Large-area Microlens Arrays Research Articles

Fabrication of Large-Area Silicon Spherical Microlens Arrays by Thermal Reflow and ICP Etching.

In this paper, we proposed an efficient and high-precision process for fabricating large-area microlens arrays using thermal reflow combined with ICP etching. When the temperature rises above the glass transition temperature, the polymer cylinder will reflow into a smooth hemisphere due to the surface tension effect. The dimensional differences generated after reflow can be corrected using etching selectivity in the following ICP etching process, which transfers the microstructure on the photoresist to the substrate. The volume variation before and after reflow, as well as the effect of etching selectivity using process parameters, such as RF power and gas flow, were explored. Due to the surface tension effect and the simultaneous molding of all microlens units, machining a 3.84 × 3.84 mm2 silicon microlens array required only 3 min of reflow and 15 min of ICP etching with an extremely low average surface roughness Sa of 1.2 nm.

Femtosecond Laser Fabrication of SiC Microlens Arrays as Integrated Light Homogenizer and Splitter

Silicon carbide (SiC) plays a vital role in special optics because of its stable physicochemical and excellent optical properties; however, making the fabrication of SiC considerably more difficult. In this study, femtosecond laser-assisted inductively coupled plasma etching (ICP) technology is proposed to achieve the efficient fabrication of large-area microlens arrays on the surface of silicon carbide. As an illustration, the microlens array was used for high-quality beam homogenization (uniformity is up to 90%) of the transmitted and reflected Gaussian light beams with a wavelength of 532 nm. The beam-splitting ratio can be dynamically adjusted (from 1.2 to 3.2) by controlling the angle of the incident light, indicating that the SiC microlens arrays can act as an integrated optical homogenizer and beam splitter. This method demonstrates the broad application prospects of SiC in miniaturized and integrated special optics and provides new ideas for further applications of SiC in integrated optical systems.

Improvements on the uniformity of large-area microlens arrays in Fused Silica.

The uniformity of large microlens arrays in Fused Silica is governed by the production process. It comprises photolithographic patterning of a spin-coated layer of photoresist on a 200mm wafer with a molten resist reflow process and subsequent dry etching. By investigating systematic influences throughout the production process we show how to steer the lens production process with a single degree of freedom to improve the uniformity of the final microlens array. To enable this we describe the optical performance of microlenses with only one parameter: the principal aberration component. It is the result of principal component analysis of the chosen optical merit function. We present the case of manufactured microlens arrays with element sizes >100 mm × 100 mm where uniformity was improved by a factor of 2.

Fabrication of microlens array with well-defined shape by spatially constrained thermal reflow

We demonstrate a novel method for fabricating a well-defined and large-area microlens array combining digital micro-mirror device (DMD)-based lithography and spatially constrained thermal reflow. Using the flexible processing ability of DMD-based lithography, a micro pillar array is easily prepared on a glass substrate and then the microlens array is formed after a thermal reflow process. Differing from conventional approaches, the proposed thermal reflow process is conducted in polydimethylsiloxane (PDMS) solution instead of in the air. During the process, the partly cured PDMS can effectively prevent the photoresist crawling in the diameter direction, which is very convenient for controlling the precise shape of the microlens. In addition, the photoresist microlens array and PDMS soft mold with a concave cavity array can be achieved in one step. The experimental results showed that the photoresist microlens was good in both shape and surface quality. The presented method may offer great promise for fabricating a high-quality microlens array with different requirements in optical applications.

Highly uniform manufacturing method for large-area microlens arrays

The microlens array is a key microstructure component in imaging systems, such as in light field cameras. A resolution of the light field camera can facilitate the use of a small-area microlens array. However, fabricating a large-area microlens array with a highly accurate and consistent surface finish is still difficult and high-cost. A low-cost manufacturing method is proposed in this study, where the mould core of the microlens array is fabricated with high precision and high uniformity by cylindrical ultra-precision diamond turning. The proposed tool path strategy is employed to achieve superior surface quality and avoid the dynamic vibration of an unsmooth path. Light field camera prototypes are developed using a commercial DSLR camera with a fabricated microlens array. The successful performance of the prototypes confirms that the proposed manufacturing method satisfies the application demands of large-area microlens arrays.

Innovative design of reel-to-reel hot embossing system for production of plastic microlens array films

This paper reports an innovative design of reel-to-reel hot embossing system for the production of plastic microlens array films. The novel system mainly consists of a reel-to-reel mechanism with tension rollers, a gas-assisted hot embossing module and a vacuum-assisted de-molding unit. During this new process operation, a thin steel mold with large-area micro-circular hole array pattern was fixed on the bottom hot plate of this system. The part of plastic film roll was placed on the mold by tension rollers. Under the proper processing conditions, the softened polymer will just partially fill the micro-circular holes of the mold and, due to surface tension, form a convex lens surface. After a proper cooling process, the plastic film with a large-area microlens array pattern was de-molded from the mold by reverse pressure activated through the vacuum system. In this way, a roll of plastic film with a large-area microlens array can be successfully fabricated. In addition, the sag height, curvature, and focal length of the fabricated microlens arrays could be changed and controlled. The embossing by gas pressure is uniform and the de-molding by a vacuum system is running smooth. This technique shows great potential for mass production of plastic optical films with various large-area microlens arrays.

Precision UV imprinting system for parallel fabrication of large-area micro-lens arrays on non-planar surfaces

This paper reports a vacuum-assisted UV imprinting system for parallel fabrication of micro-lens arrays on a spherical dome for artificial compound eye applications. Artificial compound eyes are a kind of micro-optics structure inspired by insect eyes. They offer unique optical characteristics, e.g., wide field of view (FOV), high sensitivity and light weight, in a compact arrangement with miniaturized lens arrays on a curved surface. In practice, it has been a great challenge to precisely fabricate micro-lens arrays on curved substrates at reasonable cost. The UV imprinting system was developed to ensure conformal contact between a soft elastomeric stamp and a convex substrate, i.e., dome, coated with UV-curable resins. The pressure-driven stamp enables large-area direct formation of high fill factor hexagonal micro-lens arrays through soft UV imprint processes. The geometrical characteristics, surface quality and optical properties of the micro-lens array were characterized quantitatively, e.g., NA=0.06; RMS∼λ/50. Lastly, the optical performance of the artificial compound eyes was demonstrated by an ultra-wide angle imaging experiment under uniform illumination. The vacuum imprinting system presents a new manufacturing platform for fabricating 3-dimensional structures on nonplanar substrates in a parallel and cost effective way. The fabricated compound eye structure will be particularly useful in the areas of multi-channel imaging, machine vision, and surveillance.

Direct fabrication of seamless roller molds with gapless and shaped-controlled concave microlens arrays

This Letter demonstrates the direct fabrication of gapless concave microlenses on glass cylinders, which can be used as seamless roller molds for the continuous imprinting of large-area microlens arrays. The method involves femtosecond laser exposures followed by a chemical wet-etching process. A honeycomb-like concave microlens array was fabricated on a glass cylinder with a diameter of 3 mm. We demonstrated the flexibility of the method in tuning the shape and depth of the concave structures by the arrangements of the laser exposure spots and laser powers, and examined the replicating ability of the roller mold by the polymer castling method.

Microlens Arrays: Fabrication of Microlens Arrays with Well‐controlled Curvature by Liquid Trapping and Electrohydrodynamic Deformation in Microholes (Adv. Mater. 23/2012)

Y. C. Ding, J. Y. Shao, and co-workers demonstrate a creative and economic method for manufacturing large-area microlens arrays (MLAs) with controllable curvature and supersmooth surfaces. On page OP 165, the production of a concave MLA is achieved by electrohydrodynamically deforming the surface of a photocurable prepolymer trapped in microhole array which has been etched onto a doped silicon wafer. The concave MLA is then used as a master to generate the convex MLA via vacuum micromoulding.

Large-area microlens arrays fabricated on flexible polycarbonate sheets via single-step laser interference ablation

A single-step method for the fabrication of large-area microlens arrays on flexible polycarbonate sheets is described. On areas of approximately 1 cm2, 17 million to 120 million microlenses ranging in size from sub-micrometer to several micrometers are fabricated via deep-UV pulsed laser interference ablation. The uniformity and surface quality of fabricated microlens arrays are examined and confirmed through atomic force microscopy and scanning electron microscopy. Optical imaging performance of the microlenses, and their use in massively parallel, pulsed laser nanofabrication on silicon is demonstrated. The microlens arrays can be fabricated in a matter of seconds, suggesting the potential for fast and low-cost production on flexible plastic substrates.

Fabrication of microlens arrays using a CO2-assisted embossing technique

This paper reports a method to fabricate microlens arrays with a low processing temperature and a low pressure. The method is based on embossing a softened polymeric substrate over a mold with micro-hole arrays. Due to the effect of capillary and surface tension, microlens arrays can be formed. The embossing medium is CO2 gas, which supplies a uniform pressing pressure so that large-area microlens arrays can be fabricated. CO2 gas also acts as a solvent to plasticize the polymer substrates. With the special dissolving ability and isotropic pressing capacity of CO2 gas, microlens arrays can be fabricated at a low temperature (lower than Tg) and free of thermal-induced residual stress. Such a combined mechanism of dissolving and embossing with CO2 gas makes the fabrication of microlens arrays direct with complex processes, and is more compatible for optical usage. In the study, it is also found that the sag height of microlens changes when different CO2 dissolving pressure and time are used. This makes it easy to fabricate microlens arrays of different geometries without using different molds. The quality, uniformity and optical property of the fabricated microlens arrays have been verified with measurements of the dimensions, surface smoothness, focal length, transmittance and light intensity through the fabricated microlens arrays.

Fabrication of large area resin microlens arrays using gas-assisted ultraviolet embossing

Large-area microlens arrays are becoming important components in many applications such as LCD-TV diffusers. This paper reports a uniform pressure, low temperature process for their fabrication. The process integrates gas-assisted embossing and UV-curing embossing. During the process, the 230mm x 203mm PMMA substrate is pressed against the stainless-steel stamper coated with UV-curable resin. Under the gas pressuring and UV irradiating, a large array of microlens can be formed. By using this process, high embossing temperature and high embossing pressure can be avoided. Little residual stress is observed in the embossed PMMA substrate. The uniformity of large-area fabrication and optical properties of fabricated resin microlens array have been verified. This study has successfully shown the potential of this gas-assisted UV embossing process for the replication of large-area microstructures.