Design, Fabrication, and Integration Technology of Large-Scale Microlens Array for Infrared Charge Coupled Device Application

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.

256 X 256 diffractive microlens arrays have been designed by considering the independent optical and processing parameters for 3 - 5 micrometers wavelength with a microlens size of 100 micrometers . The lens F number and array pitch are 1.5 and 100 micrometers , respectively. The diffractive microlens arrays have been fabricated on the surface of Si substrates by successive photolithography and Ar<SUP>+</SUP> ion-beam-etching technique. The practical processes and fabrication method are discussed. The optical characteristics and measurements of the diffractive microlens arrays are presented.

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.

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.

In this paper, we demonstrate the fabrication of refractive silicon microlens array with a large focal number and almost perfect spherical lens shape. Through a modified thermal reflow, photoresist microlens array of a large focal number is fabricated, which is then transferred into the silicon substrate by ion beam milling. To reach an accurate spherical lens profile, we both theoretically and experimentally study the practical factors that harm the pattern transfer fidelity, which mainly include the etching selectivity and faceting effect. Other secondary etching effects, such as the trenching effect and re-deposition effect, are also discussed. Based on these studies, a silicon microlens array with a focal number of 1.35 has been successfully obtained, with the profile error controlled well-below 0.121 μm, less than λ/6 within the whole infrared wavelength band. Besides, the fabricated microlens array exhibits a good uniformity and fine surface smoothness. The fabricated silicon microlens arrays can be applied in minatured infrared and terahertz imaging devices, or used as the master mould for soft lithography.

Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique

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.

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.

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.

We describe a simple and effective method to fabricate a uniform plastic microlens array (MLA) with high fill-factor over a large area utilizing self-aligned diffuser lithography (SADL). In order to make an intimate contact between the photomask and the positive photoresist during 3D diffuser lithography, which is crucial for obtaining a uniform MLA mold over a large area, we fabricated a self-aligned metal mask directly on top of the positive photoresist, eliminating any air gap between the metal mask and the underlying photoresist. After replication of the developed concave MLA mold onto the poly(dimethylsiloxane) (PDMS), a standard deviation of sag (height) of the MLA was observed by laser scanning confocal lithography. The standard deviation, which indicates uniformity, was reduced by as much as a factor of 6 by applying SADL compared with that obtained from conventional diffuser lithography. Using this method, we fabricated a 7 inch MLA sheet with excellent uniformity. The proposed method can be extensively applied for fabrication of large-size MLA sheets with plastic materials thanks to its simplicity and versatility.

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.

Microlens arrays play a critical role in enhancing imaging systems due to their outstanding optical performance, compact size, and lightweight nature. However, traditional fabrication methods for microlens arrays suffer from low precision, inefficiency, high costs, and a lack of adequate surface figure control. In this paper, we present a novel approach for microlens array fabrication, using a projection lithography process with mask-shifting. The method employs a 0.2× projection objective lens to enhance linewidth resolution. By employing a projection-based mask-shift filtering technique, we achieve superior surface figure accuracy while reducing the complexity of mask preparation. The experimental results for four microlenses with different aperture sizes demonstrate surface figure accuracy in the submicron range and surface roughness at the nanometer level. In addition, 3D profilometer scanning equipment was employed to measure the surface roughness of these microlens arrays, and the measurement results of these microlens arrays processed using the proposed method for their surface roughness are 18.4 nm, 29.6 nm, 34.4 nm, and 56.1 nm. Our findings indicate that this method holds great potential in microlens array fabrication, offering the ability to achieve lower linewidths and higher surface figure accuracy compared to conventional methods.

New types of microlens arrays for the IR based on inorganic chalcogenide photoresists

We demonstrate the fabrication of a refractive microlens array by using 248 nm excimer laser micromachining with coded gray-tone mask photolithography. With pre-corrections to the nonlinear exposure process, the maximum deviation from the designed shape was below 5%. The fabricated hemispherical lens of 30 µm radius was used as a solid immersion lens (SIL) and combined with a 0.54 numerical aperture (NA) objective to achieve a 0.87 effective NA through the knife-edge scanning test. The experimental results agreed with those of the simulation. Unlike the methods such as the thermal melting process, this one-step optical exposure method with a coded mask provides a relatively fast and cost-effective way to realize a microlens array in optical data storage, information processing, and optical interconnection applications.

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.