Tung-Chuan Wu

Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices

In this paper, we experimentally and theoretically investigated the optical characteristics of organic light-emitting devices (OLEDs), having different pixel sizes and attached with patterned microlens array films. For a regular microlens array, though it can extract the waveguiding light and increase luminous current efficiency for a large-pixelated OLED, we observed that it decreases the luminance to an even lower level than that of the planar OLED as its pixel size is close to the microlens dimension. Although a microlens can effectively outcouple the light rays originally at incident angles larger than the critical angle, it also can impede the outcoupling for the light rays originally at incident angles smaller than the critical angle. Enhancement or reduction of the light extraction depends on the relative positions of the light emitting point and the microlens. Therefore, we proposed a center-hollowed microlens array, of which the microlenses directly upon the pixel are removed, and proved that it can increase the luminous current efficiency and luminous power efficiency of a small-pixelated OLED. By attaching this patterned microlens array, 87% of luminance enhancement in the normal direction was observed for a 0.1x0.1 mm2 OLED pixel. On the other hand, a regular microlens array resulted in 4% decrease under the same condition.

Efficiency improvement and spectral shift of an organic light-emitting device by attaching a hexagon-based microlens array

In this paper, we present and analyze the influences of the fill factor and the sag of hexagon-based microlenses on the optical characteristics of an organic light-emitting device (OLED), such as spectral shift, CIE (abbreviation of the French ‘Commission internationale de l’´ eclairage’) coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly on increasing the fill factor of the microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinates decreased linearly on increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 35% and 40%, respectively, by attaching a microlens array having a fill factor of 0.90 and a height ratio of 0.56. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased on increasing the viewing angles, due to the microcavity effect.

Fabrication of a micro-punching head using LIGA and stacking processes

A simple stacking process to fabricate microstructures with a high aspect ratio of 30 is presented in this study. For the x-ray absorption of a substance, an analytical model of the absorption dosage (exposure dosage) along a depth of x is developed. It can accurately estimate the time and dosage required for exposure. Based on the calculated result, 1 mm thick PMMA (polymethylmethacrylate) photoresist with a separate gold (Au)/graphite mask is exposed using x-rays followed by a developing process. Then a stacking process is applied for these 1 mm thick PMMA microstructures to form a microstructure with a high aspect ratio. With this stacking method, a PMMA microstructure with a height of 3 mm has been fabricated. In addition, a nickel?cobalt (Ni?Co) alloy electroplating process is applied to fabricate a metallic mold with high hardness. The hardness of the mold by electroforming is the key factor to the quality of the micro-punch process. As an integrated micro-punch head for IC leadframes, the hardness should be higher than 500 Vickers hardness (Hv). In addition, it was found experimentally that changing the ratio of Ni sulfamate to Co sulfamate can affect the deposit hardness and internal stress. The deposit internal stress varies with Co content. Thus, it is necessary to add a stress reducer to improve the hardness and reduce internal stress. A hardness of the mold of 550?560 Hv with 26?28 wt.% of Co and zero internal stress have been made successfully after adding a stress reducer. With this electroplating technique, a micro-punching head with high hardness and high aspect ratio can be fabricated by LIGA technology.

Optical characteristics of silver film on the moth-eye structure

The development of coating optics to lower the reflected light and thereby to increase the optical efficiency of an optical system has been a very important issue for many years. Conventional solutions to this, such as multilayered alternation of high and low refractive index layers, often lead to an expensive coating process. Recently, the use of antireflection structured (ARS) surface, which is called “moth-eye structure”, has been proposed as an applicable option based on both the theoretical and experimental studies. In the current study, the experimental results of the reflectance and transmittance of two different thicknesses of silver films deposited on the moth-eye structure were carried out. The moth-eye structure arrays were fabricated by holographic exposure and photolithographic processes on the polymer film. The structure arrays were consisted of periodic length of about 300 nm, with the diameter of about 250 nm and the height of 150 nm. Compared with the silver coating film on the flat PET substrate, the optical property of the silver coating film on the moth-eye structure showed a better result for the anti-reflection application. The 25 nm-thick silver film on the moth-eye structure is suggested to be applied for the car window glass of antireflective films to obtain the high performance of heat insulation with acceptable transparency in the visible range.

Roll to Roll Processing for Flexible Nanophotonics

This article discusses the current status and achievements of R2R technology for large area nano-scaled optical devices developed at MSL/ITRI. Firstly, a single layer of nanostructure on polymer film is designed for anti-reflection purpose by finite difference time domain (FDTD) method in the visible light spectrum. The conical array with around 1 aspect ratio, like moth-eye shape and showing superior performance in the optical simulation, has been adapted for the R2R experiments. The development of R2R process includes roller machine design and fabrication, roller mold design and making, development of rolling imprint process, characterization of rolled devices. In this study, large area (200mm *200mm) Ni template was fabricated with DUV exposure, followed by dry etching and electroforming process, respectively. Then, the template was bonded on the roller mold with magnetic film to make nanostructure roller mold. With the delicate nanostructure roller mold, systematic experiments have been conducted on the home-made roller machine with various parameters, such as linear speed, dose rate, and material modifications. The duplicated nanostructure films show very good optical quality of anti-reflection (AR < 1%) and are in good agreement with the theoretical predictions. Besides, the duration of the roller mold has been highly promoted to hundreds of imprint in the UV embossing process.Copyright

A study of the self-assembled mono-layer deposition process for the anti-adhesion of nano-imprint stamps

The adhesion phenomenon of nano-imprint stamps will inevitably be one of the key issues for industrial applications of nano-imprint processes. The study of processes involving application of a thin film of chlorosilane on the surface of stamp that then attracts more and more attentions has resulted in documentation of promising solutions. However, the deposition of chlorosilane must be at under water-free atmosphere in order to achieve positive results. This study described below involves using a vacuum chamber and heating system designed to rapidly deposit high quality chlorosilane film onto silicon and nickel-based stamps through the vapor phase deposition method. The quality of anti-adhesion of the coated stamps has been investigated by imprinting experiments. The contact angles of the coated stamps after hundreds of imprints remain nearly the same as those of the as-deposited stamps.

The Effect of Sub-Wavelength Structure Morphology on Anti-Reflection Properties

In this paper, we present a new thermal reflow method to transform photresist pillars into the shapes of spherical caps and month-eyes. We also investigated the effect of this sub-wavelength structure morphology on anti-reflection properties of these photoresist patterns. The diameter, the height, and the pitch of this regularly arranged pillar array were set at 200, 250, and 340 nm, respectively. After the thermal reflow, experimental results showed that the reflectivity of the heat-treated photoresist patterns can be reduced to approximately one fourth of that of the polished surface of the silicon wafer.

Outcoupling Efficiency Improvement of Planar OLED Devices with Square-based Microlenses

In this paper, analysis of outcoupling efficiency improvement and optical characteristics of organic light-emitting devices (OLEDs) attached with square-based microlens arrays are presented. A square-based microlens array, having a base length of 35.4 μm and a fill factor of 89.6%, increases the luminous current efficiency and outcoupling efficiency of the OLED device by 42% and 47%, respectively. Optical properties of the OLED device, such as optical emitting spectrum and CIE coordinate, have also been investigated when attached with microlens arrays having different fill factors.

Fabrication of Periodic Nanostructure in Nanoimprint Process

In this paper nanoimprint molds with 50nm and 100nm feature size nanostructure were fabricated. The periodic pattern of a positive resist was formed on silicon wafer by electron beam lithography, and then the nanostructure of Si was etched using an inductively coupled plasma reactive ion etching (ICP-RIE) system. During the etching process, different ratio of C4F8gas was added to the original etching gases SF6/O2. By increasing the C4F8gas, the sidewall protection was improved. The C4F8gas also increased the etching resistance of the electron beam resist, and the nanoseale resist patterns were maintained through the etching process. The resist was removed after etching, and then the 50nm periodic nanostructure with aspect ratio 6 was obtained. To achieve nanoimprinting process with less damage, the possible sources of stresses resulting from the molding/demolding process, film solidification and thermomechanical mismatch during cooling were studied.