons resist used in traditional fiber-imprinting methods. Hundreds of fibers can be shaped simultaneously with one mold within minutes. The imprinted nanostructure arrays on optical fibers are transformed into plasmonic optrodes through metal deposition. Variation of imprint depths and mold patterns allows tailoring of the plasmonic resonances of these nanostructure arrays for high-performance refractometric sensing and on-fiber polarization. The sensitivity of 690 nm/RIU and figure of merit of 50 are both among the highest values for similar plasmonic nanostructure arrays. This resist-free nanoimprint paves the way towards a low-cost and high-throughput realization of plasmonic optrodes and their wide applications.

Optically Tunable Bifunctional Structures Fabricated by Hybrid Imprint‐Photo Lithography (HIPL)

AbstractOptically tunable bifunctional structures are created by hybrid imprint‐photo lithography (HIPL), which is a combination of photolithography and soft lithography via a structured photomask. Prismatic patterns are formed on a flexible photomask, and the structured photomask is exploited as an imprint mold to create two‐layered microstructures that show a tunable refraction property. Fabricated microstructures, such as the perpendicularly aligned cross‐prismatic island (PAC‐PI), show optically dual functionality that originates from each optical property of the upper and lower layers in two‐layered structures with symmetric light collimation in the vertical and horizontal axes along both directions of the upper and lower prismatic layers, whereas a single conventional prismatic film shows only one‐axis collimation. Moreover, tunability of the light collimation, which can be modulated by varying the area ratio of the upper and lower layers from 80% to 20% using optical simulation, is demonstrated. Such optically tunable bifunction on a single sheet could potentially reduce the number of required optical sheets, such as the liquid crystal display backlight unit (LCD BLU). This new hybrid lithography process could provide a potential pathway to construct highly complex optical components, such as planar waveguide, microlight emitting diode, virtual reality/augmented reality, and holographic display devices.

A Novel Nanoimprint Lithography Thiol-ene Resist for Sub-70 nm Nanostructures

In this paper, we propose a novel kind of UV click-polymerization thiol-ene copolymers as nanoimprint lithography resists for sub-70 nm resolution patterns. High-precision mold imprint and release are two of the most critical steps of nanoimprint lithography, which requires the resists with properties of excellent conformal replication and low surface energy. Conventional UV-curable resists used in nanoimprint lithography, such as acrylate, epoxy resin, and vinyl ether, cannot satisfy all these properties requirements because they exhibit surface oxygen inhibition during polymerization, or materials fracture and delamination during mold releasing. A novel kind of thiol-ene copolymers have been investigated in this study, which have many properties favorable for use as nanoimprint lithography resists to imprint sub-70 nm and high-aspect-ratio nanostructures. These properties include sufficiently low viscosity and high Young's modulus, low surface energy for easy demolding, polymerization in benign ambient, and in particular, high chemical-etch resistance. These excellent properties give improve nanoimprinting results.