Nanoimprint Lithography Process Research Articles

Fabrication of a Metal Roller Mold with Nanoimprinted Pattern Using Thermal Nanoimprint Lithography

In this work, fabrication of a metal roller mold with nanoimprinted pattern was demonstrated. To get metal nanopattern on a metal roller mold, thermal nanoimprint lithography (TNIL) and subsequent electroforming process were conducted. A poly(methyl methacrylate) (PMMA) nanopattern was fabricated by TNIL process using a polydimethylsiloxane (PDMS) soft stamp on a bare PMMA film. An optimal experimental condition of TNIL process was investigated for large area, uniform PMMA nanopatterning. As a result, large area PMMA line patterns with 200 nm linewidth were fabricated by large-area TNIL process. Electroforming process on the PMMA nanopatterned film resulted in nickel (Ni) nanopattern with a linewidth of 200 nm from the PMMA line pattern. A large area (360 mm by 730 mm in width and length) Ni stamp for a roller mold was fabricated by laser cutting and tiling process of the multiple electroformed Ni stamps. We successfully fabricated a Ni roller mold with feature size of 200 nm in linewidth by attaching the large area Ni stamp to the surface of a roller body.

Replication and Surface Treatment of Micro Pattern Generated by Entanglement of Nanowires

In this study, a nano-micro pattern was fabricated by a nanoimprint lithography process using a porous material, particularly anodic aluminum oxide (AAO), and polymer resin. The fabricated mold consisted of a group of nanowires forming a bundle and showing a specific micro pattern. The structures were subjected to various surface treatments to control surface conditions and wettability. UV-Ozone treatment and octadecyltrichlorosilane (OTS) coating were used as surface treatments. Through these surface treatments, the surface energy of the fabricated structure was lowered, and as a result, it could be used as a mold for nano-micro patterning. The final product was also fabricated through a nanoimprint lithography process, and the reverse image of the mold was duplicated. The surface of each structure was observed by scanning electron microscopy (SEM) and the surface properties were examined by contact angle measurement.

Facile fabrication and formation mechanism of aluminum nanowire arrays

Anodized alumina membranes (AAMs) have proven effective at making vertically-oriented and well-ordered metal nanowire arrays, which are useful in plasmonics and electrochemistry. Here, we produced Al nanowires via directed AAM pore nucleation: a patterned oxide mask on a flat Al surface directed where pores did and did not form, the pores acting to oxidize Al around the sites without pores. This left Al nanowires embedded in the AAM, and produced freestanding Al nanowires after etching the AAM. The nanowire tops had two distinct contours, smooth bowls and flat rough surfaces—suggesting that nanowires with bowl tops result from slow pore development relative to pattern-nucleated pores, not pore blockage as prior literature suggests. The observed low porosity of ∼2%, as opposed to the more typical 10%, suggests pore nucleation in the electrolyte employed may need greater local variations in electric field or pH, possibly explaining the electrolyte’s peculiar ability to make Al nanowires. Finally, a soft nano-imprint lithography process was developed here to pattern the mask without damaging the stamp, avoiding a stamp degradation problem in previous work that utilized hard nano-imprint lithography.

Molecular Dynamics Study of Poly(dimethylsiloxane) Nanostructure Distortion in a Soft Lithography Demolding Process.

This study aims at investigating the distortion of poly(dimethylsiloxane) (PDMS) nanostructures in a soft lithography demolding process using molecular dynamics simulation. Experimental results show that after peeling, PDMS nanopillars became 10–60% longer in height than the mold size. Molecular dynamics simulations have been employed to plot the stress–strain curve of the nanopillars when subjected to uniaxial stress. Three force fields (COMPASS, CVFF, and PCFF) were used for modeling. The demolding process in soft lithography and nanoimprint lithography causes significant deformation in replication. The experimental results show clear signs of elongation after demolding. Molecular dynamics simulations are employed to study the stress–strain relationship of the PDMS nanopillars. The results from the simulation show that a PDMS nanopillar at temperature T = 300 K under tensile stress shows characteristics of flexible plastic under tensile stress and has a lower Young’s modulus, ultimate tensile stress, and Poisson’s ratio.

Multidirectional monolayer metal nano-grating micro polarizer array based on nanoimprint lithography and plasma ashing process

A novel fabrication process based on nanoimprint lithography (NIL) and plasma ashing process is proposed for the efficient fabrication of multidirectional monolayer metal nano-grating micro polarizer array (MPA). In this process, the NIL resist serves as a multifunction layer: the definition of the nano-grating patterns and the sacrifice layer for the monolayer metal nano-grating MPA forming. As a demonstration, a quartz filter with large area of monolayer metal nano-grating MPA was fabricated by this novel process. The performance test results demonstrate that the extinction ratios of the four tested micro polarizers with different orientations are 10.25, 10.45, 9.37, and 10.55, and the maximum polarization transmittances are 40.28%, 42.31%, 40.31%, and 42.35%, respectively. The polarization imaging experiment results demonstrate that the angle of polarization measurement error is <0.3°; the degree of linear polarization measurements error is <2.5%.

Deep-Ultraviolet LEDs Fabricated by Nanoimprinting

In this study, deep-ultraviolet 280-nm-wavelength light-emitting diodes (DUV LEDs) with a nanopatterned sapphire substrate (NPSS) were fabricated using a nanoimprint lithography process. The output power of these flip-chip (FC) DUV LEDs was measured and analyzed in a ray-tracing simulation using NPSSs: cones with diameters of 2.5 μm (LED1), 400 (LED2) and 200 nm (LED3) as well as holes with diameters of 350 (LED4) and 750 nm (LED5). For comparison, a mirror for the c-plane FC DUV LEDs was introduced by coating Ni/Au and Al thin films. With the injection current at 350 mA, the output powers of the c-plane, LED1, LED2, LED3, LED4, and LED5 were 13.38, 14.68, 20.05, 16.79, 15.74, and 18.93 mW, respectively. The LED2 efficiency improved 49.9% compared with mirror LED and the result was consistent with ray-tracing simulation.

Disposable ultrasound-sensing chronic cranial window by soft nanoimprinting lithography

Chronic cranial window (CCW) is an essential tool in enabling longitudinal imaging and manipulation of various brain activities in live animals. However, an active CCW capable of sensing the concealed in vivo environment while simultaneously providing longitudinal optical access to the brain is not currently available. Here we report a disposable ultrasound-sensing CCW (usCCW) featuring an integrated transparent nanophotonic ultrasonic detector fabricated using soft nanoimprint lithography process. We optimize the sensor design and the associated fabrication process to significantly improve detection sensitivity and reliability, which are critical for the intend longitudinal in vivo investigations. Surgically implanting the usCCW on the skull creates a self-contained environment, maintaining optical access while eliminating the need for external ultrasound coupling medium for photoacoustic imaging. Using this usCCW, we demonstrate photoacoustic microscopy of cortical vascular network in live mice over 28 days. This work establishes the foundation for integrating photoacoustic imaging with modern brain research.

2D Dielectric Nanoimprinted PMMA Pillars on Metallo-Dielectric Films

In this work, we propose an optimized nanoimprint protocol for the fabrication of a two-dimensional (2D) array of polymethyl-methacrylate (PMMA) nano-pillars deposited on different sputtered configurations (bilayer and multi-layer) of copper (Cu) and aluminum nitride (AlN) slabs supported by a silicon dioxide (SiO2) substrate. Both the Cu/AlN bilayer and multilayer thin films were deposited by a sputtering technique. The sub-micron PMMA pillars were realized by using nanoimprint lithography (NIL). In order to optimize the NIL process, several tests were performed by varying temperature and pressure, allowing us to achieve uniform and high-resolution pillars. The fabricated periodic array enabled the phase-matching of the incident plane wave exciting optical resonances. All the fabricated devices were then optically characterized by means of an ad hoc setup, where the reflected light from the sample was analyzed. The fabricated nano-pillars are mechanically stable, and they could be fully exploited for the realization of novel metallo-dielectric core/shell structures for sensing, surface-enhanced Raman spectroscopy, and light–matter interactions.

Experimental and Numerical Study on the Characteristics of the Thermal Design of a Large-Area Hot Plate for Nanoimprint Equipment

A numerical study was conducted on the thermal performance of a large-area hot plate specifically designed as a heating and cooling tool for thermal nanoimprint lithography processes. The hot plate had the dimensions 240 mm × 240 mm × 20 mm, in which a series of cartridge heaters and cooling holes were installed. Stainless steel was selected to endure the high molding pressures. To examine the hot plate’s abnormal thermal behavior, ANSYS Fluent V15.0, which is commercial CFD code, was used to perform computational analysis. A numerical model was employed to predict the thermal behavior of the hot plate in both the heating and cooling phases. To conduct the thermal design of a large-area hot plate for nanoimprint equipment, we selected the model to be studied and proposed a cooling model using both direct and indirect cooling methods with and without heat pipes. In addition, we created a small hot plate and performed experimental and computational analyses to confirm the validity of the proposed model. This study also analyzed problems that may occur in the stage prior to the large-area expansion of the hot plate. In the case of a stainless steel (STS304) hot plate for large-area hot plate expansion, the heat pipes were inserted in the direction of the cartridge heaters to address the problems that may occur when expanding the hot plate into a large area. As a result, the heating rate was 40 °C/min and the temperature uniformity was less than 1% of the maximum working temperature of 200 °C. For cooling, when considering pressure and using air as the coolant for the ends, a cooling rate of 20 °C/min and thermal performance of less than 13.2 °C (less than 7%) based on the maximum temperature were obtained. These results were similar to the experimental results.

Direct metallic nanostructures transfer by flexible contact UV-curable nano-imprint lithography

We propose a novel flexible contact ultraviolet (UV)-curable nano-imprint lithography process which can directly transfer metallic nanostructures by UV curing bonding force. The flexible template evaporated with metal acts as a mask, allowing only the structural region to penetrate UV light and transfer surface metallic nanostructures. By this process, aluminum metal grating of different feature sizes had been transferred to abtain single-layer or even multi-layer metallic nanostructures. The minimum width and spacing of the wire grating were 50 nm and sub-20 nm, respectively. Aluminum grating polarizer had also been fabricated and tested demonstrating the practicability of the process. © 2019 The Japan Society of Applied Physics

Elastohydrodynamics of Roll-to-Roll UV-Cure Imprint Lithography

The inextensible cylindrical shell theory and lubrication theory combine into a model for the elastohydrodynamics of a rolling-imprint modality of nanoimprint lithography (NIL). Foil-bearing theory describes the formation of the lubrication gap due to relative motion between a tensioned substrate and a rigid, cylindrical surface. Reproduction of the results of foil-bearing theory for both stiff and perfectly flexible substrates validates this coupled model and reveals a highly predictable region of uniformity that provides low shear stress conditions ideal for UV-cure. These results show theoretical limitations that are used to construct an operating window for predicting rolling-mode NIL process feasibility.

Numerical Study on Thermal Design of a Large-Area Hot Plate with Heating and Cooling Capability for Thermal Nanoimprint Lithography

A numerical study is conducted on the thermal performance of a large-area hot plate specifically designed as a heating and cooling tool for thermal nanoimprint lithography processes. The hot plate has the dimensions 240 mm × 240 mm × 20 mm, in which a series of cartridge heaters and cooling holes are installed. Stainless steel has been selected to endure the high molding pressures. A numerical model based on ANSYS Fluent is employed to predict the thermal behavior of the hot plate in both the heating and cooling phases. The proportional–integral–derivative (PID) thermal control of the device is modeled by adding user defined functions. The results of the numerical computations demonstrate that the use of cartridge heaters provides sufficient heat-up performance and the active liquid cooling in the cooling holes provides the required cool-down performance. However, a crucial technical issue is raised, namely that the proposed design poses a large temperature non-uniformity in the steady heating phase and in the transient cooling phase. As a remedy, a new hot plate in which heat pipes are installed in the cooling holes is considered. The numerical results show that the installation of heat pipes could enhance the temperature uniformity both in the heating and cooling phases.

Molecular orientation and stability of poly(3-hexylthiophene) nanogratings affected by the fabricated solvent vapor.

During the nanoimprinting lithography (NIL) process, the role of solvent vapor in fabricating the pattern structure and inducing the molecular alignment of nanoimprinted polymer film has been attracting significant attention. We demonstrate here that the molecular orientation and thermal stability of poly(3-hexylthiophene) (P3HT) nanograting film can be affected obviously by the fabricated solvent vapor. A solvent-vapor nanoimprinting lithography (SV-NIL) technique based on a polydimethylsiloxane (PDMS) template is employed to fabricate a P3HT nanograting structure film successfully and solvent vapor is offered by chlorobenzene, chloroform and carbon disulphide, respectively. The molecular orientation of the polymer film is carefully characterized by grazing incidence wide angle X-ray diffraction (GIWAXD) measurements to investigate the effect of various solvent vapors on the molecular orientation of the P3HT nanograting film. For the P3HT nanograting film fabricated by chloroform and chlorobenzene solvent, the edge-on molecular orientation of the typical form II crystallographic structure is induced. However, this indicates that there are both the face-on molecular orientations of the form II and form I crystallographic conformation present for the P3HT nanograting film fabricated by carbon disulphide solvent. Therefore, the fabricated solvent vapor plays a significant role in determining the formation of the molecular orientation of the polymer nanostructure. Then, the role of thermal annealing in the stability of the molecular orientation was investigated for the P3HT nanograting film after a fixed temperature. As for the annealed nanograting film fabricated by chlorobenzene and chloroform solvent vapor, a single edge-on molecular orientation mode of the form I crystallographic structure has been obtained. However, for the annealed nanograting film fabricated by the carbon disulphide solvent, the edge-on and face-on molecular orientations of the form I crystallographic structure are still retained. This indicates that the stability of the form II crystallographic conformation is mainly dependent on the thermal annealing process. Therefore, after the annealing process, the final determining of the molecular alignment and crystallographic conformation depends significantly on the orientation type of the nanograting film before the annealing history, and it can be further inferred that the molecular orientation of the annealed polymer film is still affected by the fabricated solvent vapor significantly. Thus this will provide new understanding and guidance for the research of the topographical structure and molecular alignment of conjugated polymers.

Assessment of nano-scale tribological and mechanical properties of flexible transparent polymers based on atomic force microscopy

Transparent polymers are often used in various devices that are fabricated by nano-imprinting lithography (NIL). One of the main concerns in NIL is regarding the interfacial interaction between the mold and the polymeric part. In this work, the tribological and mechanical properties of PET and PDMS were systematically assessed with respect to temperature using an AFM. The nano-scale properties obtained were used to analyze the stress distribution of a polymeric part during NIL. These results are expected aid in optimization of NIL process to achieve higher precision and durability of polymeric parts used in bio and energy devices.

High density GaAs nanowire arrays through substrate processing engineering

GaAs nanowires (NWs) vertically aligned were successfully fabricated through substrate processing engineering. High-density vertical GaAs NWs are grown on n-type Si (111) substrate by molecular beam epitaxy. Systematic experiments indicate that substrate pretreatment is crucial to vertical epitaxial growth of one-dimensional (1D) nanomaterials. The substrates etched using diluted buffered oxide etch (BOE) were explored to improve the NW density and vertical. We also find that the substrate processing engineering strongly affect the morphology of GaAs NWs. Finally, we demonstrate fabrication of GaAs NW arrays on Si surface by field-emission scanning electron microscopy (FE-SEM). This single-step process indeed offers a simple and cost-effective way to obtain a large area of GaAs NW arrays without using e-beam lithography (EBL) and/or nanoimprint lithography (NIL) processes. This work provided a new approach for hight density NW arrays.

Developing a predictive model for nanoimprint lithography using artificial neural networks

Nanoimprint lithography (NIL) is a high-throughput and cost-effective technique for fabricating nanoscale features. However, the fine tuning of NIL process parameters is critical in achieving defect-free imprints. Currently, there exists no unified material and process design guidelines to deal with the complex set of process parameters. In this research, an artificial neural network (ANN) algorithm was developed to predict the imprint quality based on a set of input factors collected from experiments and literature. The prediction accuracy was tested in two stages by the use of three ANN models - General Regression Neural Network (GRNN), Back Propagation Neural Network (BPNN), and Probabilistic Neural Network (PNN). Higher prediction accuracy was observed for GRNN in both stages (1st stage=58.22% and 2nd stage=92.37%). A significant increase in the prediction accuracy was observed in the second stage after including additional input factors and larger data set. The average prediction accuracies for GRNN, BPNN, and PNN in the second stage were reported as 92.37%, 84.79%, and 91.19%, respectively. The prediction of the GRNN model was validated with experimental results confirming the accurate classification of all the outputs. This research lays the foundation for the development of an expert system for nanoimprint lithography.

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.

Fabrication of Broadband Antireflective Sub-Micro Structures on 4H-SiC by Mesh Patterning Etching

An approach to fabricating pseudoperiodic antireflective subwavelength structures on silicon carbide by using modified resist as etching mask is demonstrated. The etched submicron structure is a mesh-trenched pattern thus as the so called “mesh patterning etching”. The mesh patterning process is more time-efficient than the e-beam lithography or nanoimprint lithography process. The influences of the reactive-ion etching conditions and post bake temperature of the resist film to the sub-micron structure profile and its corresponding surface reflectance have been systematically investigated. Under optimal experimental conditions, the average reflectance of the silicon carbide in the range of 390-800 nm is dramatically suppressed from 40% to around 8% after introducing the mesh patterning sub-micro structures.

Self-cleaning and wear-resistant polymer nanocomposite surfaces

Superhydrophobic self-cleaning and wear-resistant nanocomposite surfaces were produced by mimicking the hierarchical structure of the lotus leaf using a combination of rapid self-assembly and a UV nano-imprint lithography (UVNIL) process with a silicone master. Two different acrylate formulations containing acrylated silica nanoparticles and an acrylated silicone surfactant were used. The presence of the silicone master did not suppress the spontaneous migration of the surfactant to the polymer surface, which increased its hydrophobic character. Adding acrylated silica particles considerably increased the viscosity of the acrylate suspensions and led to a shear-thinning behavior. However the particles did not prevent the fast migration process of the surfactant and further increased the hydrophobicity of the material, due to increased nanoscale roughness of the nanocomposite surface. The largest increase of hydrophobicity was achieved for the UVNIL printed lotus surfaces using the acrylate formulation with lowest viscosity. These surfaces became superhydrophobic for the highest investigated concentration of silica. These nanocomposite lotus surfaces were, in addition, very hard with a microhardness above 400 MPa and particularly wear-resistant, and were self-cleaning with respect to hydrophobic contamination.

High Quality 3D Photonics using Nano Imprint Lithography of Fast Sol-gel Materials

A method for the realization of low-loss integrated optical components is proposed and demonstrated. This approach is simple, fast, inexpensive, scalable for mass production, and compatible with both 2D and 3D geometries. The process is based on a novel dual-step soft nano imprint lithography process for producing devices with smooth surfaces, combined with fast sol-gel technology providing highly transparent materials. As a concrete example, this approach is demonstrated on a micro ring resonator made by direct laser writing (DLW) to achieve a quality factor improvement from one hundred thousand to more than 3 million. To the best of our knowledge this also sets a Q-factor record for UV-curable integrated micro-ring resonators. The process supports the integration of many types of materials such as light-emitting, electro-optic, piezo-electric, and can be readily applied to a wide variety of devices such as waveguides, lenses, diffractive elements and more.