Microlens Array Mold Research Articles

Study on gas trapping during precision glass molding of microlens array in a nitrogen atmosphere

AbstractMicrolens arrays will suffer from filling defects due to trapped gas when molded in a nitrogen atmosphere by precision glass molding (PGM). In this paper, a multistep molding method is proposed to avoid gas trapping and improve the accuracy of a microlens array. The defect formation mechanism of the microlens array caused by the trapped gas is investigated, and the effect of the molding pressure on the defect formation is analyzed. A numerical model of the mold‐nitrogen‐glass interface at high temperature is established to evaluate the defect evolution, and the minimum number of PGM steps required to greatly reduce defects caused by the trapped gas is predicted. The numerical model is validated by a multistep PGM experiment of D‐K59 glass material. The results show that a three‐step PGM process significantly reduced the height of the defect. The difference between the height of the microlens unit and the depth of the mold is less than 0.4%. The molded microlens array has a peak‐to‐valley value of 0.38 μm and a surface roughness Ra of 3.5 nm. This work is instructive for the fabrication of high‐precision glass microlens arrays.

Flexible metallic mold based precision compression molding for replication of micro-optical components onto non-planar surfaces

Replication of micro/nano-scale optical components onto non-planar substrates is considered technically challenging and excessively expensive via existing manufacturing approaches. A low-cost, large-volume and high-precision fabrication approach is thus highly desirable. In this paper, a novel precision compression molding approach that enables large-volume and high-resolution replication of micro/nano-scale optical features from planar to non-planar substrates is proposed. A micro-lens array is demonstrated to be transferred from a planar master mold to a non-planar substrate. In such process, a micro-lens array mold is initially fabricated by single-point diamond turning on a planar mold surface and then replicated by micro-injection molding. After that, the optical structures are transferred from micro-injected polymeric samples to a nickel foil by electro-plating, which is employed as a critical transition medium. To obtain sufficient flexibility with considerable strength, the thickness of the electro-plating mold is controlled to be around 75 μm, which is further integrated with precision molding tools to replicate micro/nano-scale optical features onto cylindrical substrates. Geometrical profile and surface roughness of the fabricated optical components in each processing step are characterized experimentally. The results indicate that the replication of the micro-lens array successfully demonstrates the process to transfer micro-optical components from planar surfaces to non-planar thermo-plastic polymeric substrates with high fidelity and optical-level surface quality. Compared with conventional precision compression molding, this novel proposed method in this research could achieve much higher flexibility and comparable fidelity with low cost on transferring optical structures from planar to non-planar surfaces. The application of this method might be widely extended to numerous replication methods, such as precision glass molding, roll-to-roll and roll-to-plane imprinting.

自己整列微粒子の形状転写によるマイクロレンズアレイの製作とモールド変形を利用した非球面化の検討

Aspheric micro-lens arrays have been attracting great attention to improve light distribution control. This study proposes a simple and efficient method for fabricating close-packed aspheric micro-lens array. First, silica particles of ϕ10 μm were self-assembled over ϕ10 mm area using sedimentation method. Then the convex profile of the self-assembled particles was replicated to a spherical micro-lens array mold. The mold material was silicone elastomer or polydimethylsiloxane (PDMS). Then, the mold profile was replicated into ultraviolet (UV) curing resin. By applying uniaxial stretching to the mold during the secondary replication, aspheric micro-lens array was fabricated. Deformation of the PDMS mold under uniaxial stretching was analyzed by using finite element analysis (FEA) for rough estimation. Experimental results showed that intended lens profile was obtained though some defects were observed due to the nonuniformity of the particle diameters. Optical characteristic of elliptic micro-lens array was made clear.

Flexible fabrication of Fresnel micro-lens array by off-spindle-axis diamond turning and precision glass molding

Fresnel micro-lens arrays with close-to-wavelength features have significant advantages in minimization of optical systems. However, in the fabrication of a Fresnel micro-lens array by conventional diamond turning, the discontinuous features of the surface profile cause interference between the tool flank face and the finished surface. In this manuscript, a novel off-spindle-axis diamond turning method is proposed to fabricate Fresnel micro-lens array mold insert and a toolpath generation algorithm is developed. The generation of each lenslet is treated as an independent turning operation, and the whole fabrication of a Fresnel micro-lens array is completed by a succession of repetitive turning operations. With this approach, the interference between the tool and the finished surface is avoided, and the form accuracy and surface integrity are improved for each lenslet. Then, the Fresnel micro-lens array mold insert is further utilized in precision glass molding to replicate the micro-lens structures onto transparent polymer substrates. The experimental results indicate that both the diamond-turned and molded Fresnel micro-lens arrays are achieved with homogeneous quality. As an application, a compact imaging system based on the molded Fresnel micro-lens array is demonstrated. The proposed machining method in this study can be employed in the fabrication of Fresnel micro-lens arrays and other micro-optics with discontinuous profiles with high accuracy and machining efficiency.

Grinding techniques for fabricating micro-lens array mold made of cemented carbide (Polycrystalline diamond tools and mold surface roughness)

Grinding techniques for fabricating micro-lens array mold made of cemented carbide (Polycrystalline diamond tools and mold surface roughness)

Fabrication of SiC concave microlens array mold based on microspheres self-assembly

Microlens array (MLA) is applied widely in imaging, lighting, and sensing fields. Among the many techniques for MLA fabrication, molding is considered to be a mass-production method with low-cost and good accuracy. Silicon carbide (SiC), which has a high mechanical hardness, high thermal conductivity, and low thermal expansion coefficient, is an ideal mold material. But to fabricate microlens array on SiC by the mechanical method is a challenging issue because of its high brittleness and hardness. In this study, a novel method is proposed and developed, in which we use self-assembled silica microspheres monolayer as a template for SU8 resist patterning, subsequently producing concave MLAs in the SiC substrates by Inductively Coupled Plasma (ICP) etching. Two sizes of hexagonal MLAs (diameter Φ11 μm and height 0.8 μm, diameter Φ33 μm and height 2.9 μm, respectively) have been created and tested to possess good imaging performance.

Manufacturing of 3D Microlens Array Mold on Bulk Metallic Glass by Self-Aligned Multi-Ball Hot Embossing

Bulk metallic glasses (amorphous alloys) exhibiting both high hardness and extraordinary thermoplastic forming ability, are perfect mold material for 3D optical microlens array. However, conventional 3D micro fabrication for metallic glass requires Si templates which are extremely costly and eco-unfriendly to machine and demold. In this study, we report a green and low-cost self-aligned multi-ball hot embossing process to create 3D micro dimple array on metallic glass by replicating spherical crowns of self-aligned precision balls. A 3D metallic glass micro dimple array with aperture of 867.2 μm and surface roughness of 8.2 nm was fabricated in 30 s. Different from conventional hot embossing that material full-filled into mold, the new process enables micro dimple fabrication with different specifications by freely controlling the down displacement of balls. Therefore, the relationship between down displacement and depth of micro dimple was revealed through exploring the material flow behavior by a verified FE model. In addition, to test the feasibility of the fabricated metallic glass mold, a PMMA compound eye structure was embossed and its focusing and imaging performance were subsequently evaluated. This research demonstrates a new manufacturing process for rapid fabrication of 3D microlens arrays metallic glass mold in a neat and economic way.

Manufacturing of a microlens array mold by a two-step method combining microindentation and precision polishing.

A novel two-step method for manufacturing microlens array molds by combining microindentation and precision polishing is proposed. Compared with conventional manufacturing methods, such as single-point diamond turning, this two-step method, as an alternative method, presents great advantages on cost and flexibility on spherical microlens array mold fabrication. Various curvatures of radii and arrangements for microlens array molds can be fabricated in the same way. In this paper, a hexagonal microlens array with 1.58 mm curvature radius was demonstrated to prove the feasibility of the proposed method. First, a large number of precise steel balls were organized in hexagonal arrangement and pressed into the mold's surface to generate multiple microdimples. Second, the pileups around the microdimples were removed from the mold surface by precision polishing. The geometrical accuracy and surface quality were investigated by an optical surface profiler. The measurement indicated that, compared with the initial surface, the surface inside the dimple had significantly higher hardness and better surface quality than that of the steel balls. Then the microlens array on the mold was further replicated to poly(methyl methacrylate) substrates by a precision compression molding process. The experimental results showed that the fabricated mold and the polymer replicas have high fidelity, great uniformity, and good surface roughness. The proposed two-step, low-cost mold fabrication method can produce highly uniform microlens arrays and is therefore suitable for high-volume fabrication of precise optical elements such as integrated light-emitting diodes and other similar micro-optics.

Structural design and fabrication of polycrystalline diamond micro ball-end mill

Since cemented carbide has been used as a micro-lens array mold, its processing method and micro-tool is one of the research priorities. In order to achieve high-quality milling of cemented carbide micro-lens array, the design and fabrication of the 0.5-mm diameter polycrystalline diamond (PCD) micro ball-end mill were studied. Based on the space vector, the mathematical model of the plane cutting edge of the PCD tool is established. According to the three processing characteristics of milling, grinding, and milling grinding, the double-edged conical surface micro ball-end mill, spherical micro ball-end mill, and single-edged hemispherical micro ball-end mill were designed respectively. Based on the line-face contact model of the grinding wheel and the movement principle of the CNC tool grinder, the grinding principles and methods of the rake face and the flank face are proposed. The high precision grinding of PCD micro ball-end mills are realized. The micro-groove milling experiment of cemented carbide was conducted; the milling performance of three PCD micro ball-end mills was studied. Considering the surface morphology and roughness of the micro-groove machining, a single-edged hemispherical PCD micro ball-end mill can avoid the phenomenon of chip blocking and depositing and can also fully exert the grinding characteristics of PCD materials to obtain relatively better processing quality.

Off-spindle-axis spiral grinding of aspheric microlens array mold inserts.

A novel approach named off-spindle-axis (OSA) spiral grinding for fabricating aspheric microlens array (AMLA) mold inserts for precision glass molding (PGM) is presented. In OSA spiral grinding, three translational motions of the grinding wheel are synchronized with the rotation of the workpiece to form a local spiral wheel path for individual lens-lets. With this approach, the form accuracy of lens-lets can be compensated within sub-micrometer by means of the on-machine measurement. The determination of wheel path and form error compensation via on-machine measurement are systematically studied. A tungsten carbide mold insert with four convex aspheric lens-lets is fabricated to evaluate the grinding performance. PGM experiments are performed to produce glass AMLA using the ground insert. The experimental results indicate that both the ground and molded AMLA with homogeneous quality are achieved. The form accuracy and surface roughness of both the mold insert and the molded AMLA were less than 0.3 µm in PV and 10 nm in Sa, respectively.

Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold

The design and manufacture of cost-effective miniaturized optics at wafer level, usingadvanced semiconductor-like techniques, enables the production of reduced form-factor cameramodules for optical devices. However, suppressing the Fresnel reflection of wafer-level microlensesis a major challenge. Moth-eye nanostructures not only satisfy the antireflection requirementof microlens arrays, but also overcome the problem of coating fracture. This novel fabricationprocess, based on a precision wafer-level microlens array mold, is designed to meet the demandfor small form factors, high resolution, and cost effectiveness. In this study, three different kinds ofaluminum material, namely 6061-T6 aluminum alloy, high-purity polycrystalline aluminum, and purenanocrystalline aluminum were used to fabricate microlens array molds with uniform nanostructures.Of these three materials, the pure nanocrystalline aluminum microlens array mold exhibited auniform nanostructure and met the optical requirements. This study lays a solid foundation for theindustrial acceptation of novel and functional multiscale-structure wafer-level microlens arrays andprovides a practical method for the low-cost manufacture of large, high-quality wafer-level molds.

Injection Compression Molded Microlens Arrays for Hyperspectral Imaging.

In this work, a polymer microlens array (MLA) for a hyperspectral imaging (HSI) system is produced by means of ultraprecision milling (UP-milling) and injection compression molding. Due to the large number of over 12,000 microlenses on less than 2 cm², the fabrication process is challenging and requires full process control. The study evaluates the process chain and optimizes the single process steps to achieve high quality polymer MLAs. Furthermore, design elements like mounting features are included to facilitate the integration into the final HSI system. The mold insert was produced using ultraprecision milling with a diamond cutting tool. The machining time was optimized to avoid temperature drifts and enable high accuracy. Therefore, single immersions of the diamond tool at a defined angle was used to fabricate each microlens. The MLAs were replicated using injection compression molding. For this process, an injection compression molding tool with moveable frame plate was designed and fabricated. The structured mold insert was used to generate the compression movement, resulting in a homogeneous pressure distribution. The characterization of the MLAs showed high form accuracy of the microlenses and the mounting features. The functionality of the molded optical part could be demonstrated in an HIS system by focusing light spectrums onto a CCD image sensor.

Fabrication of microlens array on 6H-SiC mold by an integrated microcutting-etching process

Precision glass molding (PGM) is an efficient method to mass-produce glass microlens arrays with high-precision, the key to which is the high-quality mold. Silicon carbide (SiC) is considered to be an ideal mold material, but microstructures are difficult to machine in SiC because of its high hardness and strength. In this study, a microcutting-etching method involving the combination of single-point diamond cutting with inductively coupled plasma (ICP) etching is proposed to fabricate microlens arrays on 6H-SiC. The research described in this paper focuses on the use of the proposed method for controllable processing of a microlens array mold made of 6H-SiC. The microcutting process of the mask material and the etching process of the substrate material 6H-SiC are studied. The techniques to control the accuracy of the microlens array on 6H-SiC are managed by studying the cutting process, selectivity ratio and etching condition. In this manner, a ring-shaped microlens array with excellent accuracy and consistency is generated.

Fabrication and milling performance of micro ball-end mills with different relief angles

Micro ball-end milling is an efficient and feasible method for the fabrication of microlens array molds which are widely applied in the aerospace, mechanical electronics, optics, and optoelectronics fields. The geometric structure of the micro ball-end mill plays an important role in micro dimple milling performance. The relief angle, as one of the major geometric parameters of the micro ball-end mill affects the sharpness of the cutting edge and determines friction between the tool and the workpiece. To investigate effective fabrication of a micro ball-end mill with different relief angles, this paper proposes a practical grinding method for the relief face validated by a grinding experiment, in which micro ball-end mills with relief angles of 10°, 20°, and 30° are fabricated. Furthermore, to analyze the influence of the relief angle on milling performance, micro dimple milling experiments on die steel material H13 with fabricated mills are carried out. The milling force, tool wear, and machining quality are measured and analyzed. The experimental results show that the micro ball-end mill with a 20° relief angle is more appropriate for micro dimple milling compared to the other two mills.

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.

Fabrication of spherical microlens array by combining lapping on silicon wafer and rapid surface molding

Silicon is a promising mold material for compression molding because of its properties of hardness and abrasion resistance. Silicon wafers with carbide-bonded graphene coating and micro-patterns were evaluated as molds for the fabrication of microlens arrays. This study presents an efficient but flexible manufacturing method for microlens arrays that combines a lapping method and a rapid molding procedure. Unlike conventional processes for microstructures on silicon wafers, such as diamond machining and photolithography, this research demonstrates a unique approach by employing precision steel balls and diamond slurries to create microlenses with accurate geometry. The feasibility of this method was demonstrated by the fabrication of several microlens arrays with different aperture sizes and pitches on silicon molds. The geometrical accuracy and surface roughness of the microlens arrays were measured using an optical profiler. The measurement results indicated good agreement with the optical profile of the design. The silicon molds were then used to copy the microstructures onto polymer substrates. The uniformity and quality of the samples molded through rapid surface molding were also assessed and statistically quantified. To further evaluate the optical functionality of the molded microlens arrays, the focal lengths of the microlens arrays were measured using a simple optical setup. The measurements showed that the microlens arrays molded in this research were compatible with conventional manufacturing methods. This research demonstrated an alternative low-cost and efficient method for microstructure fabrication on silicon wafers, together with the follow-up optical molding processes.

Fabrication of a Micro-Lens Array Mold by Micro Ball End-Milling and Its Hot Embossing.

Hot embossing is an efficient technique for manufacturing high-quality micro-lens arrays. The machining quality is significant for hot embossing the micro-lens array mold. This study investigates the effects of micro ball end-milling on the machining quality of AISI H13 tool steel used in the micro-lens array mold. The micro ball end-milling experiments were performed under different machining strategies, and the surface roughness and scallop height of the machined micro-lens array mold are measured. The experimental results showed that a three-dimensional (3D) offset spiral strategy could achieve a higher machining quality in comparison with other strategies assessed in this study. Moreover, the 3D offset spiral strategy is more appropriate for machining the micro-lens array mold. With an increase of the cutting speed and feed rate, the surface roughness of the micro-lens array mold slightly increases, while a small step-over can greatly reduce the surface roughness. In addition, a hot embossing experiment was undertaken, and the obtained results indicated higher-quality production of the micro-lens array mold by the 3D offset spiral strategy.

Fabrication of Microlens Array Molds by Indentation Method Using Sintered Porous Metal

Fabrication of Microlens Array Molds by Indentation Method Using Sintered Porous Metal

Replication of optical microlens array using photoresist coated molds.

A cost reduced method of producing injection molding tools is reported and demonstrated for the fabrication of optical microlens arrays. A standard computer-numerical-control (CNC) milling machine was used to make a rough mold in steel. Surface treatment of the steel mold by spray coating with photoresist is used to smooth the mold surface providing good optical quality. The tool and process are demonstrated for the fabrication of an ø50 mm beam homogenizer for a color mixing LED light engine. The acceptance angle of the microlens array is optimized, in order to maximize the optical efficiency from the light engine. Polymer injection molded microlens arrays were produced from both the rough and coated molds and have been characterized for lenslet parameters, surface quality, light scattering, and acceptance angle. The surface roughness (Ra) is improved approximately by a factor of two after the coating process and the light scattering is reduced so that the molded microlens array can be used for the color mixing application. The measured accepted angle of the microlens array is 40° which is in agreement with simulations.

2501 New Fabrication Method of Gapless Biaxial Microlens Arrays for a Light Control Film

A new method for producing gapless biaxial microlens array is proposed for a dual-directional light-control film with a high fill factor. Three steps in this production technique are included for formations of semiellipsoid microlens array with low fill factor to be caulked into gapless biaxial microlens array mold inserts. First, two photoresists with different melting temperatures method will be used to obtain the semiellipsoid microlens array with low fill factor. Second, the air-pressure electroforming caulking is applied to fill the gaps between adjacent lenses after the thermal reflow. Third, the secondary master mold of gapless biaxial microlens array is fabricated for hot embossing process to replicate the array pattern onto PMMA sheet. A gapless biaxial microlens array with long and short axes is developed to change the vertical and horizontal light emitting angles. Based on the simulated and experimental results, the horizontal light emitting angle can be increased to 30° and the vertical one is reduced to 15° by using the biaxial microlens array with high fill factor as the optical scattering film, to effectively enhance optical uniformity for the application of a rear projection display.