Nanoimprint Lithography Molds Research Articles

Scalable Nanoimprint Manufacturing of Functional Multilayer Metasurface Devices

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

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.

Fabrication and nanoindentation characterization of nickel micro-pillar mold for nanoimprint lithography

In this work, we develop a facile processing technique that tailors the microstructure of the void-free micro-pillars on a nickel substrate based on electroplating for applications in nanoimprint lithography (NIL). The obtained nickel pillars typically have an inhomogeneous microstructure consists of columnar grains and a multitude of nanocrystalline. The columnar grain size is refined when using alternating current, which is a result of the disrupted grain growth and promoted grain nucleation due to higher cathodic overpotential. The nickel micro-pillars produced by alternating current also yield a higher hardness of 3.5±0.2 GPa while maintaining a moderate plasticity index of 0.53±0.06. The enhanced mechanical strength of the micro-pillars through microstructure fine-tuning is potentially helpful for large-scale patterning by NIL for commercial manufacturing.

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.

High-throughput nanomanufacturing of synthetic antiferromagnet-polymer nanoparticles with high magnetic moment, very low remanence, and high magnetic susceptibility for biomedical applications

A high-throughput top-down nanomanufacturing approach for making metal–polymer nanoparticles with tunable magnetic properties using nanoimprint lithography is reported. The nanoparticles comprise of a layered structure that includes a Co/Ru/Co synthetic antiferromagnet (SAF) and a polymethyl methacrylate (PMMA) or a hydrogen silsesquioxane (HSQ) layer. The Co/Ru/Co structure deposited by magnetron sputtering was tuned to achieve antiferromagnetic coupling between Co layers, desired magnetic susceptibility, and saturation magnetization. Ultrahigh density positive tone thermal nanoimprint lithography molds designed to maximize the depth and minimize the thickness of imprint trenches in nanoimprint resist (PMMA or HSQ) were fabricated. The molds were used to pattern large arrays of 200 nm × 200 nm, 400 nm × 400 nm, and 600 nm × 600 nm SAF nanostructures with superparamagneticlike magnetic characteristics. The nanostructures were released from the wafers into an aqueous suspension by dissolving the sacrificial underlayers, Cu for SAF/PMMA nanoparticles and PMMA for SAF/HSQ nanoparticles.

Perfluoropolyether (PFPE) Intermediate Molds for High-Resolution Thermal Nanoimprint Lithography.

Among soft lithography techniques, Thermal Nanoimprint Lithography (NIL) is a high-throughput and low-cost process that can be applied to a broad range of thermoplastic materials. By simply applying the appropriate pressure and temperature combination, it is possible to transfer a pattern from a mold surface to the chosen material. Usually, high-resolution and large-area NIL molds are difficult to fabricate and expensive. Furthermore, they are typically made of silicon or other hard materials such as nickel or quartz for preserving their functionality. Nonetheless, after a large number of imprinting cycles, they undergo degradation and become unusable. In this paper, we introduce and characterize an innovative two-step NIL process based on the use of a perfluoropolyether (PFPE) intermediate mold to replicate sub-100 nm features from a silicon mold to the final thermoplastic material. We compare PFPE elastomeric molds with molds made of the standard polydimethylsiloxane (PDMS) elastomer, which demonstrates better resolution and fidelity of the replica process. By using PFPE intermediate molds, the nanostructured masters are preserved and the throughput of the process is significantly enhanced.

Selection of UV-resins for nanostructured molds for thermal-NIL

Nanoimprint molds made of soft polymeric materials have advantages of low demolding force and low fabrication cost over Si or metal-based hard molds. However, such advantages are often sacrificed by their reduced replication fidelity associated with the low mechanical strength. In this paper, we studied replication fidelity of different UV-resin molds copied from a Si master mold via UV nanoimprint lithography (NIL) and their thermal imprinting performance into a thermoplastic polymer. Four different UV-resins were studied: two were high surface energy UV-resins based on tripropyleneglycol diacrylate (TPGDA resin) and polypropyleneglycol diacrylate (PPGDA resin), and the other two were commercially available, low surface energy poly-urethane acrylate (PUA resin) and fluorine-containing (MD 700) UV-resins. The replication fidelity among the four UV-resins during UV nanoimprint lithograph from a Si master with sharp nanostructures was in the increasing order of (poorest) PUA resin < MD 700 < PPGDA resin < TPGDA resin (best). The results show that the high surface energy and small monomer size are keys to achieving good UV-resin filling into sharp nanostructures over the viscosity of the resin solution. When the four UV-resin molds were used for thermal-NIL into a thermoplastic polymer, the replication fidelity was in the increasing order of (poorest) MD 700 < TPGDA resin < PUA resin (best), which follows the same order of their Young’s moduli. Our results indicate that the selection of an appropriate UV-resin for NIL molds requires consideration of the replication fidelities in the mold fabrication and the subsequent thermal-NIL into thermoplastic polymers.

Modeling of Flexible Composite Mold for Nanoimprint Lithography

Modeling of Flexible Composite Mold for Nanoimprint Lithography

Basic Verification of Method for Automated Design of Capacity-Equalized Mold for Nanoimprint Lithography

Basic Verification of Method for Automated Design of Capacity-Equalized Mold for Nanoimprint Lithography

A pH-UV Dual-Responsive Photoresist for Nanoimprint Lithography That Improves Mold Release

We have developed a degradable photoresist that is responsive to pH and ultraviolet light (UV). This dual-responsive resist consists of 5,7-diacryloyloxy-4-methylcoumarin (fluorescent monomer), acrylic anhydride, and 3,6-dioxa-1,8-dithiooctane. It can be photocured using thiol–acrylate polymerization and photodimerization of coumarin moieties under 365 nm UV light exposure. The cured resist is degradable in aqueous solutions with pH > 7. The degradation process can be characterized by the change of fluorescence intensity in the aqueous solution. In this study, we have analyzed the properties of the degradation of the resist by changing the pH of the solution and its exposure time under 254 nm UV light. This UV exposure can induce photocleavage of the coumarin dimers. We then used these materials to fabricate micropatterns through nanoimprint lithography (NIL) process. Compared with other conventional degradable materials capable of NIL, the dual-responsive resist can help to clean the NIL mold easily at room temperature. This resist is also more environmentally friendly, is relatively low cost, can be faster to degrade, and is easier to characterize. It also has low volume shrinkage, which may have a valuable and positive effect on the development of NIL.

Simple and scalable preparation of master mold for nanoimprint lithography

Nanoimprint lithography (NIL) is one of the most prominent bottom-up techniques for duplicating nanostructures with a high throughput. However, fabrication of starting master mold commonly requires expensive equipment of top-down techniques, or additional steps to transfer the fabricated patterns from bottom-up methods. Here we demonstrate that a SiO2 nanostructure manufactured from a self-assembled block copolymer, polystyrene-b-polydimethylsiloxane (PS-b-PDMS), directly serves as a master mold for NIL without further modification. A hexagonally aligned pattern over the entire substrate is established using a simple technique; solvent annealing and etching. Etching also plays an important role in endowing fluorine on the surface of SiO2, thus promoting smooth demolding upon imprinting. The obtained pattern of the SiO2 nanostructure is transferred to a polymer surface using UV nanoimprint. Identical patterns of the SiO2 nanostructure are elaborately reproduced on Ni and Cu nanodot arrays via electroplating on the polymer transcript, which was verified by morphological observations. The uniformity of the replicated Ni nanodot array is evaluated using spectroscopic ellipsometry. The measured optical response of the Ni nanodot is validated by electromagnetically simulated results, indicating that the pattern transfer is not limited to a small local area. In addition, the durability of the SiO2 mold pattern is corroborated after the imprinting process, thus guaranteeing the reusability of the fabricated nanostructure as a master mold. The proposed approach does not require any high-end lithographic techniques; this may result in significant cost and time reductions in future nanofabrication.

Importance of Molds for Nanoimprint Lithography: Hard, Soft, and Hybrid Molds

Nanoimprint lithography has attracted considerable attention in academic and industrial fields as one of the most prominent lithographic techniques for the fabrication of the nanoscale devices. Effectively controllable shapes of fabricated elements, extremely high resolution, and cost-effectiveness of this especial lithographic system have shown unlimited potential to be utilized for practical applications. In the past decade, many different lithographic techniques have been developed such as electron beam lithography, photolithography, and nanoimprint lithography. Among them, nanoimprint lithography has proven to have not only various advantages that other lithographic techniques have but also potential to minimize the limitations of current lithographic techniques. In this review, we summarize current lithography techniques and, furthermore, investigate the nanoimprint lithography in detail in particular focusing on the types of molds. Nanoimprint lithography can be categorized into three different techniques (hard-mold, soft-mold, and hybrid nanoimprint) depending upon the molds for imprint with different advantages and disadvantages. With numerous studies and improvements, nanoimprint lithography has shown great potential which maximizes its effectiveness in patterning by minimizing its limitations. This technique will surely be the next generation lithographic technique which will open the new paradigm for the patterning and fabrication in nanoscale devices in industry.

Theoretical study of fabrication of line-and-space patterns with 7 nm quarter-pitch using electron beam lithography with chemically amplified resist process: III. Post exposure baking on quartz substrates

Electron beam (EB) lithography is a key technology for the fabrication of photomasks for ArF immersion and extreme ultraviolet (EUV) lithography and molds for nanoimprint lithography. In this study, the temporal change in the chemical gradient of line-and-space patterns with a 7 nm quarter-pitch (7 nm space width and 21 nm line width) was calculated until it became constant, independently of postexposure baking (PEB) time, to clarify the feasibility of single nano patterning on quartz substrates using EB lithography with chemically amplified resist processes. When the quencher diffusion constant is the same as the acid diffusion constant, the maximum chemical gradient of the line-and-space pattern with a 7 nm quarter-pitch did not differ much from that with a 14 nm half-pitch under the condition described above. Also, from the viewpoint of process control, a low quencher diffusion constant is considered to be preferable for the fabrication of line-and-space patterns with a 7 nm quarter-pitch on quartz substrates.

Fabrication of Ni mold for nanoimprint lithography by combining two supercritical fluid-based deposition technologies

A nano-scale three-dimensional (3D) Ni mold fabrication technique was developed for nanoimprint lithography by filling Ni into a patterned polymer resist film, 100 nm in width and with an aspect ratio of 1. This was achieved by a combination of two supercritical fluid-based technologies: supercritical fluid impregnation (SCFI) of Pd and supercritical fluid deposition (SCFD) of Ni. Pd nanoparticles were formed primarily on the surface of the 3D patterned resist film by SCFI to provide a catalytic effect on the resist surface, which initiated the subsequent Ni-SCFD. Ni was then filled into the patterned resist by SCFD, exploiting its superior gap-filling capabilities. Our proposed scheme has the potential to be applied to structures scaled down to below 100 nm.

OrmoStamp mold fabrication via PBW for NIL

Nanoimprint lithography (NIL) is a promising technology that can fabricate structures with high resolution and high throughput. To replicate patterns through NIL, a high quality master mold is needed. A UV patternable inorganic–organic hybrid polymer, OrmoStamp with high resolution is promising for the fabrication of NIL molds. Here, OrmoStamp molds were fabricated by proton beam writing (PBW) in resist followed by OrmoStamp casting. In this paper, different resists (HSQ, PMMA, SU-8 and SML) were evaluated by PBW for OrmoStamp mold fabrication. The results show that HSQ gives the smallest line width down to 30nm. These lines have subsequently been successfully replicated in OrmoStamp. High aspect ratio structures in OrmoStamp and Ni were obtained from proton beam written resist samples, featuring an aspect ratio of ∼7 and ∼70, respectively. Finally thermal NIL has been demonstrated in PMMA using OrmoStamp molds.

Anti-adhesive characteristics of CHF3/O2 and C4F8/O2 plasma-modified silicon molds for nanoimprint lithography

The anti-adhesive characteristics of a plasma-modified silicon mold surface for nanoimprint lithography are presented. Both CHF3/O2 and C4F8/O2 plasma were used to form an anti-adhesive layer on silicon mold surfaces. The gas mixing ratios of CHF3/O2 and C4F8/O2 were experimentally changed between 0% and 80% to optimize the plasma conditions to obtain a low surface energy of the silicon mold. The plasma characteristics were examined by optical emission spectroscopy (OES). In order to investigate the changes in surface energy and surface chemistry of the anti-adhesive layer during repeated demolding cycles, contact angle measurements and X-ray photoelectron spectroscopy (XPS) were performed on the plasma-modified silicon mold surface. Simultaneously, the surface morphology of the demolded resists was evaluated by field-emission scanning electron microscope (FE-SEM) in order to examine the effect of the anti-adhesive layers on the duplicated patterns of the resists. It was observed that the anti-adhesive layer formed by CHF3/O2 plasma treatment was worn out more easily during repeated demolding cycles than the film formed by C4F8/O2 plasma treatment, because CHF3/O2 gas plasma formed a thinner plasma-polymerized film over the same plasma treatment time.

Defect analysis and lifetime evaluation of a release-coated nanoimprint mold

Nanoimprint lithography (NIL) is very useful technique for the fabrication of nanopatterns. In ultraviolet NIL, a release coating on the mold surface prevents the adhesion of the replicating resin. However, this release coating gradually deteriorates as the number of repetitions of NIL transfer increases. It is therefore important to evaluate the lifetime of the release agent. Measurements of the contact angle and release force are normally used in lifetime evaluation; however, these values do not clearly characterize the lifetime behavior because they tend to approach saturation as the number of repetitions increases. We introduce the error rate as a new means of evaluating the lifetime of release coatings. The error rate is derived by statistical treatment of transferred dots patterns. We found that mixing of various types of release agent is effective in reducing the error rate and, therefore, in increasing the lifetime of the NIL mold.

A master-mold fabrication by electron beam lithography followed by nanoimprinting and self-aligned double patterning

As a new scheme of master-mold fabrication, a half pitch (hp) 12 nm line and space (L/S) pattern was fabricated from hp 24 nm L/S resist mandrels, which were prepared by electron beam (EB) writing as well as nanoimprinting, followed by the self-aligned double-patterning (SADP) technique. It was observed that the line width roughness (LWR) was reduced and improved by single and multiple nanoimprintings in the new scheme of the master-mold fabrication to make hp 24 nm resist mandrels. We have studied the phenomena and revealed that the resist pattern of nanoimprinting had sharper and smoother shoulders and bottom edges in cross section than those of the EB resist. These shoulder shapes of nanoimprinting seemed to be reflected in its LWR improvement. The new scheme has advantages of resolution enhancement and better pattern quality of LWR on a master mold for nanoimprint lithography, in comparison with conventional optical and EB lithography technologies.

Injection Compression Molding of Replica Molds for Nanoimprint Lithography

As a breakthrough in the cost and durability of molds for nanoimprint lithography (NIL), replica molds are fabricated by injection compression molding (ICM). ICM is commonly used for optical disks such as DVDs or Blu-ray disks and is also a practical fabrication method for nanostructures. In this paper, I successfully demonstrated the fabrication of cycloolefin polymer replica molds with structures smaller than 60 nm by ICM. Furthermore, ultraviolet (UV)-NIL using these replica molds was demonstrated. UV-cured resist was replicated over an area of 60 mm diameter. The degree of replication by UV-NIL in the first usage of each replica mold had good repeatability. Because ICM is a high-throughput, low-cost process, the replica mold can be disposed of after a certain time for UV-NIL. This method leads to a high-integrity UV-NIL process of patterned media because multiple large-area replica molds can be fabricated simultaneously.

Study on porous silicon template for nanoimprint lithography

The template for naoimprint lithography having a nano-sized structure was usually fabricated by traditional lithography such as extreme ultraviolet (EUV) lithography, focused ion beam (FIB) lithography, electron beam (EB) lithography. However, these approaches are always time-consuming and inefficiency which limits the potential application in nanoimprint lithography. To find a simple and low-cost method to fabricate the mold for nanoimprint lithography, and to improve the application in nanoimprint lithography have become the research focus. Instead of being formed by traditional lithography, the anodic aluminum oxide (AAO), with highly regular structures and high pore density, is the mold to achieve periodic structures for nanoimprint lithography. In this work, we successfully transfer a 2D nanoporous array structure to the Si surface via the nanoimprint lithography and AAO. The pore diameter and the interpore distance of the porous silicon (PS) are well consistent with that of AAO template. The interval, the diameter, and the height of the hexagonal array structure are 350–560 nm, 170–480 nm, and 200 nm, respectively. We have tested the Raman spectrum under the excitation by lasers of wavelength 514 nm. According to the results, two samples each exhibits a peak at 520 cm-1 and no frequency shift is observed with the Si characteristic Raman peak, indicating that the PS was not extensively damaged by the ICP etching process. Raman intensity in the structured Si is almost enhanced by a factor of 12 as compared with the case on polished Si, which will greatly benefit the application of Si-based optical devices. Thus, we have realized the replica of the PS template and obtained a nanopillar soft template via the hot embossing lithography.