Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS)

In this present work, we aim to improve the hydrophobicity of a polydimethylsiloxane (PDMS) surface. Various heights of 3D PDMS micropillars were fabricated via grayscale photolithography, and improved wettability was investigated. Two approaches of PDMS replication were demonstrated, both using a single master mold to obtain the micropillar arrays. The different heights of fabricated PDMS micropillars were characterized by scanning electron microscopy (SEM) and a surface profiler. The surface hydrophobicity was characterized by measuring the water contact angles. The fabrication of PDMS micropillar arrays was shown to be effective in modifying the contact angles of pure water droplets with the highest 157.3-degree water contact angle achieved by implementing a single mask grayscale lithography technique.

A Study on Fabrication Process of Gold Microdisk Arrays by the Direct Imprinting Method Using a PET Film Mold

In this study, an efficient nanofabrication process of metal microdisk arrays using direct imprinting was developed. This process was comprised of three steps; sputter etching on the quartz glass substrate, gold thin film deposition on an etched surface of a substrate, and transfer imprinting using a polyethylene terephthalate (PET) film mold on the Au thin film. A new idea to utilize a PET film mold for disk patterning by the nano transfer imprinting was examined. The PET film mold was prepared by thermally embossing the pillar pattern of a master mold on the PET film. The master mold was prepared from a silicon wafer. The PET film mold was used for transfer imprinting on a metal film deposited on a quartz substrate. The experimental results revealed that the PET film mold can effectively form gold micro-disk arrays on the Au film despite the PET film mold being softer than the Au film. This method can control the distribution and orientation of the nano-arrays on the disk. The plasmonic properties of the gold micro-disk arrays are studied and the absorbance spectrum exhibit depends on the distribution and orientation of gold micro-disk patterns. The nano-transfer imprinting technique is useful for fabricating metallic microdisk arrays on substrate as a plasmonic device.

Research of replication accuracy in some elastomer materials with different Young’s modulus

In this work we observed three different elastomeric materials with different Young’s modulus: Silastic T-4, Plat Set 30, and Lasil-C. Their usage makes it possible to overlap the range of rigidity obtained for Silgard under different curing conditions, without high temperatures and long curing time. The results obtained during the replicas formation using a brass master mold with micro-sized structures for these elastomers were presented. The quality of the replication in materials with low Young’s modulus turned out to be better than for Silgard, and for the hard Silastic T-4 – it is comparable to it.

Study On Cutting Force and Tool Vibration For Optimization of Depth of Cut in Ultra-Precision Planing Process of the Master Mold For Retro-Reflection Film

The micro-triangular pyramid patterns are widely used in advanced optical components with retro-reflection characteristic. The performance of the retro-reflection is affected by an effective area, and it can be maximized by machined surface without defects such as edge blunt, burr, surface roughen. The ultra-precision planing process is well-known that can fabricate superior surface when the depth of cut (DOC) is applied to minimum depth above the critical value to prevent the size effect. However, it was very difficult to determine a DOC without comparing of quality of machined surfaces through the ultra-high magnification measuring instrument such as SEM. In this study, the critical DOC which is key parameter was analyzed using cutting force and tool vibration signals. These signals were converted to specific cutting resistance and frequency spectrum, respectively. As a result, spectrum frequency signal was more effective and accurate than specific cutting energy, and critical DOC was determined to 1µm. This proposed process was validated by comparing the quality variation of the machined surfaces with analysis result based on cutting signals. Finally, a master mold with area of 250mm2 for fabrication of the retro-reflection film was manufactured by applying optimized DOC, and the retro-reflection film was fabricated by press molding process. This retro-reflection film was clearly recognized at a distance of 100m from light source with low power.

Shrinkage-Considered Mold Design for Improvement of Micro/Nano-Structured Optical Element Performance

Polymer shrinkage in nano-imprint lithography (NIL) is one of the critical issues that must be considered in order to produce a quality product. Especially, this condition should be considered during the manufacture of optical elements, because micro/nano-structured optical elements should be controlled to fit the desired shape in order to achieve the intended optical performance. In this paper, during NIL, we characterized the shrinkage of polymeric resin on micro lens array (MLA), which is one of the representative micro/nano-structured optical elements. The curvature shape and optical performance of MLA were measured to check the shrinkage tendency during the process. The master mold of MLA was generated by the two-photon polymerization (2PP) additive manufacturing method, and the tested samples were replicated from the master mold with NIL. Several types of resin were adjusted to prepare the specimens, and the shrinkage effects in each case were compared. The shrinkage showed different trends based on the NIL materials and MLA shapes. These characterizations can be applied to compensate for the MLA design, and the desired performance of MLA products can be achieved with a corrected master mold.

Fabrication of Flexible Interposer Using Printing Method

In recent years, flexible electronic devices such as printed electronics are gathering attention. To make flexible connect between one circuit device and another circuit device, interposer is necessary. However, most of conventional interposers are not flexible, because there are made of silicon or glass substrate. To solve this problem, we have been developed fabrication process of flexible interposer. Master mold was fabricated by photolithography process. First, SU-8 resist was coated on silicon substrate with 5μm thickness. Then, photolithography process was carried out to SU-8 resist. After development, pillar shape master molds with diameters of 10 or 20 μm were obtained. After release coating of master molds, hole patterns for vias were transferred by UV nanoimprint lithography. The obtained hole patterns were diameter of 10 μm or 20 μm, and pitch of 21.0 μm and 40.1 μm, respectively. Next, these holes were filled with silver ink by roll press method. Then, sintering process was carried out to evaporate of solvent of silver ink. After that, flexible interposer was obtained. As a result, we have been succeeded in filling the holes array with silver ink. Obtained interposer vias, which were silver region, were 8.2 μm diameter and 3.3 μm height, or 20.3 μm diameter and 5.3 μm height for 10 mm square size.

Fabrication of an Antireflective Structure on a Lenticular Lens

In our previous study, various lenses with antireflective structures (ARSs) were fabricated by ultraviolet nanoimprint lithography (UV-NIL) and a thin flexible replica mold, which had ARSs. However, in the case of a lens with a large uneven surface such a lenticular lens, it is difficult to transfer an ARS. In this paper, the improvement of the transfer process for a lenticular lens with an ARS is investigated. A master mold of ARSs was fabricated by irradiating glassy carbon with an oxygen ion beam. A flexible replica mold with ARSs was transferred from the ARS master mold by UV-NIL. In this case, the thickness of the flexible replica mold was 210 μm. A lenticular lens with the reverse shape was also transferred from the original lenticular lens by UV-NIL. To add an ARS to the lenticular lens with the reverse shape, it was covered with a flexible replica mold coated with a UV-curable resin. In addition, the lenticular lens was placed on the back side of the flexible replica mold to contact the lens curve properly. Afterwards, the setting samples were pressed with a roll press and simultaneously UV-cured. After this roll press method was carried out, a lenticular lens with the reverse shape and an ARS was obtained. Using this mold, a lenticular lens with an ARS was replicated by UV-NIL and the reverse-shape mold with an ARS.

Improvement of Transfer Durability of a Pillar-Shaped Release-Agent-Free Replica Mold in Ultraviolet Nanoimprint Lithography

Ultraviolet nanoimprint lithography (UV-NIL) can be used to fabricate nanoscale patterns with high throughput. It is expected to serve as a low-cost technique for the production of items in large numbers. However, master molds for UV-NIL are expensive and laborious to produce, and there are problems associated with the deterioration of the master mold and damage to its nanopattern due to adhesion of the UV-curable resin. Consequently, the UV-curable resin has to combine low-viscosity characteristics for coatability with an antisticking property. Coating a master mold with a release layer is important in preventing damage to the master mold or adhesion between the mold and the UV-curable resin. However, the released layer deteriorates as the master mold is repeatedly used to fabricate nanopatterns. By contrast, the use of a replica mold is a valuable technique for preventing the deterioration of the master mold, and there have been several studies on the fabrication of replicas of master molds with the use of UV-curable resins. In many cases, the fabrication of nanopatterns with replica molds requires the use of a release agent. In a previous study, we developed a material for replica molds that does not require a release agent. This material consisted of a UV-curable resin with an antifouling effect that was prepared from cationically polymerizable UV-curable and epoxy-modified fluorinated resins. With the use of this material, replica molds with patterns of pillars or holes were fabricated with UV-NIL. The lifetime of the mold with the nanopattern of pillars was shorter than that with holes. In addition, the replica mold with the pillar-shaped nanopattern had numerous defects and allowed adhesion of the transfer resin after repeated efforts. Herein, we describe an improved release-agent-free hard replica mold. We transferred large numbers of nanopatterns of pillars from the replica mold, and evaluated the error rate and contact angle of our improved release-agent-free hard replica mold. The resulting release-agent-free replica mold with a nanopattern of pillars was capable of transferring up to 1000 sequential imprints. In addition, to improve the release properties of the transfer resin, we included an additive to the transfer resin that contained a reactive fluorinated material. This material improved the release properties of the transfer resin and mitigated the deterioration of the contact angle and increase in the error rate.

A Microlens Array with Different Focal Lengths Fabricated by Roll-To-Roll UV Lithography

A simple, highly efficient and low cost roll-to-roll (R2R) UV imprinting lithography facility was achieved for fabricating micro-structures. Firstly, a novel microlens array with focuses distributed on a curved surface was designed and analyzed by an optical software ZEMAX. Then an ultra-precision diamond machine was applied to generate the freeform microlens array features on the master mold, and a belt-type polydimethylsiloxane (PDMS) mold with a microlens array pattern was prepared from the machined master mold. The R2R process was employed to replicate the microlens arrays, followed by an evaluation of their profiles and optical properties. Our experiments demonstrate that the applied method is reliable and efficient for producing the polymeric microlens arrays.