Fabrication and Optical Characterization of Polymeric Aspherical Microlens Array Using Hot Embossing Technology

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.

The cylindrical microlens array (CMLA) is an important micro-optical component, it has very significant application effects in the fields of naked eye three-dimensional display, laser line generation, and deformed beam shaping. However, the high efficiency and low-cost manufacturing of large-area CMLA is still challenging for industry and academia. Based on the electric-field-driven (EFD) jet printing technique, a new method for manufacturing a large-area CMLA with high efficiency and low cost has been proposed. We use the theoretical analysis and numerical simulation to investigate the basic principle of EFD jet printing. The influence of process parameters on the formation of CMLA was revealed by experiments. Using optimized process parameters, the fabrication of a CMLA with a patterned area of 50 × 50 mm2, a line width of 70 μm, and a period of 100 μm was achieved. The geometric and optical properties of the fabricated CMLA were characterized, showing that the products have the good geometric appearance and focusing effect. The results show that the proposed fabrication method provides a new manufacturing process for large-area manufacturing of CMLA with high efficiency and low cost.

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.

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 research, the manufacturing processes of non- spherical refraction microles array by gray-scale mask is investigated. Compared to the conventional multi-lithography fabricating method, the gray-scale mask approach requires only a single lithography action to fabricate a non- spherical refraction microlens array. In the firs part of this research, we emphasize the gray-scale mask based microlens array fabrication processes through the UV-LIGA approach. Furthermore, a two-stage process-modeling scheme is proposed to reduce the time-consuming trial-and-error parameters tuning labor works. At the first stage, a multi- layer feedforward neural-network is employed to model the relationships between the diameter and height of the microlens are obtained, the surface profile of this microlens can be predicted by an empirical equation. The empirical equation is derived through the regressing analysis method with data points sampled from the real microlens surface profile. Experimental results demonstrate that the proposed two-stage scheme can precisely predict the surface profile of the gray-scale mask based microlens.

In this paper we describe the work on application of hot embossing process to fabrication of micro-optical elements transmitting in the visible, near infrared and mid-infrared part of spectrum, using multi-component soft glasses as starting materials. Description of used hot embossing (HE) process is provided, followed by discussion of rheological and viscosity properties of several glass types considered for demonstrator fabrication experiment. Demonstrator microlenses were fabricated from lead-bismuth-gallium oxide (PBG08) and tellurite glasses (TWPN/I/6), with transmission windows extending from the visible up to the mid-infrared (up to around \(6.5\,\upmu {m}\)). Fabricated elements, which included diffractive and refractive lenses and lens arrays, were examined in context of homogeneity (quality of pattern replication) and obtained optical properties (\( {M}^{2}\), focal length, resolution). A demonstrator microscope objective was constructed using refractive plano-concave, plano-convex and biconvex lenses fabricated with the HE setup and its optical characteristics were provided.

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.

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.

Functional micro-structures are gaining more and more importance in the industrial technologies during the last few years. For example, deep micro holes find a wide range of applications in inkjet printer nozzles, spinnerets holes, turbine blades cooling channels, diesel fuel injection nozzles, drug delivery orifices, etc. Another important example is microlens arrays. The ability of microlens arrays to focus incident light into a series of beam spots makes them useful as important optical elements that are widely used in the area of optoelectronic and optical communications. Due to the wide applications of these functional micro-structures in optical, biomedical engineering and microelectromechanical systems (MEMS), micro fabrication techniques have gained growing interest from researchers and engineers. Recently, the use of reaction-bonded silicon carbide (RB-SiC) as a material in manufacturing optical molding dies for aspherical lenses and microlens arrays has become a new research focus, due to its superior material properties, such as high hardness and strength at elevated temperature, high thermal conductivity, chemical stability, wear resistance and low density. However, due to its high hardness (Vickers hardness 25-35 GPa), it is typically difficult to be machined. Although abrasive machining processes, such as lapping, polishing and grinding can produce a fine surface finish, but the machining efficiency is low and the production cost is high. Diamond cutting is able to produce a high material removal rate, but the severe tool wear in diamond cutting of RB-SiC is the main obstacle that limits its wide application in the industry. Compared to the aforementioned methods, micro electrical discharge machining (micro-EDM) has emerged as a possibly effective machining tool to fabricate complex micro-structures on hard and difficult-to-cut materials like RB-SiC. However, the use of conventional micro-EDM alone to obtain micro-structures with good surface finish and high form accuracy in RB-SiC, is still a challenging issue. The high resistivity of the RB-SiC workpiece is the main limiting factor for the discharge current of micro-EDM, which directly affects the machining efficiency of RB-SiC. The main objective of this study is to develop hybrid machining processes based on micro-EDM for improving the machinability and surface finish of RB-SiC. Carbon nanofibers assisted micro-EDM was proposed to replace the conventional micro-EDM. The effects of carbon nanofibers addition in the dielectric fluid were confirmed experimentally. The machining mechanism and the material migration phenomenon with the addition of carbon nanofibers were investigated and clarified. Next, a novel machining process, namely hybrid micro-EDM process by combining ultrasonic cavitation and carbon nanofibers was proposed, in order to improve the machining efficiency of RB-SiC ceramic material and to prevent from tool material deposition on workpiece. The effectiveness of the hybrid process was verified through the fabrication of micro-structures on RB-SiC. This thesis consists of five chapters. Chapter 1 gives an overview of the background of this research. Firstly, an introduction of the RB-SiC ceramic material, including the fabrication process, material properties, applications and previous machining processes of RB-SiC material were given. EDM was chosen as a method to fabricate micro-structures on the RB-SiC, which can overcome the shortcomings of the abrasive machining process. Then, an overview of the principle of micro-EDM/EDM process was discussed. The problem of machining RB-SiC by using conventional EDM was also pointed out. Some recent developments for enhancing the machinability of hard and brittle ceramic materials by micro-EDM/EDM were reviewed. Finally the objectives and organization of this thesis were stated.

As a technology for polymer surface fabrication, roll-to-roll hot embossing has been widely used because of its advantages, such as its low cost and high efficiency. However, the metal micro-mold is a major factor that determines the hot embossing of the polymer. In this study, a new metal micro-mold manufacturing method is proposed. The metal micro-mold is produced using laser shock imprinting (LSI) on the surface of metal foil. It has the characteristics of good thermal stability and high strength. During our LSI experiment, the strength of the mold increased after laser shocking. In this study, copper foils of different thicknesses were selected for LSI experiments. Through the analysis of the profile and forming depth of the microstructure, combined with the numerical simulation of the forming mechanism of copper foils with different thicknesses using ABAQUS software(Abaqus 2021), a copper foil with a flat back was selected as the final metal micro-mold. On this basis, copper molds with different microstructure shapes were created. Then, the mold was used in the hot-embossing experiment to manufacture the microstructure on the surface of polyethylene terephthalate (PET) and to study the fidelity and integrity of the molded microstructure. The deformation resistance of the copper mold under hot-embossing conditions was verified through a nano-indentation experiment. The final results show that the metal micro-mold produced via LSI had a high accuracy and molding stability and has potential applications in the field of roll-to-roll hot embossing.

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.

We present in this paper a method for obtaining a low cost and high replication precision 2D (two dimensional) nanofluidic chip with a PET (polyethylene terephthalate) sheet, which uses hot embossing and a thermal bonding technique. The hot embossing process parameters were optimized by both experiments and the finite element method to improve the replication precision of the 2D nanochannels. With the optimized process parameters, 174.67 ± 4.51 nm wide and 179.00 ± 4.00 nm deep nanochannels were successfully replicated into the PET sheet with high replication precision of 98.4%. O2 plasma treatment was carried out before the bonding process to decrease the dimension loss and improve the bonding strength of the 2D nanofluidic chip. The bonding parameters were optimized by bonding rate of the nanofluidic chip. The experiment results show that the bonding strength of the 2D PET nanofluidic chip is 0.664 MPa, and the total dimension loss of 2D nanochannels is 4.34 ± 7.03 nm and 18.33 ± 9.52 nm, in width and depth, respectively. The fluorescence images demonstrate that there is no blocking or leakage over the entire micro- and nanochannels. With this fabrication technology, low cost polymer nanochannels can be fabricated, which allows for commercial manufacturing of nano-components.

Micro-lens arrays, due to their unique structure and outstanding optical performance, have been widely applied in advanced domains such as display technology, optical imaging, and laser processing. Among the manufacturing techniques, embossing molding is considered an efficient and precise method for fabricating micro-lens arrays. However, given the intricate structural characteristics of these arrays, precise control over the process becomes paramount. This study introduces a digital twin system for micro-lens array embossing manufacturing, grounded on a process knowledge base. The system relies on the rich process knowledge within the backend database combined with real-time sensor data, providing a powerful drive for the digital twin, thereby ensuring the precision and stability of the embossing manufacturing process. Additionally, the digital twin technology not only offers real-time monitoring and feedback for the production process of the micro-lens arrays but also aids researchers in optimizing process parameters, further enhancing product quality and consistency.

This paper reports a simple and effective method to fabricate microlens arrays with the ultraviolet-curable resins, and a soft mold of micro-holes array. During capillary forming operation, the surface of the soft mold of micro-holes array is being pressed against the ultraviolet-curable resin layer coated on the plastic substrate. An array of convex lense can be formed in the circular holes of the soft mold due to the capillary filling and surface tension. The microlens arrays have smooth surface and uniform focusing function. The shape and height of micolens can be controlled with a proper combination of pressing pressure, pressing duration and UV curing dose. This technique shows great potential for fabricating polymer microlens arrays with high productivity and low cost.