The specific metallic blue of Morpho butterfly is structural color having high reflectivity produced by an interference effect. However, a single color in too wide angular range (> ± 40° from the normal) contradicts the optical interference. After we have proven the principle of the mystery by reproducing the specific nanostructures of their scales, we found the artificial Morpho-color to serve wide applications, because it can provide a brilliant single color in wide angular range with high reflectivity without chemical pigment, which is resistant to fading caused by chemical change for long time. We have developed various techniques for applications of the specific color, such as mass-production processes, simulation and control of its optical properties, and substrate-free color materials. One of the remaining key issues is to produce the large-area flexible film, because the flexible film was limited to small area, and large-area Morpho-color material have long been accompanied with the solid substrate, which has limited the variety of applications. By combining these processes to fabricate the small flexible film and solid large-area Morpho-color material, we developed a simple process to mass-fabricate the large-area flexible Morpho-color material. The method was quite simple, whereas this process needed to develop a flexible mold for nano-imprint, because the large-area nano-imprint gives serious stress and damage to the solid mold. Finally, by developing a process to produce a large-area flexible mold, we have successfully replicated the flexible large-area Morpho-color film, which can be realized by immersion into hot water. The fabricated large-area Morpho-color film was flexible, resistant against stress and strain, and found to maintain the original optical properties of Morpho-color. This process will expand the applications of the Morpho-color, which enables the coloring without limit of shape and area.
In order to minimize the defects formation when using nano-imprinting process we investigated the efforts
applied on the resist during the release of the template. Lift-off release has already been characterized accurately
but for peeling studies are still lacking. However from experimental results it has been observed that peeling
offers better performances when it comes to limit the defects. Using finite element method we simulated
imprinting on PMMA resist by a silicon template and extracted the maximal release force and the induced stress
in the resist in regard to the template stiffness and the number of patterns imprinted. Compared to lift-off
method we found that maximal release force was much lower and decided to investigate the induced stress
behaviour. We observed that using peeling the maximal release force doesn’t increase linearly in function of the
template size as in lift-off but instead saturates beyond a certain template size, that saturating point depending
on the template stiffness, a low stiffness meaning a lower maximal release force. However we found an opposite
trend when we extracted the induced stress in the resist which decreases as the template stiffness increases,
theoretically resulting in fewer defects. This seems to be due to the smaller bending of the more rigid template
that put less constraint on the imprinted features during the releasing and thus avoid breaking them in the
process. Therefore according to these results, to minimize defects when peeling release method is employed we
should use a highly rigid template.
Novel photo-lithography is newly proposed named built-in lens mask lithography. The method emulates optical propagation plane in exposure system using binary transmittance and phase mask instead of projection lens. The performance of the built-in lens mask lithography is studied by numerical simulation and experimental study using conventional proximity exposure system. The result shows resolution enhancement in deep focus plane.
Cost effective micro lithography tool is demanded for fine micro devices. However, resolution of a conventional proximity exposure system is not sufficient below several micron feature size for deep focus depth. On the other hand, a reduction projection system is sufficient to resolve it but the cost of the tool is too much high compared to proximity exposure systems. To enhance the resolution of photolithography, there has been proposed a number of novel methods beside shorting of wave length. Some of them are utilized in current advanced lithography systems, for example, the immersion lithography<sup>1</sup> enhances effective NA and the phase shift mask<sup>2</sup> improves optical transmittance function. However, those advanced technology is mainly focused on improvement for advanced projection exposure systems for ultra-fine lithography. On the other hand, coherence holography pattering is recently proposed and expected for 3-dimentional pattering<sup>3-5</sup>. Also, Talbot lithography<sup>6-8</sup> is studied for periodical micro and nano pattering. Those novels pattering are based on wave propagation due to optical diffraction without using expensive optical lens systems. In this paper we newly propose novel optical lithography using built-in lens mask to enhance resolution and focus depth in conventional proximity exposure system for micro lithographic application without lens systems. The performance is confirmed by simulation and experimental works.
Molecular dynamics simulation is performed to study the yield stress and fracture mechanism of single crystalline silicon mold with notch-defect structures. From the stress distribution, it is found that the stress is concentrated near the notch defect and the notch acts as a trigger of the crucial mold fracture. The yield stress with a nano scale notch on the mold sidewall deteriorates more than 7.5 % compared to a defect-free mold. It is found that a surface damage such as notch defect is significant for strength deterioration of the mold. This result shows that the surface defects on the sidewall, which could be induced during the mold fabrication process such as dry etching process, causes serious failure.
Two types of dimethacrylate which have hemiacetal ester moiety in a molecule were synthesized from difunctional
vinyl ethers and methacrylic acid. UV curing of the monomers and photo-induced degradation of the UV cured resins
were investigated. On UV irradiation at 365 nm under N<sub>2</sub> atmosphere, these dimethacrylates containing
2,2-dimethoxy-2-phenylacetophenone and triphenylsulfonium triflate became insoluble in methanol. The UV cured
resins degraded if acids were generated in the system. Present resins were applied to make a plastic replica of mold
for imprint lithography and the plastic replica was prepared in good form. The effect of imprint conditions on volume
shrinkage of methacrylates was investigated. Dimethacrylate that has adamantyl unit showed a low-shrinkage
Hybrid patterning by thermal and UV nanoimprint lithography is newly proposed to fabricate micro-nano mixture
structures. The SU-8 resist is thermally imprinted using the quartz mold, which has fine nano structures and micro Cr
blank patterns. After the thermal nanoimprint, UV is exposed keeping the mold on the resist through the mold. Then,
the mold is detached and the resist is developed to fabricate micro structures. Using this process, micro gratings having
40 μm in width and 20 μm in depth nano dots pattern, which has 200 nm feature size is successfully demonstrated.
The brilliant metallic blue in wings of <i>Morpho</i> butterflies has a mysterious feature. The blue luster is produced from the
butterfly's scale, which does not contain a blue pigment at all. The origin of the coloration is then attributed to a
microscopic structure that can also explain its high reflectivity. However, its optical characteristics on the scattered
wavelength contradicts obviously the grating or multilayer, because it appears blue from wide angle. The mystery of the
lack of multi-coloration has recently been explained using a model with a peculiar optical structure, and experimentally
proven by fabricating the optical film by controlling the parameters in nanoscale. The reproduced <i>Morpho</i>-blue was
found to be important from viewpoint of a wide variety of applications. However, the fabrication process of the nano-
structure is too costly due to conventional lithography method. To solve the problem, nano-casting lithography (NCL)
was newly applied using UV curable polymer to replicate the nanostructure and improve heat-resistance for the
following process of deposition. After fabrication of the nano-patterned polymer structure by the NCL, TiO<sub>2</sub> and SiO<sub>2</sub>
layers were deposited and the <i>Morpho</i>-blue structure was successfully replicated in low cost. The reflective characteristic
of the replicated structure was found to reproduce the basic properties of the natural <i>Morpho</i>-blue, as well as the
originally fabricated <i>Morpho</i>-blue.
Anti-reflection structure having sharpened corn shape is reproduced by nano casting method. Firstly, the master
quartz structure is transferred to UV curable resin to fabricate the replicated mold of the master structure using nano
casting method. Next, the anti-reflection structure is transferred to PMMA film using the replicated mold by the nano
casting method. To avoid the defects at the mold releasing, thin sacrifice layer is coated on the replicated mold.
Also, the molecular weight of the PMMA is optimized to improve the yield of the releasing process and transferred
pattern shape. Fine anti-reflection structure is fabricated by the proposed process using the nano casting method
without damages to the master structure.
SU-8 (Kayaku Microchem Co., Ltd.) provides well-defined resist profiles with high aspect ratios, and is also suitable for use as a permanent resist. SU-8 has been widely used for many years in the MEMS (Micro Electro Mechanical System), IC package (bump, insulator, encapsulation), micro fluid (inkjet, micro reactor, biochips), and optical device (waveguide, optical switch) fields. SU-8 is a chemically amplified negative resist based on epoxy resin. This resist generates a strong acid during exposure, and PEB (Post Exposure Baking) induces the crosslinking reaction of the resin with the acid working as a catalyst to insolubilize the resist. In our study, we sought to investigate the potential application of SU-8
3000NIL, the most commonly used resist for the MEMS process, to imprint lithography. The results we obtained indicate that SU-8 3000NIL can indeed be applied to imprint lithography after optimizing
process conditions for imprinting.
Numerous methods are available for lithography below the 100 nm node scale, including F2, 193 nm immersion, EB, EUV, and imprint lithography. Among these methods, imprint lithography has attracted significant attention because it does not require expensive exposure equipment. Imprint lithography can be performed by one of two primary methods: the thermal method or the UV curing method. In thermal imprinting, the resin is softened above Tg before being formed by a mold. In UV imprinting, a transparent mold is applied to a liquid resin, which is then exposed to UV light for curing. Thermal imprinting requires a pressure of 10 MPa and consumes throughput (to increase and reduce the temperature) time ["requires time for throughput (i.e., time required to increase and reduce temperatures)"]. In contrast, UV imprinting does not require high pressure, since the resin is basically a viscous liquid and soft enough to be deformed. However, since the resin is in liquid form, the UV imprinting process is sensitive to the flatness of the substrate and mold. Problems of non-uniformity (i.e., interference patterns) have been noted in residual film distribution. In response, we developed what we call the PEP method, which combines the advantages of both thermal and UV imprinting. We have performed various experiments to examine the consequences of the PEP approach. The Pre-Exposure Process method essentially consists of a type of UV imprinting, but one in which the resin is subject to extremely weak exposed prior to the pressing ["exposed to very weak UV radiation before pressing"], which slightly hardens the resist and increases rigidity. The mold is then pressed to shape the resin, followed by the primary exposure. This process allows the resin to maintain softness equivalent to that at or above Tg in thermal imprinting, while allowing processing, as in UV imprinting. We also examined the relationship between exposure and crosslinking ratios, using FT-IR equipment with an exposure function, to determine the optimal crosslinking ratio for the PEP method. The results of these examinations are also reported.
A fine grating with high aspect rate pattern is one of the essential elements for advanced nano optical devices such as a quarter wave plate. To fabricate high aspect ratio pattern having sub wavelength feature size, nanoimprint lithography is applied. However, fatal defects caused by mechanical stress and friction between the mold and polymer are significant problems. To eliminate the defects, the process sequence, pressure and temperature conditions are optimized. Using Si based mold, sub wavelength grating having 200nm in width and over 1.7 micron in height is demonstrated using PMMA thin film on quartz substrate. This method is a promising technology for industrial production of advanced nano optical elements having high aspect ratio structure.
Fabrications of free form surface and well array plate demonstrated using glass material by imprint lithography. A metal mold is processed by a Diamond cutting. Using a low Tg glass plate, the mold is pressed to the glass surface and transferred the free form surface. The feature error of the proceed glass surface is within 200nm in 3mm x 6mm field. On the other hand, deep well array for bio-chemical device is transferred on glass surface using ceramics mold. Micro wells with 200 x 200μm in square and 50 μm in depth is achieved without fatal defect using optimized imprint process conditions.
Low-cost micro fabrication technique is indispensable for mass production of micro optical elements. Si very large-scale integration (VLSI) fabrication technology such as electron beam lithography or advanced photo lithography is generally applied to fabricate micro structures, however the production cost is too expensive for mass production of optical elements. To overcome the problem, imprint lithography is one of the promising method to fabricate micro structures at low cost. Various shaped mold is fabricated by semiconductor process technology. Process conditions such as temperature or pressure are designed to avoid fracture of the glass plate. Fine gratings, fine blazed and various shaped structures are successfully achieved on the glass surface without fatal defects. The cross-sectional profile of the imprinted structure is fairly good and surface roughness is less than a few nano meters.
In this paper, fabrication of a fine gold grating on glass substrate is demonstrated using imprint lithography for optical elements. A Si mold with fine grating patterns is prepared using conventional IC’s process. The line widths of the gratings are varied from 1.0μm to 200nm. About 20nm thick Si<sub>3</sub>N<sub>4</sub> film is coated on the mold surface by LP-CVD to improve hardness of the mold. The Si mold is pressed to a gold film on a glass substrate at room
temperature. To eliminate fatal fracture of the sample in pressing, the form of the sample is just aligned to the mold to avoid stress concentration at the mold edges. The gold film is plastically deformed and fine gold grating with 200nm in line width and 300nm in height is successfully fabricated on the glass plate. The cross sectional profile of the gold pattern is fine rectangular shape. Using room temperature direct imprint lithography, metal gratings are
successfully fabricated on a glass plate. This method is a promising way to fabricate fine micro optical elements by low cost.
Fabrication of a fine diffractive optical element on a Si chip is demonstrated using imprint lithography. A chirped diffraction grating, which has modulated pitched pattern with curved cross section is fabricated by an electron beam lithography, where the exposure dose profile is automatically optimized by computer aided system. Using the resist pattern as an etching mask, anisotropic dry etching is performed to transfer the resist pattern profile to the Si chip. The etched Si substrate is used as a mold in the imprint lithography. The Si mold is pressed to a thin polymer (poly methyl methacrylate) on a Si chip. After releasing the mold, a fine diffractive optical pattern is successfully transferred to the thin polymer. This method is exceedingly useful for fabrication of integrated diffractive optical elements with electric circuits on a Si chip.
Proximity correction is an important technique to fabricate diffractive optical elements with the direct-writing electron-beam lithography. For the precise proximity correction, the absorbed energy distribution is calculated with an electron scatter simulator based on the Monte Carlo method, and a resist profile is estimated with a resist development simulator based on the cell removal model. In this paper, we calculated the optimum electron dosage for a chirped-period diffraction grating by use of such a precise proximity correction. To reduce the calculation time, we set the cell size 200nmx200nm. The resultant resist profile, however, was much more precise than the cell size because of the interpolation. It took 24 hours to optimize the electron dosage of a grating with a width of 5mm and the minimum grating period of 4micrometers . Moreover the grating, which was fabricated according to the calculated dosage, had a profile that agreed well with the calculated profile.
This paper introduces the novel concepts of 'multistage phase shifter' and 'comb-shaped shifter' for resolving the problems of a transparent type phase shifting mask. The use of a multistage shifter decreases the light intensity dip at the shifter edges. The use of the comb- shaped shifter enables control of the pattern width. The effectiveness of a multistage shifter and a comb-shaped shifter were demonstrated by experiments and simulations. These technologies make it possible to fabricate a wide range of patterns for VLSI using the transparent phase shifting mask.