Thermal Imprint Research Articles

Immersion effect of fluoride liquid into cavities of negative mold in thermal imprint

Around each concave imprinted pattern, the compressed air that is caused by the imprint pressure, generates capillary bridges; and in case of a convex imprinted pattern the trapped air remaining inside the concave mold pattern gives rise to bubble defects. Thus, in order to remove such defective moldings, we proposed an immersion nanoimprint method where a compressible gaseous material is replaced by perfluorotributylamine (Fluorinert FC-43) as an incompressible liquid material. However, the insertion steps result in an excessive amount of FC-43 left behind after its utilization, and continues to remain in the cavity of the negative mold. This problem was resolved by controlling the amount of FC-43 liquid by applying three kinds of insertion techniques, namely dropping, spraying, and evaporation. In the cases of concave and convex imprinted patterns on a 460-nm-thick PMMA film, the defective moldings processes resulting in capillary bridges and bubbles defects were addressed. Among the FC-43 insertion techniques, in case of evaporation method, only a limited amount of inserted FC-43 was left behind, that resulted in the formation of some very small-sized capillary bridges and bubble defects. These results showed that in case of negative molds, complete filling can be achieved if air could be completely replaced by FC-43 which can be done by minimizing the amount of FC-43 liquid.

Fast and continuous patterning on the surface of plastic fiber by using thermal roller imprint

The authors succeeded in producing high-speed continuous patterning on the surface of plastic fiber at a feeding speed of 20 m/min by using a system they developed employing thermal roller imprint methodology. In this method, a cylindrical mold with seamless microstructures formed on its surface was used, wherein a plastic fiber is pressed between the cylindrical mold and a backup roller and is then run for imprinting. Here, the movement of the cylindrical mold in the direction of space-change between the mold and the backup roller can be precisely controlled, and the press force during the imprinting can be measured by a load cell located beneath the backup roller. The cylindrical mold and the backup roller are heated up to 250 °C and are rotated synchronously, imparting a forward-linear motion to the plastic fiber and thereby making continuous patterning of microstructures possible. Since the press force feedback system during high-speed imprinting cannot be adequately controlled, the authors devised a scheme where the system memorizes the press positions corresponding to the rotating angles of the cylindrical mold at low feeding speed under a controlled press force. Here, the periodic variation of the center-to-center distance between the cylindrical mold and the backup roller can be measured at set intervals and, by using the memorized relationship between the rotational angle and press position, the position of the cylindrical mold is then moved. Finally, the authors imprinted 50 μm high microstructures onto the surface of the plastic optical fiber whose diameter was 250 μm. As a result, when the press position was fixed, the standard deviation of the press force was 6.86 N; however, this value dropped to 2.80 N when the improved control method was employed. The range of depth variation was 8.0 μm when the press position was fixed, and this number went down to 2.2 μm when the improved control method was employed.

Preparation of random stamps for thermal nanoimprint

Tailoring of surfaces or interfaces by patterning is a well-known approach used for the improvement of optical and electrical properties of thin film optical devices, and nanoimprint is a strong candidate to realise this at low cost over large areas. For such purposes we provide stamps with random texture. Contamination masking is used instead of lithography to further reduce the costs. Reactive ion etching in SF6/O2 at high oxygen content is effective to provide pyramidal structures of about 300-500nm in height. Loading affects the etch result and has to be considered when establishing a process suitable for large area processing. The geometry of the pyramids is characterised by SEM and AFM measurements, and also by thermal imprint to show the shape of the grooves in between the pyramids. Optical characterisation in a single-axis goniometer-type set-up documents a substantial broadening of the angular distribution of the intensity of the reflected light compared to a flat surface, well suited for the purpose envisaged.

Integration of rotated 3-D structures into pre-patterned PMMA substrate using step & stamp nanoimprint lithography

In this work, we fabricated hybrid micron-scale blazed grating structures on top of linear nanoscale gratings by superimposing thermal nanoimprint process. A poly(methyl methacrylate) (PMMA) substrate was at first pre-patterned with a nanoscale binary line grating by thermal NIL. The second imprint was added into the linear surface-gratings by sequential thermal imprinting using a stamp with micron-scale blazed gratings. The rotation controlled patterning was realized by Step and Stamp Imprint Lithography (SSIL) using NPS300 nanoimprinting stepper with a rotation imprint head with ability to control X/Y positioning and angular orientation of the stamp. In this paper, we demonstrate the potential of the hybrid imprint process for fabrication of e.g. hybrid bio devices or optical elements with arbitrary orientation.

Effect of buffer materials on thermal imprint on plastic optical fiber

We are advancing the development of a smart fiber using a plastic optical fiber (POF) as a fibrous substrate with micro-electro-mechanical-systems (MEMS) patterned on its surface. We employed hot embossing and thermal nanoimprinting techniques for patterning on the surface of POF, although, no work has so far been reported on the molding characteristic of POF. And moreover, achieving any high quality imprinted patterns on POFs has also proven to be difficult. We have been studying the effect of molding on POFs under various heating temperatures and press depths by sandwitching the POF between patterned face of a mold and a buffer material. When a soft buffer material with its hardness less than that of the polymethyl methacrylate (PMMA) core of a POF fiber was used, a reasonable patterning on the clad layer covering the surface of the POF was achieved without any sign of deformation of PMMA core. On the other hand, when the hardness of a buffer material happened to be equal to, or higher than that of the PMMA core, then deep concave pattern could be processed by purposefully deforming the POF. We successfully transferred a pseudo MEMS pattern with a width of 20 μm on the surface of a 250-μm-diameter POF. Also, under another kind of optimized molding conditions combined with buffer material, we fabricated an arc-shaped weaving guide structure on POF with the weaving guide`s bottom width of 300 μm. The investigation of the molding characteristic of POF by examining any change in the cross-sectional shape is a unique one. The experimental results thus obtained, add significantly to the database for the processing of a fibrous substrate by thermal deformation.

Fabrication of glass-like carbon molds to imprint on glass materials by MEMS processing technologies

We processed a precise relief structure on the surface of a glass-like carbon (GC) substrate by applying micro-electro-mechanical-systems (MEMS) technologies, and made a high temperature resistant mold for thermal imprinting on glass materials. An attractive feature of GC is its chemical stability at high temperatures (above 1,000 °C). The down side is its brittleness that makes microfabrication with GC a difficult task. We investigated to find if photolithography combined with reactive-ion-etching (RIE), which are generally used in MEMS fabrication, could be applied for the fabrication of GC molds. In our work with the RIE process, we made masking layers using Au and a positive-tone photoresist. By taking advantage of the difference between the etching rates of the masking materials and GC, we fabricated convex mold patterns with vertical and curved sidewalls. From the experimental results imprinted on Pyrex glass and on quartz, the practicability of using both kinds of GC molds appeared to be quite promising. We believe that in the near future these techniques will be successfully applied in the fabrication of large-size GC molds.

Experimental and Numerical Analysis of Polymer Deformation in Thermal Nanoimprint Lithography

Nanoimprint lithography has been widely used in various applications as a promising micro/nanofabrication tool. In the thermal nanoimprint process, optimization of important process parameters such as temperature, pressure, and pressure holding time is greatly needed to successfully imprint micro-and nanoscale patterns in a resist material. In this study, both experimentation and numerical simulation were performed to extend the understanding of the behaviour of polymer materials and to optimize process condition effectively during the thermal imprint process. The effects of the process conditions on the polymer deformation are discussed to achieve a rapid and high throughput thermal imprint process.

2D photonic gratings from thermal imprinting of ITO-based films

A nanocomposite system based on titania-bound indium tin oxide (ITO) nanoparticles has been developed as directly patternable material for the realization of two dimensional (2D) plasmonic gratings through nanoimprint lithography. An optical and electrical characterization of the ITO-based films has been performed. The imprinting process has been optimized in order to obtain a faithful negative replication of an array of nanopyramids on a hard master, in terms of period and depth of the structures. The morphology of the fabricated 2D gratings has been characterized with Atomic Force and Scanning Electron Microscopy. Experimental measurements and theoretical simulations of the far-field properties of the ''naked'' photonic grating, before coupling with an emitter or depositing a thin metal film on top, have been carried out for incident light in the 300-1500nm wavelength range. A good qualitative agreement for transmittance data has been obtained. Simulated data for reflectance properly reproduce the experimental result only at long wavelengths, in the near-infrared region. Suggestions for further model implementations are proposed. The plasmonic architecture will be eventually designed and realized by applying the suitable metal coating.

Stop grating for perfect replication of micro Fresnel lens by thermal imprinting

A stop grating concept is proposed to improve polymer filling in the thermal imprinting of a micro Fresnel lens structure. The stop grating consists of line and space structures outside the Fresnel lens pattern zone area. The experimental results have proved that the stop grating can help to achieve the complete filling of a mold, at the same time acting as a stop to prevent possible damage to the mold surface relief structures during imprinting press. A computer simulation was carried out to identify the phenomena of micro-holes at the edge of imprinted pattern. By removing the cavity between the pattern area and stop grating, perfect imprinting results have been achieved.

Fabrication of Mg-based bulk metallic glass molds by thermal imprint process

Bulk metallic glasses (BMGs) have exhibited high strength, low surface energy and superior glass-forming ability. At temperatures above the glass transition temperature, BMGs become supercooled viscous materials and can be formed into complicated shapes or patterns on micro- or nano-scales. Therefore, BMGs have been considered to replicate molds for micro/nano forming applications. In this study, Mg58Cu31Y6Nd5 bulk metallic glasses were applied to study the fabrication of BMG molds from nickel master molds with micro structures by thermal imprint process. Theoretical and experimental studies were performed to understand the filling process of mold replication. A commercial computational fluid dynamics software based on the finite volume method (FVM) was used for simulating the filling behavior, where the FVM predictions reasonably well agreed with experimental measurements. With adequate operation parameters, both simulation and experimental results showed that filling rate was up to 97%. The replicated BMG molds were then employed to fabricate micro structures on PMMA substrates. It was found that the fabricated micro structures were easy to be released from the BMG molds and they gave almost the same geometrical features as that on the original master molds. Mg-based bulk metallic glasses showed good formability for replicating molds with fine structures in this study and they could be applied to micro/nano imprint applications in the future.

Surface Patterning of Glass via Electrostatic Imprinting Using a Platinum Mold

Electrostatic imprinting is a highly suitable process for patterning large area and high efficiency glasses because it enables glass patterning at low temperatures with low pressures. Because high DC voltage bias is applied to the mold and glass during the thermal imprinting, the mold materials should have electrical conductivity, appropriate glass adhesion properties, and excellent thermal and electrochemical stability. In this study, thin Pt/Ni molds were fabricated via Si micromachining and electroforming techniques and were then used in the electrostatic imprint process in order to evaluate their feasibility as molds. Under the investigated process conditions, the pattern transfer to glass was accomplished without noticeable degradation of the mold. Furthermore, the process parameter effects on replication fidelity and potential defects were investigated.

Freestanding smooth micron-scale polydimethylsiloxane (PDMS) membranes by thermal imprinting

We report on a novel fabrication process based on thermal imprinting for the formation of micron-scale, freestanding, dielectric layers of poly(dimethylsiloxane). This technique is the basis for three-dimensional elastomeric membrane micro-electro-mechanical system applications where the structural material is part of the actuator and the lateral expansion is by vertically applied bias. We have fabricated freestanding smooth defect-free membranes with thicknesses in the range of 0.4–4.8 μm and with diameters of centimeters order of magnitude. A curve was plotted to calibrate the thickness of the elastomer layer to the pressure of the imprint. The adhesion between the polymer and the silicon (Si) chip's surface was reduced by the deposition of a hydrophobic dodecyl-trichlorosilane monolayer on the chips prior to imprinting. The ability to detach the membrane from the chips after imprinting is critical for the production of layers that are freestanding. Additionally, we demonstrate the feasibility of patterning the membranes at the time of imprinting to create freestanding patterned micron-scale membranes. A simple device made up of a freestanding circular membrane with electrodes on the circumference demonstrating the application of the method is presented here. The device's electromechanical characteristics are presented as well.

Direct release of synthetic antiferromagnetic nanoparticles fabricated by defect-free thermal imprinting

We report an improved process, using a defect-free thermal imprinting process that directly releases homogeneously patterned lithographic nanoparticles from a large-area wafer to water. Our method has a substantially larger yield and avoids all the pitfalls that are associated with the sacrificial layers in the traditional imprinting methods. By using an ethylene tetrafluoroethylene mold and a bi-layer resist lift-off, defect-free imprints were achieved in areas larger than 1 × 1 cm2. Illustrating the effectiveness of this method, high-quality planar Fe/Ta multi-layered magnetic nanoparticles, 300 nm in diameter, were deposited by ion-beam sputtering and their basic magnetic properties are characterized. These particles prepared by direct release with tunable magnetic properties and good engineering possibilities are ideal for a wide range of applications, including those specifically for biomedicine.

Performance enhancement of polymer electrolyte membrane fuel cell by employing line-patterned Nafion membrane

On purpose to enhance the performance of polymer electrolyte membrane fuel cell, micrometer- to nanometer-scale line patterns were introduced onto the electrolyte membrane using thermal imprint lithography. Single cell performance test and electrochemical analysis were conducted for the membrane electrode assembly containing patterned and pristine Nafion membranes. At a cell voltage of 0.6V, current density of the single cells containing micro-patterned membrane was larger by as large as 30% than that of the single cell containing untreated membrane. The performance enhancement appeared to result from the enlarged membrane/catalyst interfacial area. The performance of single cell containing nano-patterned membrane was lower than that containing pristine membrane, which is due to the poor interfacial contact between large catalyst particles and small gaps of the pattern.

Alumina Templates on Silicon Wafers with Hexagonally or Tetragonally Ordered Nanopore Arrays via Soft Lithography

E-mail: shin@snu.ac.kr Received October 15, 2011, Accepted November 3, 2011Due to the potential importance and usefulness, usage of highly ordered nanoporous anodized aluminum oxidecan be broadened in industry, when highly ordered anodized aluminum oxide can be placed on a substrate withcontrolled thickness. Here we report a facile route to highly ordered nanoporous alumina with the thickness ofhundreds-of-nanometer on a silicon wafer substrate. Hexagonally or tetragonally ordered nanoporous aluminacould be prepared by way of thermal imprinting, dry etching, and anodization. Adoption of reusable polymersoft molds enabled the control of the thickness of the highly ordered porous alumina. It also increasedreproducibility of imprinting process and reduced the expense for mold production and pattern generation. Asnanoporous alumina templates are mechanically and thermally stable, we expect that the simple and cost-effective fabrication through our method would be highly applicable in electronics industry. Key Words : Imprint lithography, AAO, Self-assembly, Anodization, ElectroplatingIntroductionDuring the recent decades, there has been an extensiverange of study for manufacturing of nanomaterials due to itspotential application to electronics, photonics, informationstorages, or filters.

Thermal imprint techniques for preparation of superhydrophobic polymer coatings

The self-cleaning property of superhydrophobic surfaces has raised considerable interest among scientists. Many preparation methods require complex and expensive fabrication techniques that cannot be used to directly modify substrates. We designed an imprint process using a micro/nano-structured mold to emboss several polymer coatings in order to introduce superhydrophobicity and still maintain their transparent/translucent properties. Polydimethylsiloxane (PDMS) elastic molds with various micro and nano features were fabricated using different grades of sandpaper as master molds, increasing the water contact angles (WCAs) on the PDMS surfaces from 120° (plain) to more than 170°. The PDMS molds were then applied to replicate some commercial polymer coatings at an elevated temperature of about 140°C to transfer the mold pattern to the coatings. The hydrophobic property of those polymer coatings was then enhanced to a WCA of greater than 150°. After fluoroalkylsilane deposition on the surfaces, the surfaces displayed an improved lipophobic property (hexadecane contact angle (HCA) went from 90° to 110°). The arbitrarily-rough surfaces displayed ‘sticky superhydrophobicity’ due to the water drops being adsorbed and not rolling off from the surfaces. Atomic force microscopy (AFM) was employed to measure the morphologies of imprinted coatings and also estimate the roughness, occupying polymer ratios, and critical CA to evaluate the wetting of sample surfaces. From the evaluation of surface properties, all the samples using sandpaper as templates exhibited stable Cassie or metastable wetting. The rougher surfaces provided some large protrusions to contact with and be immersed into water droplets, leading to high water penetration depth and thus stickiness. Additionally, the rougher the mold was, the greater the WCA was, but the transparency of the coating was reduced. However, the imprinted polymer surfaces could still maintain sufficient transparency for use in applications requiring preservation of the appearance of the underlying surface.

Silver front electrode grids for ITO-free all printed polymer solar cells with embedded and raised topographies, prepared by thermal imprint, flexographic and inkjet roll-to-roll processes

Semitransparent front electrodes for polymer solar cells, that are printable and roll-to-roll processable under ambient conditions using different approaches, are explored in this report. The excellent smoothness of indium-tin-oxide (ITO) electrodes has traditionally been believed to be difficult to achieve using printed front grids, as surface topographies accumulate when processing subsequent layers, leading to shunts between the top and bottom printed metallic electrodes. Here we demonstrate how aqueous nanoparticle based silver inks can be employed as printed front electrodes using several different roll-to-roll techniques. We thus compare hexagonal silver grids prepared using either roll-to-roll inkjet or roll-to-roll flexographic printing. Both inkjet and flexo grids present a raised topography and were found to perform differently due to only the conductivity of the obtained silver grid. The raised topographies were compared with a roll-to-roll thermally imprinted grid that was filled with silver in a roll-to-roll process, thus presenting an embedded topography. The embedded grid and the flexo grid were found to perform equally well, with the flexographic technique currently presenting the fastest processing and the lowest silver use, whereas the embedded grid presents the maximally achievable optical transparency and conductivity. Polymer solar cells were prepared in the same step, using roll-to-roll slot-die coating of zinc oxide as the electron transport layer, poly-3-hexylthiophene:phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) as the active layer and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the top electrode, along with a flat bed screen printed silver grid. The power conversion efficiency (PCE) obtained for large area devices (6 cm(2)) was 1.84%, 0.79% and 1.72%, respectively, for thermally imprinted, inkjet and flexographic silver grids, tested outside under the real sun. Central to all three approaches was that they employed environmentally friendly solvents, i.e. water based nanoparticle silver inks.

Intact triacylglycerol profiles of fats and meats via thermal imprinting easy ambient sonic-spray ionization mass spectrometry

Thermal imprinting (TI) on a paper surface, using minimal solvent amounts, followed by direct analysis of the triacylglycerols (TAG) content via easy ambient sonic-spray ionization mass spectrometry (EASI-MS) is shown to provide a fast protocol to analyze TAG in meats and fats. The technique is simple, fast and eco-friendly requiring no hydrolysis, derivatization or chromatographic separation. The entire TI-EASI-MS protocol is performed in a few minutes and with minimal sample handling and solvent consumption. The TAG profiles obtained via TI-EASI-MS are shown to be quite similar to those obtained using GC and MALDI-MS analyses, and the imprinting and mailing of the imprinted paper in a sealed plastic bag is proposed for remote TI-EASI-MS analysis of meat and fat samples.

Diffusion driven optofluidic dye lasers encapsulated into polymer chips

Lab-on-a-chip systems made of polymers are promising for the integration of active optical elements, enabling e.g. on-chip excitation of fluorescent markers or spectroscopy. In this work we present diffusion operation of tunable optofluidic dye lasers in a polymer foil. We demonstrate that these first order distributed feedback lasers can be operated for more than 90 min at a pulse repetition rate of 2 Hz without fluidic pumping. Ultra-high output pulse energies of more than 10 μJ and laser thresholds of 2 μJ are achieved for resonator lengths of 3 mm. By introducing comparatively large on-chip dye solution reservoirs, the required exchange of dye molecules is accomplished solely by diffusion. Polymer chips the size of a microscope cover slip (18 × 18 mm(2)) were fabricated in batches on a wafer using a commercially available polymer (TOPAS(®) Cyclic Olefin Copolymer). Thermal imprinting of micro- and nanoscale structures into 100 μm foils simultaneously defines photonic resonators, liquid-core waveguides, and fluidic reservoirs. Subsequently, the fluidic structures are sealed with another 220 μm foil by thermal bonding. Tunability of laser output wavelengths over a spectral range of 24 nm on a single chip is accomplished by varying the laser grating period in steps of 2 nm. Low-cost manufacturing suitable for mass production, wide laser tunability, ultra-high output pulse energies, and long operation times without external fluidic pumping make these on-chip lasers suitable for a wide range of lab-on-a-chip applications, e.g. on-chip spectroscopy, biosensing, excitation of fluorescent markers, or surface enhanced Raman spectroscopy (SERS).

Adaptation to hot climate and strategies to alleviate heat stress in livestock production

Despite many challenges faced by animal producers, including environmental problems, diseases, economic pressure, and feed availability, it is still predicted that animal production in developing countries will continue to sustain the future growth of the world's meat production. In these areas, livestock performance is generally lower than those obtained in Western Europe and North America. Although many factors can be involved, climatic factors are among the first and crucial limiting factors of the development of animal production in warm regions. In addition, global warming will further accentuate heat stress-related problems. The objective of this paper was to review the effective strategies to alleviate heat stress in the context of tropical livestock production systems. These strategies can be classified into three groups: those increasing feed intake or decreasing metabolic heat production, those enhancing heat-loss capacities, and those involving genetic selection for heat tolerance. Under heat stress, improved production should be possible through modifications of diet composition that either promotes a higher intake or compensates the low feed consumption. In addition, altering feeding management such as a change in feeding time and/or frequency, are efficient tools to avoid excessive heat load and improve survival rate, especially in poultry. Methods to enhance heat exchange between the environment and the animal and those changing the environment to prevent or limit heat stress can be used to improve performance under hot climatic conditions. Although differences in thermal tolerance exist between livestock species (ruminants > monogastrics), there are also large differences between breeds of a species and within each breed. Consequently, the opportunity may exist to improve thermal tolerance of the animals using genetic tools. However, further research is required to quantify the genetic antagonism between adaptation and production traits to evaluate the potential selection response. With the development of molecular biotechnologies, new opportunities are available to characterize gene expression and identify key cellular responses to heat stress. These new tools will enable scientists to improve the accuracy and the efficiency of selection for heat tolerance. Epigenetic regulation of gene expression and thermal imprinting of the genome could also be an efficient method to improve thermal tolerance. Such techniques (e.g. perinatal heat acclimation) are currently being experimented in chicken.