Mask-Shifting-Based Projection Lithography for Microlens Array Fabrication

Microlens projection lithography is a new lithography method that uses a microlens array with aperture stops to image a single pattern on a mask onto a resist layer, forming an array of the miniaturized pattern. However, the fabrication of a microlens array with aperture stops is complicated, time-consuming and requires expensive facilities. In this paper, we report a single-step fabrication method of plastic microlens arrays with aperture stops. By using gas pressure to hot press a stainless-steel shadow mask with a microhole array onto a thermoplastic film, the softened polymer material is partially filled into the holes of the shadow mask, and forms a convex surface due to surface tension. After cooling, while keeping the microlens array and shadow mask attached, a plastic microlens array with aperture stops is obtained. To verify its capacity, a microlens projection lithography facility has been designed, constructed, and tested. Arrays of µm-sized patterns have been successfully fabricated using transparency masks of a mm-sized pattern.

Microlens arrays (MLAs) have attracted wide attention due to their crucial applications in optics, optoelectronics, and biochemistry. In this paper, we present a simple and green approach for the economical fabrication of MLAs with controlled curvature based on water condensing. By controlling the input current and working time of initiative cooling and the viscoelasticity of UV-curable polymer, uniform porous films with adjustable morphology were prepared. MLAs with aspect ratios of 1.41, 1.01, and 0.69 were fabricated by micromolding the porous film templates. Furthermore, the fluoropolymer encapsulations with the MLAs were applied for the packaging of deep ultraviolet light-emitting diodes (DUV-LEDs). Consequently, the light output powers of DUV-LEDs are enhanced by 7.1%, 10.2%, and 15.4%, respectively, by using these MLAs at the driving current of 350 mA.

Tradition lithographic techniques to produce micrlens array are complicated and time consuming. Due to the capability to replicate nanostructures repeatedly in a large area with high resolution and uniformity, hot embossing has been recognized as one of the promising approaches to fabricate microlens array with high throughput and low cost. This paper introduces processes to realize fabricating microlens array in mass production by direct hot embossing on silicon substrate. The mold is fabricated by multi-photolithography and etching steps and polymethyl methacrylate (PMMA) is chosen as the resist. Processes include coating, heating, pressing, etc. Fidelity and optical performance of the embossed microlens array were measured. High fidelity and fine optical performance of the embossed microlens array demonstrate the possibility of hot embossing to fabricate microlens array in mass production.

Microlens arrays have been widely used as an essential component in many optical systems. The conventional fabrication techniques, such as ink-jet printing, photoresist thermal reflow, grayscale photolithography, and LIGA process have some drawbacks in aspects of process complexity and high fabrication cost. This paper presents a low-temperature, rapid and cost-effective method to successfully generate SU-8 microlens arrays on biocompatible Parylene-C surfaces. Vapor-induced SU-8 dewetting has been achieved on chemically modified Parylene-C surfaces that were selectively pre-treated with O(2) or SF(6) plasma. By this technique, the SU-8 droplets are self-organized onto desired positions to form a microlens array. This method is also suitable for production of microlens array on curved surfaces. The progress of dewetting has been studied by both experiment and simulation.

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.

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.

This paper presents the fabrication of a micro-lens array surface with a single-mask process and two etching steps with KOH water solution. Numerical analysis of optics was used to determine the optimal design parameters such as curvature sagitta and radius. The dimension of each lens is 20μm x 20μm. We used anisotropic etching of <100> silicon through a circular and squar mask to produce a pyramidal pit formed by four (111) planes. The oxide mask is stripped and the immersion of the sample in the etchant solution favors the etching of (411) plane transforming the pit into a smooth hemispherical cavity. An intermediate stage exists when a wider 19.470 <411> - face pyramid replaces the initial 54.740 inverted pyramid. The dependence of surface roughness on concentration and temperature of KOH is investigated in the range of 25%-40% and 60°C-80°C, respectively, and compared between them. The surface profiles and roughness was characterized by AFM. The etching depth and radius of micro-lens array was obtained from the SEM images and AFM data. Also, the array of concave depressions was directly used as a mould for replication of KER-2500 transparent polymeric silicon from Shin-Etsu with a refractive index n=1.41. The perfectly matched array of micro-lenses can be detached from substrate and used as a local solar concentrator. Optical properties such as the focal length of the plano-convex micro-lens array, obtained by replication, are measured and analyzed.

In this paper, we present a new roll-to-roll method to fabricate visible light transparent microlens arrays on a glass substrate by using soft and cost-effective polydimethylsiloxane (PDMS) molds. First, we fabricated microlens array master molds by photoresist thermal reflow processes on silicon substrates. We then transferred the pattern to PDMS molds by a spin coater. After making the PDMS molds, we used a two-wheel roll-to-roll printing machine to replicate ultraviolet resin microlens arrays on glass substrates. The PDMS molds can be made easily at a low cost compared with traditional electroplating metal molds. We studied the quality of microlens arrays that were replicated by different rolling pressures of 20, 200 and 500 N cm−2. We also identified the relation between the pressure and the shape of the microlens arrays. The results showed that the best yield rate and replication performance were achieved with a pressure of approximately 200 N cm−2 and 4 min of ultraviolet light exposure.

An optical head based on the Talbot effect with an aspheric microlens array for an optical read-only-memory (ROM) card system was designed and fabricated. The mathematical expression for the wavefield diffracted by a periodic microlens array showed that the amplitude distribution at the Talbot plane from the focal plane of the microlens array was identically equal to that at the focal plane. To use a reflow microlens array as a master pattern of an ultraviolet-imprinted (UV-imprinted) microlens array, the reflow microlens was defined as having an aspheric shape. To obtain optical probes with good optical qualities, a microlens array with the minimum spherical aberration was designed by ray tracing. The reflow condition was optimized to realize the master pattern of a microlens with a designed aspheric shape. The intensity distribution of the optical probes at the Talbot plane from the focal plane showed a diffraction-limited shape.

Hot embossing has been widely used in fabricating microlens arrays because of its low cost, high efficiency, and high quality. The process parameters such as molding temperature, molding pressure, and holding temperature affect the microlens array’s replication quality. This work selected the stainless steel S136H tool steel as the mold material to process an aspheric microlens array structure through ultra-precision milling. Polymethyl methacrylate (PMMA) microlens arrays with different surface replication were prepared by controlling the molding temperature, molding pressure, and holding temperature. By analyzing the surface quality, contour replication, and optical imaging of hot-embossed samples, the optimal molding temperature of PMMA for optimal replication of aspheric lens arrays was determined as 130 °C. Besides, the internal elastic recovery of PMMA affected the dimensional accuracy and optical performance of the lens. The results showed that, at the molding pressure of 400 N and the holding temperature of 60 °C, the surface defects were eliminated, and the aspheric lens array had perfect replication with a profile deviation of only 4 μm. The aspheric microlens array with good quality was eventually achieved by these optimal process parameters, which provides a foundation for producing aspheric microlens arrays in a low-cost and high-efficiency way.

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.

Retroreflectors are an important optical component, but current retroreflector structures and manufacturing processes are relatively complex. This paper proposes a rapid, low-cost, large-area method for fabricating retroreflectors based on microlens arrays. Tunable microlens arrays with adjustable curvature, fill factor, and sizes were prepared using photolithography and thermal reflow techniques. Subsequently, a two-step nanoimprinting process was used to create a flexible reverse mold and transfer the structure onto the desired substrate. The microlens arrays, with a diameter of 30 μm, a period of 33 μm, a curvature radius ranging from 15.5 to 18.8 μm, and a fill factor ranging from 75.1% to 88.8%, were fabricated this way. In addition, the method also fabricated microlens arrays with diameters ranging from 10 to 80 μm. Retroreflectors were made by sputtering a layer of silver on the MLAs as a reflecting layer, and tests showed that the microlens-based retroreflector exhibited superior retroreflective performance with a wide-angle response of ±75°. Microlens-based retroreflectors have the advantages of simple operation and controllable profiles. The fabrication method in this paper is suitable for large-scale production, providing a new approach to retroreflector design.

A microlens array has become an important micro-optics device in various applications. Compared with traditional manufacturing approaches, digital light processing (DLP)-based printing enables fabrication of complex three-dimensional (3D) geometries and is a possible manufacturing approach for microlens arrays. However, the nature of 3D printing objects by stacking successive 2D patterns formed by discrete pixels leads to coarse surface roughness and makes DLP-based printing unsuccessful in fabricating optical components. Here, we report an oscillation-assisted DLP-based printing approach for fabrication of microlens arrays. An optically smooth surface (about 1 nm surface roughness) is achieved by mechanical oscillation that eliminates the jagged surface formed by discrete pixels, and a 1-3 s single grayscale ultraviolet (UV) exposure that removes the staircase effect. Moreover, computationally designed grayscale UV patterns allow us to fabricate microlenses with various profiles. The proposed approach paves a way to 3D print optical components with high quality, fast speed, and vast flexibility.

In this paper, a method for the rapid design and fabrication of multifocal microlens arrays based on photomultiplier digital micromirror devices is presented. In the method of adopting digital micro lens device instead of conventional hot reflux method, to avoid the hot reflux method the characteristics of the complex process, high costs, the use of special digital gray mask instead of the binary mask, point by point correction more focusing microlens array of sphere, to avoid the binary mask multi-exposure cause accumulated error serious etc. The results show that the actual profile agrees well with the design profile by using grayscale mask, and this method provides a convenient way to prepare high quality multi-focus microlens array.

The high filling factor double side micro lens array (MLA) for laser beam shaping has been widely applied in optoelectrical applications. In this paper, we demonstrated the double side MLA for the laser beam shaping process. The point laser source has been successfully transformed into a two dimension uniformity light imaging. The ultra-precision slow tool servo (STS) diamond shaping and plastic injection method for MLA fabrication had been studied. The complexity micro structure of high filling factor MLA via the planning of cutting tool path is used in this research. The high alignment accuracy of both sides MLA is obtained by artful fixture design. The form accuracy and surface roughness are less than 0.1μm and 10nm, respectively. The alignment error of both sides MLA is less than 10μm.