Shih-Yu Hung

Applying ANN/GA algorithm to optimize the high fill-factor microlens array fabrication using UV proximity printing process

An artificial neural network and genetic algorithm were used to achieve a high quality microlens array fabrication using a UV proximity printing process in this study. The UV proximity printing process can precisely control the geometric profile of the microlens array in the fabrication process without thermal reflow. The major objective in using the robust design is to reduce the variations in the focal length of the microlens array, allowing improved focus and enhanced illumination brightness. The artificial neural network was used first to characterize the nonlinear relationship between the manufacturing parameters and the properties derived from experimental data. The manufacturing parameters which affect microlens array uniformity include: (1) geometrical ratio, (2) printing gap, (3) spin coating revolution speed, (4) exposure time and (5) developing time. It is very important to control these parameters to decrease the sensitivity to noise. The L18 orthogonal array was used as the learning data for the artificial neural network to construct a system model that could predict the focal length for arbitrary setting of parameters. Then, the genetic algorithm was applied to obtain the robust setting of parameters. The results showed that the microlens array quality could be significantly improved in comparison with the original design.

Fabrication of a microlens array electroformed mold with low roughness and high hardness

A systematic approach to achieve a LRHH (low roughness and high hardness) electroforming process is developed in this study. Because electroformed molds with low roughness and high hardness are required for microlens array fabrication using the LIGA-like process, the invented Ni?Co alloy-pressurized electroforming process is used to fabricate a metallic micro-mold for microlens array molding. The electrolyte parameters such as Co content, current density, brightener content, pH value and temperature will be examined with respect to roughness and hardness. An artificial neural network (ANN) is used to construct a system model that accurately predicts the responses for arbitrary parameter settings. A genetic algorithm (GA) is applied to minimize the surface roughness and improve the micro-mold hardness. The results show that the micro-mold surface roughness and hardness could be significantly improved using the ANN/GA approach. A LRHH electroforming process is carried out using parameter design to improve the surface morphology and increase the service life of the micro-mold during the forming process.

Semiellipsoid microlens fabrication method using UV proximity printing

We present a new semiellipsoid microlens fabrication method that controls the printing gap in the UV lithography process without thermal reflow. The UV proximity printing method can precisely control the curvature radius ratio of the semiellipsoid microlens in the fabrication process. The proposed fabrication method facilitates mass production to achieve a high-yield and high-coupling semiellipsoid microlens that is suitable to be used in commercial fiber transmission systems. A semiellipsoid microlens can be tipped on a single-mode fiber end to improve power coupling efficiency from laser diodes. The semiellipsoid microlens allows increasing the fiber spot size and numerical aperture. It is very important to control the geometric parameters in the assembly procedure to increase the optical coupling efficiency between the laser diode and single-mode fiber. Wide misalignment tolerance, low loss, and low manufacturing cost could be achieved by the proposed fabrication method. The theoretical model is first developed to predict the optical coupling efficiency for various microstructure geometries of semiellipsoid microlens and assembly parameters in this study. Then, the Taguchi method is applied to obtain the optimal geometric parameters setting. The results show that optical coupling efficiency could be significantly improved by using the optimal geometric parameters setting.

Semiellipsoid microlens fabrication method using the lift-off and alignment exposure processes

We present a new semiellipsoid microlens fabrication method using the lift-off and alignment exposure processes. The lift-off method is used to create an elliptical base before the thermal reflow process. During the photoresist thermal reflow process, the elliptical base can precisely define the bottom shape of the liquid photoresist, and fabricate the semiellipsoid microlens array with a large height and small radius of curvature. The prolate spheroid approximation method is developed to estimate the thickness of the elliptic photoresist column required by the semiellipsoid microlens of a certain height, with the error being controlled within ±3%. Electroforming technology is then used to convert the photoresist patterns into a metallic mold for a PDMS ellipsoidal microlens. The experiment results show that the geometries of a semiellipsoid microlens can be accurately defined by three parameters: the length of the major axis of the elliptical base, the length of the minor axis of the elliptical base and the thickness of the elliptical photoresist column. The proposed fabrication method facilitates mass production to achieve a high-yield and high-coupling semiellipsoid microlens that is suitable for use in commercial fiber transmission systems

Optimal design using thermal reflow and caulking for fabrication of gapless microlens array mold inserts

The thermal reflow process is widely used in microlens array fabrication. However, the resulting arrays are commonly criticized for their low fill factor. In this work, caulking is applied to fill the gaps between adjacent lenses. The experimental results prove that a gapless microlens array with a 100% fill factor could be successfully produced and the caulking time precisely controlled. Furthermore, an artificial neural network and genetic algorithm are used to achieve high quality using the thermal reflow and caulking. The L18 orthogonal array is used as the learning data for the artificial neural network to construct a system model that could predict the results (e.g., S/N, focal length, and roughness) for arbitrary parameter settings. The genetic algorithm is then applied to obtain the optimal parameter settings. The major objectives in using the optimal design are to reduce the variation in the focal length and the surface roughness for a microlens array. This allows improved focus and enhanced illumination brightness. The results show that microlens array quality could be significantly improved in comparison with the original design.

Tilted microlens fabrication method using two photoresists with different melting temperatures

A new fabrication method for a tilted microlens array for light control films was developed to increase the efficiency of a liquid crystal display so that lateral light sources are collected and the dazzling light problem at certain viewing angles is improved. The thermal reflow with two layers of different photoresists is used to fabricate the tilted microlens array in this study. After the lithography process, the round photoresist column with two layers of different photoresists can be obtained. During the thermal reflow processing, the upper photoresist layer (AZ-4620) reaches the glass transition temperature, which is transformed from a glassy state into a rubbery state. Since the glass transition temperature of the lower photoresist layer (AZ-5214E) is higher than the temperature of thermal reflow, the lower photoresist layer is still able to maintain its solid state. The lower layer creates a round base during the thermal reflow process. The experimental results show that the photoresist mold of tilted microlens may be produced under gravity by inverting and tilting the substrate during the thermal reflow process. After the wafer is placed inversely and obliquely, the base can not only restrict the bottom shape of the liquid photoresist to a round shape but also prevent the sliding of liquid photoresist during the thermal reflow process. Compared with the conventional thermal reflow without a base, the photoresist base is relatively reliable and effective in preventing lenses from sliding.

Optimization on hardness and internal stress of micro-electroformed NiCo/nano-Al2O3 composites with the constraint of low surface roughness

In this paper, Ni?Co/nano-Al2O3 composite electroforming was used to make the metallic micro-mold for a microlens array. The microstructures require higher hardness to improve the wear resistance and lifetime. Nano-Al2O3 was applied to strengthen the Ni?Co matrix by a new micro-electroforming technique. The hardness and internal stress of Ni?Co/nano-Al2O3 composite deposit were investigated. The results showed that the hardness increased with the increasing Al2O3 content, but at the cost of deformation. Increasing the Al2O3 content in the composite was not always beneficial to the electroformed mold for microlens array fabrication. This work will concentrate on the relationship between important mechanical properties and electrolyte parameters of Ni?Co/nano-Al2O3 composite electroforming. Electrolyte parameters such as Al2O3 content, Al2O3 particle diameter, Co content, stress reducer and current density will be examined with respect to internal stress and hardness. In the present study, low stress and high hardness electroforming with the constraint of low surface roughness is carried out using SNAOA algorithm to reduce internal stress and increase service life of micro-mold during the forming process. The results show that the internal stress and the RMS roughness are only 0.54 MPa and 4.8 nm, respectively, for the optimal electrolyte parameters combination of SNAOA design.

A new fabrication method for an asymmetric microlens array light control film using inclined exposure and an incomplete thermal reflow process

This study presents a novel fabrication method for asymmetric microlens arrays for light control films that increase the efficiency of liquid crystal displays used to collect lateral light sources and improve the brightness within the angle of view. Initially, the pattern allows exposure through the placement of a photoresist-coated substrate on an inclined fixture. An inclined photoresist column array with a round cross section is constructed using a photolithography technique. During the incomplete thermal reflow processing, only the surface part of the photoresist column reaches the glass transition temperature (Tg) and is transformed from a glassy state into a rubbery state. In order to minimize the structural surface energy and reduce the surface area, the surface of the inclined photoresist column forms the shape of a lens. With proper control of the thermal reflow temperature and processing time, the photoresist at the base maintains its original inclination and glassy state, because it does not reach the glass transition temperature. The experimental results show that the inclined exposure from different angles precisely controls the declination angle of the inclined photoresist column. Asymmetrical microlens arrays with tilt angle larger than 33° can be fabricated using the incomplete thermal reflow method.