Glass Microlens Arrays Research Articles

Deformation mechanism of glass microlenses and microlens arrays in contactless hot embossing

AbstractContactless hot embossing has been demonstrated to possess the potential for cost‐effective production and precise mounting concepts in fabricating glass microlenses and microlens arrays due to the reduced difficulty of mold fabrication and the possibility of obtaining self‐aligned assemblies. This study aims to provide experimental evidence for understanding the forming mechanism of glass microlenses and microlens arrays in the contactless hot embossing process. The effects of process parameters, diameter and position of the micro‐holes, hole diameter, and pitch of the micro‐hole array mold on the filling deformation of glass in contactless hot embossing were comprehensively investigated. It is found that placing the micro‐hole farther away from the mold center renders decrease in both filling height and tip curvature but increase in the eccentricity of the embossed glass microlens. As a result, the formed glass microlens array shows a nonuniform distribution of filling height and tip curvature. Furthermore, reducing the pitch of micro‐hole array mold can significantly improve the uniformity of formed microlens array. Based on these experimental results, the forming mechanism of microlenses and microlens arrays in contactless hot embossing process is summarized. Finally, a glass microlens array with decent uniformity in the center area was hot embossed by using a SiC micro‐hole array mold.

Conformal polishing of a glass microlens array through oxyhydrogen flame-induced viscoelasticity flow at nano/microscale

Glass microlens arrays with small unit sizes and high integration capacities are widely utilized in various optical systems, but the femtosecond laser milling produces microscale residual steps and nanoscale rough topography on microlens surface. Accordingly, an oxyhydrogen flame polishing is proposed after femtosecond laser milling, aiming to efficiently fabricate the ultrasmooth surface without damaging shape accuracy by triggering a viscoelasticity flow at nano/microscale. Through simultaneous modelling of the polishing process, a large thermal power results in a high viscoelasticity flow velocity for nanoscale topography smoothing, followed by microscale step melting. Then, a single-point polishing experiment is conducted to determine the critical transition temperatures for nanoscale smoothing and microscale melting. It is further experimentally applied to achieve an ultrasmooth surface of cylindrical microlens array with its roughness less than 0.2 nm and no hydroxyl introduction. This process also effectively repairs subsurface damage and reduces the form error to ±0.5 μm, improving the surface quality and shape accuracy. Similar results are also observed for the polishing of spherical microlenses. In conclusion, the method of combining femtosecond laser milling and oxyhydrogen flame polishing holds promise for high-efficiency and high-quality fabrication of glass components with micro features.

An Ultraviolet-Lithography-Assisted Sintering Method for Glass Microlens Array Fabrication.

Glass microlens arrays (MLAs) have tremendous prospects in the fields of optical communication, sensing and high-sensitivity imaging for their excellent optical properties, high mechanical robustness and physicochemical stability. So far, glass MLAs are primarily fabricated using femtosecond laser modification assisted etching, in which the preparation procedure is time-consuming, with each concave-shaped microlens being processed using a femtosecond laser point by point. In this paper, a new method is proposed for implementing large-scale glass MLAs using glass particle sintering with the assistance of ultraviolet (UV) lithography. The glass particles are dispersed into the photoresist at first, and then immobilized as large-scaled micropillar arrays on quartz glass substrate using UV lithographing. Subsequently, the solidified photoresist is debinded and the glass particles are melted by means of sintering. By controlling the sintering conditions, the convex microlens will be self-assembled, attributed to the surface tension of the molten glass particles. Finally, MLAs with different focal lengths (0.12 to 0.2 mm) are successfully fabricated by utilizing different lithography masks. Meanwhile, we also present the optimization of the sintering parameter for eliminating the bubbles in the microlenses. The main factors that affect the focal length of the microlens and the image performance of the MLAs have been studied in detail.

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.

Rapid fabrication of highly integrated and high numerical aperture chalcogenide glass microlens arrays

The fast growing of infrared imaging system for non-contact thermography, night surveillance and night driving assistance demands highly integrated optical lenses with miniaturization feature for performance improvement. Herein, we fabricated refractive type of microlens arrays (MLAs) with high numerical aperture (NA) and high integration on commercial chalcogenide glass (As2Se3) by combining photoresist thermal reflow and soft mold imprinting process. PDMS-type concave MLAs (PDMS-MLAs) soft mold is prepared by transfer printing process on photoresist (PR) MLAs fabricated by standard UV exposure and thermal reflow. The microlens on PDMS-MLAs could be further transferred to the surface of As2Se3 glass through the hot imprinting technique, forming chalcogenide glass-based MLAs (ChG-MLAs). Morphology detection reveals that microlens in the MLAs has an average aperture and sag height of approximately 13.4 and 1.7 μm. Moreover, the optical performance investigation demonstrated that the ChG-MLAs with 700 × 700 array size (10.5 × 10.5 mm) showed good structure uniformity and focusing capability, and through the FDTD simulation, it further evidenced that focal lengths of 6.3–8.1 µm with high numerical aperture of 0.64–0.73 (wavelength range: 3–5 μm) was achieved.

Quasi-periodic micro-lens array via laser-assisted wet etching

A close-packed micro-lens array (MLA) is widely used not only in novel optical systems but also in various engineering fields, such as semiconductors and display devices. In this paper, we present a simple and efficient method for fabricating MLAs on a glass substrate via laser ablation with a single femtosecond laser pulse and chemical wet etching in a hydrofluoric acid solution. The shapes of concave micro-lenses were optimized for laser pulse energy and etching time, and ∼70 000 micro-lenses with 15 µm diameter were formed. The shape of the micro-lens varies significantly with respect to the etching time in accordance with an initial ablated area (or laser energy), which is a key feature in fabricating micro-lenses of several tens of μm. Using the glass MLA as a mold, a polydimethylsiloxane convex-plano lens array was replicated, and the performance of the optical imaging and beam integrator was then examined.

Fabrication of high aspect-ratio aspheric microlens array based on local spiral diamond milling

Ultraprecision mold machining combined with precision glass molding is the most promising technology for the mass production of glass microlens arrays (MLAs). However, fabricating an MLA with a high aspect-ratio (AR) (ratio of height to aperture), submicron scale profile accuracy, nanoscale surface finish, and good consistency remains a considerable challenge. A local spiral diamond milling (LSDM) method is proposed to generate a high AR MLA on a nickel-phosphorous (Ni-P) plated mold. A spiral toolpath generation algorithm that considers the tool edge radius and performs profile error (PV) compensation is developed. Each lenslet at any local position can be precisely machined by controlling the servo motions of the three translational axes. Then, the entire MLA can be generated by shifting the toolpath to go through the centers of all lenslets according to the MLA layout. The tool parameters are optimized to avoid local and global tool interference during machining. The generated toolpath provides steady slide axis movements to avoid dramatic changes in cutting speed and acceleration. An MLA with an AR of 0.166 is fabricated. The results show that the fabricated mold and the molded lenses have a profile accuracy (PV) of less than 1 μm and a surface roughness (Ra) of less than 14 nm. The cross-sectional profiles indicate that the fabricated MLA has high machining consistency.

Boundary effect of the glass microlens array in partial-filling hot embossing

Partial-filling hot embossing (PFHE) is a flexible and rapid method to fabricate the glass microlens array (GMLA) with high efficiency and mass production. However, the sidewall of the microhole mold strongly affects the deformation modes and optical performance of the GMLA during PFHE. The microholes of a mold fabricated by electrical discharging machining (EDM) and focused ion beam (FIB) were employed for hot glass embossing to reveal the boundary effect of the GMLA in PEHE. The detailed characterizations were implemented for 3 × 3 microhole arrays by white-light interfering profilometer and scanning electron microscope to highlight the anomaly in the sidewall topography of different mold cavities. Also, the three-dimensional topography and optical quality of the GMLA were measured by a white-light interfering profilometer, parallel light system, and ultraviolet spectrophotometer. Experimental results indicated that the sidewall of the mold cavity significantly affected the filling modes, appearance, and optical quality of the GMLA. The phenomenon of sidewall slip occurs at the edge of the GMLA in the FIB-mold. The sidewall quality of the EDM-mold strongly affects deformation modes and light transmittance of the GMLA. The work provides a better understanding of fabricating the GMLA in PFHE.

Tungsten carbide molds for precision glass molding process: Mechanism of high-temperature degradation

Tungsten carbide (WC) is a well-established material used in glass molding field due to its low thermal expansion and high compressive strength at elevated temperatures. During glass molding process, the WC mold has to withstand thousands of thermo-mechanical coupled interactions in the aerobic environment. Understanding the degradation mechanism of WC substrate during precision glass molding is very important for the design and preparation of protective coatings and the optimization of glass molding process. In this study, surface failure analysis of WC substrate was carried out using barrel compression test and ball-to-disk friction test. The microstructures, element compositions, surface morphology of the WC substrates were measured by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and white light interferometer (WLI). Damaging phenomena such oxidation, interdiffusion, spallation, glass sticking, and surface roughening on the WC substrate were observed. The experimental results showed that the WC substrate degraded very rapidly at above 600 °Cand air environment. The fundamental degradable reason of WC substrate is the oxidation, which directly leads to the decrease of the material strength, the aggravation of the wear, occurring of the glass sticking, and the increase of surface roughness. Furthermore, the WC mold microhole array without protective coatings can be used to fabricate the glass microlens array in a suitable glass molding environment. This work provides engineering guidance for the fabrication of glass optics during glass molding process.

Flexible fabrication of optical glass micro-lens array by using contactless hot embossing process

Abstract This paper reported the fabrication of optical glass micro-lens array (MLA) by using contactless micro-embossing technology. Firstly, the micro-hole array in the tungsten carbide (WC) substrate was made by the micro-electrical discharge machining (micro-EDM). Then, to improve the oxidation resistance, anti-sticking properties and chemical inertness of mold surface, Cr-W-N coating was deposited on the mold insert by plasma-enhanced sputtering technique. Finally, controllable MLA were fabricated with the coated EDM-mold by using the precision glass molding machine from Shenzhen University (SZU’s PGMM30). The surface roughness, three-dimensional morphology and optical performance of glass MLA were measured by ContourGT-X 3D optical profiler, ultraviolet spectrophotometer, and optical imaging system, respectively. The experimental results indicated that the contactless hot embossing process has the capability of fabricating high-quality and multi-scale glass MLA. This work provides an effective method for the fabrication of optical glass MLA.

CO2 laser thermal reflow shaped convex glass microlens array after Bessel picosecond laser inscribing and hydrofluoric acid processing.

In this paper, a convex micro-glass lens array fabrication process that utilizes ${{\rm CO}_2}$CO2 laser thermal reflow in the Bessel picosecond laser inscribing and hydrofluoric acid processed micro-glass pillars array is presented. The Bessel picosecond laser permits high tolerance and precise micro-pillar fabrication. In the thermal reshape process, the ${{\rm CO}_2}$CO2 laser power, relative defocus length, and scanning velocity are three crucial parameters to the microlens array's focal length. By using this method, microlens arrays with focal length ranging from several tens of micrometers to several hundred micrometers can be created. This research provides another way to fabricate convex micro-glass lens arrays with several hundred micrometers focal length in good utility.

Replication of high refractive index glass microlens array by imprinting in conjunction with laser assisted rapid surface heating for high resolution confocal microscopy imaging

In a multi optical probe confocal imaging system utilizing a microlens arrays as an objective lens, a high numerical aperture is required to improve resolving power. Glass microlens arrays are suitable for high-resolution imaging since they provide outstanding optical properties with a high refractive index. We demonstrated the rapid fabrication of microlens arrays on a high refractive index optical glass substrate via laser assisted thermal imprinting. The optical performance of the fabricated glass microlens arrays were evaluated and compared to that of a polymer microlens. In contrast to the polymer, the real image afforded by, and the calculated resolution of, the imprinted glass microlens arrays were significantly better, at about 0.73 µm compared to the polymer (∼1.56 µm). Our results reveal the considerable potential of direct thermal imprinting as a rapid, single-step, low cost fabrication method for replication of glass microlens array of high dimensional accuracy affording excellent optical performance.

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.

Replication of a glass microlens array using a vitreous carbon mold.

A low-cost fabrication method for a high-surface-quality glass microlens array (MLA) was proposed using a glass molding technique with a vitreous carbon (VC) mold. A VC mold with a high-surface-quality MLA cavity was fabricated, and the glass MLA with a root mean square surface roughness of 4.59 nm was replicated using the VC mold. To obtain the glass MLA with high replication quality, the effects of molding conditions were examined. The surface quality was not degraded during the proposed VC mold fabrication method and glass molding process. The focused beam spot of the glass molded MLA was analyzed; it showed a diffraction-limited characteristic of the glass molded MLA.

Fabrication method of quartz aspheric microlens array for turning mask

In order to solve the two difficult problems of the poor processing controllability and the low surface accuracy of quartz aspheric microlens array processing, a fabrication method of quartz aspheric microlens array for turning mask is proposed. This method mainly uses single point diamond turning technology and reactive ion etching technology, studies the turning and etching properties of the mask material, and optimizes the mask material by experiment. Finally, the fabrication of an aspherical glass microlens array with an area of 5 mm×5 mm was carried out. The experimental results are compared with the expected parameters. The analysis shows that the error root mean square of the quartz glass component is 1.155 nm, and the surface accuracy error is 0.47%.

Fabrication of the glass microlens arrays and the collimating property on nanolaser

The influence of the glass microlens array on the collection efficiency and beam collimating with the ZnO/GaN nanoheterojunction nanolaser is investigated. Ultraviolet random lasing behavior from the ZnO/GaN nanoheterojunction array has been demonstrated with both optical and electrical pumping. The refractive microlens arrays are designed and fabricated on the glass slide using the microfabrication procedures and integrated on the nanolaser. We show that, for the nanolaser with divergent output beams, the introduction of glass microlens arrays reduces the beam divergence and increases transmission. The detected optical power is enhanced by more than 1.8 times for the nanolaser integrated with glass microlens arrays. The drastic increase in the optical collection efficiency is resulted from the laser beam collimating.

Soda-lime glass microlens arrays fabricated by laser: Comparison between a nanosecond and a femtosecond IR pulsed laser

We present the manufacturing of microlens arrays on soda-lime glass substrates by using two different IR pulsed lasers: a nanosecond Nd:YVO4 laser (1064nm) and a femtosecond laser based on Ytterbium crystal technology (1030nm). In both cases, the fabrication technique consists of the combination of a direct-write laser process, followed by a post-thermal treatment assisted by a CO2 laser. Through the analysis of the morphological characteristics of the generated microlenses, the different physical mechanisms involved in the glass ablation process with a nanosecond and a femtosecond laser are studied. In addition, by analyzing the optical features of the microlenses, a better result in terms of the homogeneity and quality of the spot focuses are observed for those microlenses fabricated with the Nd:YVO4 nanosecond laser. Microlens arrays with a diameter of 80 and 90µm were fabricated.

Enhanced light extraction from UV LEDs using spin-on glass microlenses

In this paper, we present a cost-effective method for fabricating spin-on glass (SOG) microlens arrays (MLAs) on ultra-violet light-emitting diodes. The SOG MLA is formed using thermal reflow molds and multiple replication processes, which can reduce the cost of the solution process. In this paper, we fabricate SOG MLA of different sizes, where the diameter of each microlens is approximately 50, 100, 150, and 200 μm. In each case, the light extraction efficiency is improved by 21.86%, 14.01%, 10.35%, and 7.31%, respectively. We also discuss the effects of different-shaped SOG microlenses, namely circular, square, and hexagonal. The light extraction efficiency is improved by 7.31%, 9.60%, and 13.80% for the circular, square, and hexagonal SOG MLAs, respectively. By applying an optimized lens pattern, an increase in light extraction efficiency of 21.86% is achieved.

Hot-rolled embossing of microlens arrays with antireflective nanostructures on optical glass

sGlass is an excellent material for optical devices requiring optical and mechanical properties better than those of a polymer. Because traditional micro- or nano-fabrication on glass substrates is typically slow and costly, the fabrication of micron- or nanometer-scale structures on glass has been a challenge. For a novel continuous fabrication of glass microlens arrays with antireflective nanostructures (AR-MLA), we have designed and implemented a hot-rolled embossing facility with synergetic heating. The mold of a hybrid micro/nanostructure was fabricated in two steps. First, a micron-scale lens cavity was embossed on a nearly pure aluminum sheet with an electroplated nickel mold; an anodic aluminum-oxide process was used to create a nanostructure on the microlens array cavity. The mold was then mounted on a roller and used in a hot embossing machine to fabricate AR-MLA on optical glass. The optical properties of the fabricated microlens with nanostructures have been verified. The image of focused spots of fabricated AR-MLA shows an effective converging ability, and the reflectance at 550 nm was decreased from 7.63 % to 0.53 % with the fabricated nanostructures. This proposed technique has proven its potential for hot-rolled embossing of glass components with micro/nanostructures.

Large-scale high quality glass microlens arrays fabricated by laser enhanced wet etching.

Large-scale high quality microlens arrays (MLAs) play an important role in enhancing the imaging quality of CCD and CMOS as well as the light extraction efficiency of LEDs and OLEDs. To meet the requirement in MLAs' wide application areas, a rapid fabrication method to fabricate large-scale MLAs with high quality, high fill factor and high uniformity is needed, especially on the glass substrate. In this paper, we present a simple and cost-efficient approach to the development of both concave and convex large-scale microlens arrays (MLAs) by using femtosecond laser wet etching method and replication technique. A large-scale high quality square-shaped microlens array with 512 × 512 units was fabricated.The unit size is 20 × 20 μm² on the whole scale of 1 × 1 cm². Its perfect uniformity and optical performance are demonstrated.