Flexible Stamp Research Articles

Fabrication of high-performance lens arrays for micro-concentrator photovoltaics using ultraviolet imprinting

Micro-concentrator photovoltaics (micro-CPV) is a cutting-edge CPV approach aimed at increasing the efficiency and reducing the cost and carbon footprint of solar electricity by downscaling concentrator solar cells and optics. The reduced size of micro-CPV provides several advantages over conventional CPV, including shorter optical paths and lower temperature and resistive losses in the cell, resulting in higher electrical efficiencies. This may increase the energy yield per area compared to conventional CPV or silicon modules. Cost reduction is achieved through material savings and the use of continuous manufacturing methods enabled by the tiny size of cells and optics, such as roll-to-roll (R2R) and roll-to-plate (R2P) ultraviolet (UV) imprinting for optics production. However, adapting these processes to large-area arrays of Fresnel micro-lenses with no wasted areas and high efficiency remains a challenge. In this study, we present a comprehensive methodology for the development of micro-CPV optics with full area coverage—from design and mastering to up-scaling, tooling, and replication. The methodology involves designing a non-rotationally symmetric elementary insert tailored to ultraviolet imprinting. Crucially, multiple inserts are originated via precision machining and recombined to form a single array master mold without wasted areas. The master is then replicated into a flexible working stamp for UV imprinting of Fresnel lens arrays, utilizing different UV curable materials. The functional characterization of the lenses demonstrates an optical efficiency of 80% at 178X under collimated white light, representing the highest effective concentration achieved using UV-imprinted Fresnel lenses. Furthermore, initial reliability tests confirm the absence of degradation during thermal cycling or outdoor exposure. This methodology paves the way for continuous high-throughput manufacturing of micro-lens arrays using R2R or R2P methods, presenting a significant step forward in micro-CPV.

Elastohydrodynamic lubrication of soft-layered rollers and tensioned webs in roll-to-plate nanoimprinting

This work presents the development of a numerical model for the elastohydrodynamic lubrication of roll-to-plate nanoimprinting with flexible stamps. Roll-to-plate nanoimprinting is a manufacturing method to replicate micro- and nanotextures on large-area substrates with ultraviolet-curable resins. The roller is equipped with a relatively soft elastomeric layer, which elastically deforms during the imprint process. The elastic deformation is described by linear elasticity theory. It is coupled to the pressure build-up in the liquid resin film, which is described by lubrication theory. The flexible stamp, which is treated as a tensioned web, is pre-tensioned around the roller. The elastic deformation of the tensioned web is described by the large-deflection bending of thin plates equations, considering its non-negligible bending stiffness. A Fischer–Burmeister complementarity condition captures the contact mechanics between the tensioned web and the roller. The governing equations combine in a coupled elastohydrodynamic lubrication model, which is fully described by a set of non-dimensional numbers. These are used in a parameter study to investigate the effect on the pressure and film height distributions. It is shown that the bending stiffness of the tensioned web results in an additional hydrodynamic pressure peak and corresponding minimum in the film height, near the inlet of the roller contact. An increase of the bending stiffness corresponds to a decrease in film height. The numerical results are compared with benchmarks from literature and experimentally validated with layer height measurements from flat layer imprints. Good agreement is found between the numerical and experimental results.

Fabrication of Ideally Ordered Anodic Porous Alumina on Glass Substrates by Stamping Process Using Flexible Stamps

Establishing a process to fabricate anodic porous alumina with an ordered array of uniform-sized pores on a substrate is an important challenge of fabricating various functional devices. In this study, ordered anodic porous alumina was fabricated on glass substrates by a stamping process using a flexible stamp. This process enables the formation of a resist mask at a low pressure on the sample surface to pattern the starting point for pore generation during anodization. Therefore, it is possible to pattern samples without breaking the glass substrate, and subsequent anodization can generate ordered anodic porous alumina on the substrate. The stamping process using flexible stamps can be applied regardless of the roughness of the Al film on a substrate because the stamp deforms according to the Al surface shape, even when there is a protruding structure on the Al surface. In addition, it was shown that this process could be used to form ordered anodic porous alumina even on Al surfaces with three-dimensional curvatures, such as convex lenses.

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.

Simulating the Residual Layer Thickness in Roll-to-Plate Nanoimprinting with Tensioned Webs.

Roll-to-plate nanoimprinting with flexible stamps is a fabrication method to pattern large-area substrates with micro- and nanotextures. The imprint consists of the preferred texture on top of a residual layer, of which the thickness and uniformity is critical for many applications. In this work, a numerical model is developed to predict the residual layer thickness (RLT) as a function of the imprint parameters. The model is based on elastohydrodynamic lubrication (EHL) theory, which combines lubrication theory for the pressure build-up in the resin film, with linear elasticity theory for the elastic deformation of the roller material. The model is extended with inextensible cylindrical shell theory to capture the effect of the flexible stamp, which is treated as a tensioned web. The results show that an increase in the tension of the web increases the effective stiffness of the roller, resulting in a reduction in the RLT. The numerical results are validated with layer height measurements from flat layer imprints. It is shown that the simulated minimum layer height corresponds very well with the experimental results for a wide range of resin viscosities, imprint velocities, and imprint loads.

Large-area surface-patterned Li metal anodes fabricated using large, flexible patterning stamps for Li metal secondary batteries

The use of surface-patterned lithium (Li) metal has been proposed as a promising strategy for inhibiting the formation of Li dendrites during repeated Li plating/stripping processes. Nevertheless, the conventional Li metal patterning process is complex, expensive, incompatible with mass production, and incapable of producing finely controlled patterns on the Li metal surface. A large, flexible patterning stamp capable of large-area patterns is developed using a silicon (Si) wafer-based chemical etching process, and its effect on the electrochemical performance of a Li metal anode is investigated. The newly developed stamps have 5,000% larger patterning area compared to the conventional stainless-steel stamps. Furthermore, when compared to conventional surface-patterned Li metal fabricated with conventional stainless-steel stamps (SP-LM), the surface-patterned Li metal fabricated with large and flexible patterning stamps (LAP-LM) demonstrates improved electrochemical performance and stable morphological properties. As a result, the LAP-LM is able to retain up to 85.2% of its initial discharge capacity (85.9 mAh g −1 ) after 200 cycles at 3C (3.96 mA cm −2 ), while the SP-LM shows a severe capacity decay after 150 cycles (94.0 mAh g −1 and 13.0 mAh g −1 at the 150th cycle and 200th cycle, respectively). • A large, flexible patterning stamp capable of large-area patterns was developed. • The newly developed stamps achieved 5,000% larger patterning area for Li metal. • The new large-area stamp produced stable morphological properties for Li metal. • The new large-area stamp showed significantly improved electrochemical properties.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

AFM-Based nanofabrication and quality inspection of three-dimensional nanotemplates for soft lithography

Soft lithography is one of the most promising nanofabrication technologies for scalable nanomanufacturing. It uses flexible molds or stamps to fabricate extremely high-resolution nanopatterns in a low cost and rapid fashion on both flat and curved surfaces. Although two-dimensional soft lithography has been widely investigated and applied for some applications, there is a lack of quantitative research on using nanometer scale 3D patterns fabricated by atomic force microscope (AFM) for 3D soft lithography. In this paper, we demonstrated the effectiveness of ultrasonic vibration assisted AFM-based nanomachining in fabricating 3D master nanotemplates for 3D soft lithography. We successfully applied Polydimethylsiloxane (PDMS) daughter molds, casted from a nanometer scale 3D master template, using a soft lithography technique solvent-assisted micromolding. In addition, we investigated the quality and reusability of the master nanotemplates, and the quality of the PDMS patterns by quantifying the volume differences using an automatic imaging registration algorithm. Results show that there are zero or little PDMS residuals in the polymethyl methacrylate (PMMA) master nanotemplate with a simple contour for the first and second daughter mold casting, but 7% residuals for the third casting. And a more complex master nanotemplate shows slightly higher residual levels.

Simulation Analysis of Control System of Synchronous Flexible Stamping Device Based on AMESim

Simulation Analysis of Control System of Synchronous Flexible Stamping Device Based on AMESim

Fabrication of High Aspect Ratio Micro-Structures with Superhydrophobic and Oleophobic Properties by Using Large-Area Roll-to-Plate Nanoimprint Lithography.

Bio-inspired surfaces with superamphiphobic properties are well known as effective candidates for antifouling technology. However, the limitation of large-area mastering, patterning and pattern collapsing upon physical contact are the bottleneck for practical utilization in marine and medical applications. In this study, a roll-to-plate nanoimprint lithography (R2P NIL) process using Morphotonics’ automated Portis NIL600 tool was used to replicate high aspect ratio (5.0) micro-structures via reusable intermediate flexible stamps that were fabricated from silicon master molds. Two types of Morphotonics’ in-house UV-curable resins were used to replicate a micro-pillar (PIL) and circular rings with eight stripe supporters (C-RESS) micro-structure onto polycarbonate (PC) and polyethylene terephthalate (PET) foil substrates. The pattern quality and surface wettability was compared to a conventional polydimethylsiloxane (PDMS) soft lithography process. It was found that the heights of the R2P NIL replicated PIL and C-RESS patterns deviated less than 6% and 5% from the pattern design, respectively. Moreover, the surface wettability of the imprinted PIL and C-RESS patterns was found to be superhydro- and oleophobic and hydro- and oleophobic, respectively, with good robustness for the C-RESS micro-structure. Therefore, the R2P NIL process is expected to be a promising method to fabricate robust C-RESS micro-structures for large-scale anti-biofouling application.

Numerical modeling of the problem of indentation of elastic and elastic-plastic massive bodies

The paper considers the dynamic pressing-in indenter in a plane elastic-plastic statement. A general statement of the problem of a rigid body under a flexible stamp pressing-in is given; it allows considering the elastic and plastic properties of the material and the process of the stress-strain state formation. The problem of a flexible stamp action on the outer boundary is realized using the finite-difference method. By solving the problem for a quadrangular profile, the distribution of the stress-strain state of the body under consideration and the formation of stresses over time are shown. The effect of plastic properties of the material on the stress state of the body is assessed by comparative analysis.

Control and Patterning of Various Hydrophobic Surfaces: In-situ Modification Realized by Flexible Atmospheric Plasma Stamp Technique

Wettability plays a vital role in fundamental researches and practical applications. Wettability control and patterning have been widely studied in various fields. Although researches have grown rapidly, the methods are still restricted by limitations including complicated processes, high equipment requirements and shortage of methods to treat complex surfaces. Here we report a simple, low cost, array-based wettability control and patterning method via in-situ modification by flexible plasma stamp. Wettability control and patterning on surfaces of superhydrophobic aluminum, superhydrophobic PDMS and silicon, even plant leaf and fruit are achieved. The relationships between the wettability and the treatment time are investigated. We elucidate that the wetting states can also be reversible. The surface modification mechanism of in-situ plasma treatment is further investigated. Utilizing the step by step treatment, gradient and arbitrary wettability patterning on surfaces have been obtained. Notably, the patterned wettability on the inner surface of a tube has been realized, which has never been reported. Finally, in-situ wettability patterning is applied to achieve microfluidics channels on the inner surface of superhydrophobic tube. This work will bring new insights into the study of wetting field and stimulate more applications on wettability control and patterning.

Development of Micropatterns on Curved Surfaces Using Two-Step Ultrasonic Forming.

Nanoimprint lithography (NIL) is a micro/nanoscale patterning technology on thermoplastic polymer films, and has been widely used to fabricate functional micro/nanoscale patterns. NIL was also used to develop micro/nanoscale patterns on curved surfaces by employing flexible polymer stamps or micropatterned metal molds with macroscopic curvatures. In this study, two-step ultrasonic forming was used to develop micropatterns on a curved surface out of a flat metal stamp, by connecting ultrasonic imprinting and stretching processes. Ultrasonic imprinting was used to replicate functional micropatterns on a flat polymer film, using a flat ultrasonic horn and micropatterned metal stamps with prism and dot micropatterns. An ultrasonic stretching process was then used to form a curvature on the patterned film using a curved ultrasonic horn and a soft mold insert, to avoid damage to the pre-developed micropatterns. The ultrasonic horn was designed to have three different tip radii, and the resulting forming depth and curvature formation were investigated experimentally. As a result, three different curved surfaces containing two different micropatterns were obtained. The developed curved films containing micropatterns were then evaluated optically, and showed different optical diffusion and illumination characteristics according to the film curvature and micropattern type. These results indicate that the proposed technology can extend the functionality of conventional micropatterned products by imposing appropriate curvatures.

18.4: Nanoimprinting Technology Applied on Display Devices

This paper demonstrates a roll‐to‐roll nanoimprinting process for the mass production of structured films used on display devices. Nanoimprint lithography is one of the novel techniques for nanofabrication. It is especially suitable for mass production due to its high throughput with comparable resolutions to photo‐ and e‐beam lithography. In this work, it is applied to the production of e‐ink film that has “microcups” to fill in e‐ink pigments. The dimensions of the microcups could be adjusted by designing stamp geometries. With several flexible stamps joint together to a sleeve on the roller, this nanoimprinting process could be extended to a roll‐to‐roll equipment. For mass production of structured films, the pattern transfer quality should be consistent with very low defect rates. This is to be achieved by improving the materials for resist and stamp, the nanoimprinting conditions and sleeve fabrication, etc.

Layer-by-Layer Assembly of Three-Dimensional Optical Functional Nanostructures.

Nanotransfer printing (nTP) technology can generate highly functional three-dimensional (3D) nanostructures in a low-cost and high-throughput fashion. Nevertheless, the fabrication yield and quality of the transferred nanostructures are often limited by the merging of the surface patterns of replica stamps during transfer printing. Here, an nTP technology was developed to fabricate large-area and crack-free 3D multilayer nanostructures. Instead of directly depositing materials on the patterned flexible stamp in conventional nTPs, we transferred the nanostructures straightforwardly onto an attached polydimethylsiloxane slab by removing a sacrificial water-soluble poly(acrylic acid) film, which can avoid the cracking of metal film and the failures of printing nanostructures onto target substrates. Based on this approach, subwavelength-thick polarization rotators working at infrared wavelengths were fabricated. Excellent performance of linear polarization rotation over a broadband was realized. This nTP approach could complement existing fabrication techniques and benefit the development of various functional nanostructures with complex multilayer hierarchies.

Control of thermal radiation in metal hole array structures formed by anisotropic anodic etching of Al

An efficient preparation process for Al hole array structures emitting wavelength-selective thermal radiation that is based on the anisotropic anodic etching of Al was demonstrated. The formation of an ordered hole array was achieved by a masking process prior to the anodic etching. The present process allows the preparation of large samples because the masking of the Al foil has a high throughput owing to the simple printing process using a flexible stamp. The thermal radiation properties of the Al hole array could be controlled by adjusting the depth and aperture size of the holes.

Automated wet compression moulding of load-path optimised TFP preforms with low cycle times

This paper describes the implementation of a wet compression moulding process using TFP preforms. The fibre paths of these preforms are derived from the actual load paths which are numerically analysed taking into account multiple loading conditions. Preforms are created using a Tajima embroidery machine. This approach allows an innovative way of textile forming in the following process step. This step is based on wet compression moulding using a flexible stamp made of silicone rubber. The wet compression moulding is implemented in the draping test rig of the Institute of Aircraft Design, University of Stuttgart. It is able to produce structures with varying thicknesses which therefore stand out for their high lightweight properties.

Stability of flexible composite stamps with thermal nanoimprint

Flexible composite stamps are commonly used in low pressure and low temperature processes, e.g., UV-assisted nanoimprint, as they provide a good conformal contact between stamp and substrate. The composite stamps investigated here consist of two layers, a thin hard top layer to enable stable nanometre-scaled structures with high aspect ratio and a soft backplane to ensure conformal contact. Stamps with two different material combinations were investigated, OrmoStamp /PDMS (polydimethylsiloxane) and h-PUA/s-PUA (polyurethane acrylate). The stability of both composite stamps was tested under harsh imprint conditions in a process at elevated temperature and pressure. Temperature and pressure loading results in strain in the top layer and may lead to break when the tensile strength is exceeded. To vary the stress level in the top layer, two different stamp designs were investigated, one with a thin backplane and one with a thicker backplane. The experimental results clearly show that a thin stamp (low stress level) is more stable than a thick stamp. Moreover, composite stamps are also suitable for a thermal imprint process when temperature and pressure remain limited.

Overlay accuracy limitations of soft stamp UV nanoimprint lithography and circumvention strategies for device applications

In this work multilevel pattering capabilities of Substrate Conformal Imprint Lithography (SCIL) have been explored. A mix & match approach combining the high throughput of nanoimprint lithography with the excellent overlay accuracy of electron beam lithography (EBL) has been exploited to fabricate nanoscale devices.An EBL system has also been utilized as a benchmarking tool to measure both stamp distortions and alignment precision of this mix & match approach. By aligning the EBL system to 20 mm × 20 mm and 8 mm × 8 mm cells to compensate pattern distortions of order of 3 μm over 6 in. wafer area, overlay accuracy better than 1.2 μm has been demonstrated. This result can partially be attributed to the flexible SCIL stamp which compensates deformations caused by the presence of particles which would otherwise significantly reduce the alignment precision.

Multiphysics modelling of manufacturing processes: A review

Numerical modelling is increasingly supporting the analysis and optimization of manufacturing processes in the production industry. Even if being mostly applied to multistep processes, single process steps may be so complex by nature that the needed models to describe them must include multiphysics. On the other hand, processes which inherently may seem multiphysical by nature might sometimes be modelled by considerably simpler models if the problem at hand can be somehow adequately simplified. In the present article, examples of this will be presented. The cases are chosen with the aim of showing the diversity in the field of modelling of manufacturing processes as regards process, materials, generic disciplines as well as length scales: (1) modelling of tape casting for thin ceramic layers, (2) modelling the flow of polymers in extrusion, (3) modelling the deformation process of flexible stamps for nanoimprint lithography, (4) modelling manufacturing of composite parts and (5) modelling the selective la...