Fabrication of Microlens Array and Its Application: A Review
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Microlens arrays are the key component in the next generation of 3D imaging system, for it exhibits some good optical properties such as extremely large field of view angles, low aberration and distortion, high temporal resolution and infinite depth of field. Although many fabrication methods or processes are proposed for manufacturing such precision component, however, those methods still need to be improved. In this review, those fabrication methods are categorized into direct and indirect method and compared in detail. Two main challenges in manufacturing microlens array are identified: how to obtain a microlens array with good uniformity in a large area and how to produce the microlens array on a curved surface? In order to effectively achieve control of the geometry of a microlens, indirect methods involving the use of 3D molds and replication technologies are suggested. Further development of ultraprecision machining technology is needed to reduce the surface fluctuation by considering the dynamics of machine tool in tool path planning. Finally, the challenges and opportunities of manufacturing microlens array in industry and academic research are discussed and several principle conclusions are drawn.
KeywordsMicrolens array Ultraprecision machining 3D image system MEMS
Natural compound eyes are extensively prominent in the biological optical systems of many diurnal insects or deep-water crustaceans, and such eyes consist of a mosaic of hexagonal ommatidia that work as tiny optical units [1, 2, 3, 4]. Unlike single aperture eyes, natural compound eyes are characterized as having extremely large field of view angles, low aberration and distortion, high temporal resolution and infinite depth of field [3, 4]. However, the compound eye image system has intrinsic low resolution and sensitivity . The image resolution is subject to both the number and size of the ommatidia. If the image resolution of compound eyes increases to the same level as the human aperture eye, the radius of the overall lens would be at least 1 meter .
Although the image resolution and sensitivity of compound eyes are relatively low, microlens arrays, the artificial counterpart of natural compound eyes, still have crucial potential in a variety of applications in image systems, under the condition that high-resolution is not always required. For example, microlens array are more suitable in the extremely miniaturized imaging systems and 3D light field cameras . In addition, the high quality microlens arrays were applied in color imaging systems, 3D image acquisition systems and fingerprint identification systems [7, 8, 9].
Since the 1980s, the fabrication of microlens array is realized by different methods, such as Micro-electromechanical Systems (MEMS) based technologies [10, 11, 12, 13, 14, 15, 16, 17] and ultraprecision machining technologies [18, 19, 20]. However, little work has been focused on the comparison of these method in terms of the surface finish, form error and the efficiency of production. One of the major challenge in the fabrication of the microlens array is the fabrication and assembly accuracy in a large area [19, 20, 21]. As the image resolution of a compound eye optical system is increased with the number of microlens and the radius of each microlens unit, enlarging the overall size of a microlens array can make up the deficiency. However, to achieve the required uniformity in a large area is very difficult . Another challenge for microlens fabrication is producing microlens array on a flexible layer or a curved surface. The curved artificial compound eye is similar to the eye of the fruit fly Drosophila, which is more compatible and has a larger Field of View (FOV) . Such curved compound eye imaging systems may have great potential in terrestrial aerial vehicles, visual reality systems, surveillance etc. Image detectors, such as conventional complementary metal-oxide-semiconductor (COMS) and charge-coupled device (CCD), are planar and not suitable for curved image systems. Recent developments  in flexible technologies enable the formation of microlens arrays on flexible substrates which are bent to a spherical surface. Similar to the problem in the fabrication of planar compound eye, the requirement of precise alignment of the photodetector and microlens is hard to achieve.
In the light of the above, this paper aims to review the latest research on the progress of microlens array fabrication technologies. In Section 2, the operation principle of the compound eye is briefly introduced to provide background for the design of microlens array. In Section 3, the state-of-art technologies, including the direct and indirect methods for fabricating microlens array are reviewed and compared. Section 4 describes the applications of microlens array. Finally, the challenges and opportunities of manufacturing microlens arrays in industry and academic research are discussed and several principle conclusions are drawn in Section 5.
2 Principle of Compound Eyes
Surface roughness is another important parameter in evaluating the optical performance of a lens. And it is strongly affected by the fabrication process. Optics with large surface roughness may suffer from scattering issues, decreasing the efficiency of the contrast and light collection . The array uniformity is of great importance in the imaging system, especially when the area of microlens array is large, and light retrace is needed for further process. The array uniformity can be described by the standard deviation of the height and the radius of curvature .
3 Fabrication Methods
The fabrication methods for microlens arrays are categorized into direct methods and indirect methods. The direct method does not need to fabricate a mask or a mold insert with concave 3D microstructures. The shape of microlens is usually formed based on the surface tension effect when the material is in a thermoplastic state or liquid state resulting a super smooth surface (arithmetic average roughness R a less than 1 nm) [27, 28, 29]. More importantly, these methods involve simple and cost-effective processes, which are preferred in industry. However, it is still very difficult to control the microlens precision because the geometry of the microlens is only determined by the controlling parameters such as temperature, wettability, pressure and process time. The indirect methods need to fabricate the mold with concave microlenses and produce the final lenses by replication technologies, such as hot embossing, compact molding and injection molding. Using the indirect method, the shape of microlens array can be well-controlled but the process is complex.
3.1 Direct Fabrication Methods
3.1.1 Thermal Reflow Method
3.1.2 Microplastic Embossing Method
3.1.3 Microdroplet Jetting Method
3.2 Indirect Method
In contrast with direct methods for the fabrication of compound eye microlens arrays, indirect methods need a mold with a concave spherical microlens. The final microlens array is produced by replication technologies such as injection molding, hot embossing or UV molding. The key to the indirect methods is how to generate concave microlenses with precise geometry. The potential technologies are divided into two categories, i.e., MEMS based technologies and ultraprecision machining technologies.
3.2.1 MEMS Based Methods
To overcome this problem, direct writing technologies have been developed. For example, femtosecond laser wet etching was developed in which the patterns are directly generated on the wafer by laser processing. Then, in the wet etching process, the concave microlens are formed as the induced region has a higher selective etching rate than the other parts. Other examples of direct writing techniques includes focused ion beam writing [47, 48] and electron beams writing [49, 50].
Direct laser writing can be used to produce patterns on a spherical substrate. Curved artificial compound eyes (CACEs) are realized by this method . However, the optical system is hard to be miniaturized because it needs overlapping of the concave and the convex bulk lenses (the radius of the spherical lenses is 40 mm). Another approach to fabricate the CACEs is by transforming the 2D microlens-pattern films into a 3D shape by the negative pressure deformation process [52, 53, 54, 55]. However, the uniformity of the microlenses need to be improved because in the negative pressure deformation process, some microlens at the marginal areas may be damaged .
3.2.2 Ultraprecision Machining Methods
Ultraprecision machining technologies, such as diamond micro-milling and single point diamond turning (SPDT) are effective methods to fabricate microstructures and nanostructures with good uniformity in a large area [18, 21, 56, 57, 58]. Such ultraprecision technologies are integrated into the process chain for mass fabrication of microlens arrays. Ball-ending milling usually utilizes half-arc single crystal diamond tools to removal metallic materials [56, 57]. Metallic materials such as OFHC-Cu, AlMg3 and NiP can be processed. The achieved surface roughness (Ra) is as low as 5 nm. However, this method needs to machine the microlenses one by one, which severely extends the operation time and increases the cost. To our knowledge, the bottom of the each concave microlens is affected by the alignment error (around 1 μm) between the vertex of the cutting edge and the spindle axis. With regard to this, the single point diamond turning (SPDT) method is performed to produce microlens arrays with high quality by a slow slide servo [18, 19, 58] or a fast tool servo [21, 59, 60].
Ultraprecision machining was also applied to fabricate 3D compound eye lens arrays [63, 64]. It was reported that 601 individual compound eye microlenses (aperture of 0.58 mm) and the related microprisms were produced in a 20 mm diameter area, providing a large light deviation angle of 18.43° and maximal FOV of 180°, if the entire hemispherical surface is fabricated with microlenses. The microprism array and microlens array were precisely fabricated on a curved and a flat surface respectively, with a combination of single point diamond turning, diamond broaching and micromilling processes . However, the intensity of the microlens on the hemispherical surface is low, therefore the measured FOV is much smaller than the maximum theoretical value.
5 Conclusions and Outlook
The direct fabrication technologies, including the thermal reflow method, microplastic embossing and microdroplet jetting method, are simple and low-cost processes which are suitable for mass industrial production, but it is very difficult to control the accuracy of the microlenses shape based on the direct fabrication methods.
Compared with MEMS based technologies, ultraprecision machining is more suitable in terms of producing microlens array on a mold with good uniformity in a large area. The shape of microlens can be well controlled by ultraprecision machining. Further improvement is needed to reduce the surface fluctuation and surface roughness by optimizing the tool path considering the dynamics of machine tools.
Both the MEMS based technologies and ultraprecision machining are able to fabricate curved compound eye microlenses, but the production quality needs to improve.
3D imaging systems inserted with microlens array can be used to capture the light field information, but the spatial resolution is much lower compared with that of photos captured by 2D camera. These 3D imaging systems may be applied in the situation that the high spatial resolution is not required.
WY carried out the studies in the reviews of the principles of compound eye and the fabrication methods. He wrote the draft. LL investigated the applications of microlens array and she also contributed to the review of indirect fabrication method including molding process. WL and CC shared many fundamental ideas in the ultraprecision machining technologies, thermal reflow method and UV molding technology. CC conducted proof reading and made some critical revisions. All authors read and approved the final manuscript.
Wei Yuan born in 1990, is currently a PhD candidate at State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic University, China. He received his bachelor degree from Hefei University of Technology, China and master degree from The Chinese University of Hong Kong, China. His interests include metal cutting theory, ultraprecision machining processing and robotics. Li-Hua Li born in 1981, is current a research assistant at State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic University, China. She earned her PhD degree in measurement science and technology from Tsinghua University, China, in 2012. Dr. Li’s research focuses on the study of design theory of optical elements, fabrication and measurement technology of optics. Wing-Bun Lee born in 1951, is currently the Head of State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic University, China. His teaching and research interests include advanced manufacturing technology, materials processing, ultra-precision machining, manufacturing strategy and knowledge management systems. Chang-Yuen Chan born in 1965, is current the project manager at State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic University, China. He earned his PhD degree in mechanical engineering from Hong Kong University, China, in 1995. Dr. Chan’s research focuses on the study of ultraprecision machining technology and 3D imaging processing.
Supported by Shenzhen Science, Technology and Innovation Commission of China (Grant No. JCYJ20150630115257902), the Research Grants Council of the Hong Kong Special Administrative Region of China (Grant No. ITS/339/13FX), and Research Committee of The Hong Kong Polytechnic University, China (Grant No. RUK0).
The authors declare that they have no competing interests.
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