Glass Microlens Research Articles

An integrated hot embossing and thermal reflow method for precision manufacture of plano-convex glass microlens arrays

Plano-convex glass microlens arrays (MLAs) have broad applications in imaging, sensing, illumination, and communication systems. This paper proposed a three-step method to precision manufacture of plano-convex glass MLAs, which involves fabrication of SiC microhole arrays (MHAs), hot embossing of glass micropillar arrays (MPAs) and thermal reflow of glass MLAs. The effects of embossing temperature, force, and time on the replication accuracy of glass MPAs are evaluated. Subsequently, the glass MPAs with decent replication accuracy were subjected to thermal reflow experiments for studying the effects of reflowing temperature and reflowing time on the geometric features of formed glass MLAs. It is found that the height and tip curvature of reflowed glass microlenses can be controlled by adjusting the reflowing temperature and time. The warpage amplitudes and mean birefringence of most reflowed glass substrates are less than 5 μm and 27 nm/cm, respectively. Furthermore, the reflowed glass microlens arrays shows a decent uniformity in an area with a diameter of ∼4 mm. Finally, the feasibility of the integrated hot embossing and thermal reflow method in producing glass nano-lens arrays is also demonstrated. As a result, the hybrid forming technology that combines hot embossing with thermal reflow not only avoids the difficulties of fabrication of MLA and NLA mold inserts, but also possesses the advantages of high efficiency and low cost, which is expected to be a promising mass production technology for glass micro/nano-lens arrays.

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.

Experimental and numerical simulation of hot embossing for fabrication of glass microlens array

In this work, a hot embossing process for replication of glass microlens array (MLA) on optical glass substrate is proposed. Both numerical simulation and experimental validation were employed to investigate the filling behavior of the glass material into the microstructure mold cavities. The finite element method (FEM) was carried out using Ansys software after the establishment of the model layout and boundary condition. For validation, the K-PG375 optical glass samples were pressed at a temperature above the glass transition temperature Tg, using a home-made hot embossing setup. Then, the correlation between two- dimensional (2D) FEM analysis and experiments were established, which provide a guideline for understanding of the glass filling behavior in the viscoelastic region. The replication quality of the embossed glass was confirmed using scanning electron microscopy (SEM) and atomic force microscope (AFM). Overall, the simulation result gave a very useful insight to predict the optimum processing condition for the thermal replication process primarily the temperature, pressure and holding time.

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.

Pneumatically Actuated Thin Glass Microlens for On-Chip Multi-Magnification Observations

This paper presents a self-contained micro-optical system that is magnification-controlled by adjusting the positions of the microlens in the device via pneumatic air pressure. Unlike conventional dynamic microlenses made from a liquid or polydimethylsiloxane (PDMS) that change their shapes via external actuation, this system combines a fixed-curvature glass microlens, an inflatable PDMS layer, and the external pneumatic air pressure supply as an actuator. This device showed several advantages, including stable inflation, firm structure, and light weight; it achieved a larger displacement using the glass microlens structure than has been reported before. This fixed-curvature microlens was made from 120 µm-thick flat thin glass slides, and the system magnification was manipulated by the deflection of a 100 µm-thick PDMS layer to alter the distance from the microlens to the microfluidic channel. The system magnification power was proportional to the air pressure applied to the device, and with a 2.5 mbar air pressure supply, a 2.2X magnification was achieved. This optical system is ideal for combining with high resolving power microscopy for various short working distance observation tasks, and it is especially beneficial for various chip-based analyses.

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.

Correction of spherical surface measurements by confocal microscopy

Refractive microlenses are nowadays widely used in optical systems. Characterizing their surface is essential to ensure their quality and to optimize their fabrication process. This is realized by optical surface profilers thanks to their vertical resolution, short measurement time and areal information. However, when measuring non-flat surfaces, errors appear caused by aberrations of the microscope objective used in such systems, which significantly limit the achievable quality of the manufactured spherical surfaces. Approaches have been proposed to tackle these errors, but none of them demonstrated its validity for measurements of high quality microlenses. In this work, we demonstrate that the surface error depends on the surface position within the field of view of the microscope objective and on the surface slope. We then explain how to record the value of this error experimentally: this can be done by measuring a reference ball placed at different positions in the field of view. We finally use a machine learning algorithm to fit the experimental data in order to correct subsequent measurements. We apply this approach to measurements performed by a 20× numerical aperture 0.6 microscope objective of a confocal microscope. The effectiveness of the proposed method is demonstrated by showing that the surface error corresponds to a RMS wavefront error of λ/7 before correction and of λ/50 after correction for glass microlenses used in the visible range. This method thus allows the use of high numerical aperture microscope objectives for an accurate characterization of microlenses. Likewise, the fabrication capability of microlenses in terms of slope and quality is greatly extended, which is especially important for aspheres or freeforms.

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.

Experimental and simulated analysis of glass deformation and filling modes during partial-filling hot embossing process

In the partial-filling hot embossing process (PFHE), the stress distribution and geometry of a glass micro-lens influence its optical performance. The deformation behavior of glass is very complex during PFHE. In order to investigate the final shape and stress distribution of glass micro-lens during PFHE, both simulations and experiments were conducted. The experiment results revealed that different glass squeeze flow conditions gave rise to various cavity filling modes, including single peak, dual peak, and sag. The simulations were conducted to analyze the deformation and filling modes of glass under various mold geometries and processing conditions by using Abaqus. The results showed that variations in the mold geometry and process parameters during PFHE led to different flow driving mechanisms. The flow behaviors of the glass, which is a highly viscoelastic state above the transition temperature, have been proved to influence the profile of micro-lens. Especially, the single peak micro-lens showed a lower surface roughness than the other two filling modes. This work provided a better understanding for the relationship between design and fabrication of glass micro-lens during PFHE.

Hybrid refractive-diffractive microlenses in glass by focused Xe ion beam

The combination of refractive and diffractive components in a single optical element provides miniaturization of optical systems and enhancement of their performance. Thus, hybrid singlet lenses with diffractive structures added on top of the refractive curved surface were shown to have reduced chromatic and spherical aberration. Optical systems based on such hybrid lenses have reduced dimensions as they require fewer lenses for aberrations-correction. Diffractive elements provide additional possibilities of light manipulation and enable the realization of miniaturized multifocal systems, spectrometers, and other devices. Glass hybrid lenses are typically realized by diamond turning or glass moulding. These techniques, however, are not applicable for the fabrication of lenses in brittle materials or microlenses (hundreds of micrometers in diameter or less). On the other hand, direct writing techniques, such as focused ion beam (FIB) milling (typically Ga), offer a high resolution and flexibility of patterning on curved lens surfaces made of a great variety of materials. The disadvantages of FIB milling are its slow speed and Ga implantation that may alter or degrade the optical performance of fabricated components. FIB systems based on high brightness plasma ion sources provide more than an order of magnitude increase in milling rates with noble gas ions (e.g., Xe) compared with Ga FIBs. Here, the authors demonstrate the feasibility of rapid, direct milling of hybrid refractive-diffractive microlenses in glass using >60 nA of Xe ion current. Microlenses with up to 300-μm diameter were milled and diffraction gratings were realized on top of their curved surfaces. The performance of the lenses was characterized by mapping the transmitted intensity at different positions. Due to the introduction of diffraction gratings on the surface of the lenses, their optical performance is modified with the emergence of additional focal spots spatially separated by distances consistent with the theoretical and simulated values. The results indicate the applicability of the plasma focused ion beam systems for rapid fabrication of high-quality hybrid optical elements directly in hard substrates.

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.

Technological Platform for Vertical Multi-Wafer Integration of Microscanners and Micro-Optical Components.

We describe an original integration technological platform for the miniaturization of micromachined on-chip optical microscopes, such as the laser scanning confocal microscope. The platform employs the multi-wafer vertical integration approach, combined with integrated glass-based micro-optics as well as micro-electro-mechanical systems (MEMS) components, where the assembly uses the heterogeneous bonding and interconnecting technologies. Various heterogeneous components are disposed in vertically stacked building blocks (glass microlens, MEMS actuator, beamsplitter, etc.) in a minimum space. The platform offers the integrity and potential of MEMS microactuators integrated with micro-optics, providing miniaturized and low cost solutions to create micromachined on-chip optical microscopes.

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.