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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.
© 2014 Optical Society of America
1. Introduction
Microlens arrays (MLAs) are crucial optical devices because of its broad applications in micro-optical systems, optical fiber coupling, artificial compound eye structures, light diffusers and optical sensing technology [1–6]. However, the scale of MLAs remains a big obstacle, which restricts its further practical applications. Large-scale MLAs play an irreplaceable role in high resolution imaging and enhancement of light extraction efficiency of light emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) [7–9]. Notably, most of the image sensors are packed with square unit cells. In order to match with them, the MLAs should be fabricated square-shaped with high fill factor. Image sensors used in the high definition imaging field nowadays, like charge-coupled device (CCD) and complementary metal oxide semiconductor (CMOS), can hardly achieve a 100% pixel aperture ratios. The square-shaped MLAs can help to improve the ratio by focusing light on the photosensitive areas [10]. That will enhance the CCD and CMOS devices’ signal to noise ratio (SNR), and improve its imaging capability. Furthermore, MLAs help to lower image sensors’ sensitivity threshold, achieving the approximate imaging effect of starlight CCD by using ordinary CCD. In solid state light market, low light out-coupling efficiency restricts the wide application of OLEDs and LEDs. In order to improve the output efficiency, microlens array is combined with OLEDs and LEDs. The output power of OLEDs and LEDs covered with microlens array could be enhanced by 2 times [11].
In the past decides, varieties of methods have been adopted for fabricating MLAs, including thermal reflow [12], micro-jet printing method [13], UV lithography method [14], droplet method [15], hot embossing [16], laser interference lithography [17], laser direct writing (LDW) [18], and so on. Among these methods, micro-jet printing method was believed hardly to fabricate square-shaped MLAs and thermal reflow method had poor performance in terms of precision and morphology. Recently, femtosecond (fs) laser has become a popular tool in fabricating three-dimensional (3D) microstructures. In 2009, we used femtosecond laser direct writing (FLDW) method to fabricate spherical microlens on silica glass [19]. In 2010, H. Sun and his associates used femtosecond-laser-induced two-photon polymerization (TPP) method to fabricate hexagonal MLAs with 100% fill factor [20]. However, these laser direct writing methods are inefficient. 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 [21]. Among all the materials suitable for fabricating MLAs, glass is the most ideal material, which achieves great optical performance. However, high quality large-scale MLAs on glass substrate with high fill factor is still urgent and remains a big challenge.
In this paper, we present a femtosecond laser wet etching technique [22,23], to fabricate high quality large-scale concave MLAs on glass. By improving the crafts and parameters, we solve the problems of efficiency, uniformity and quality. A large-scale concave MLAs with more than 260,000 units was fabricated.
The convex MLAs can be easily obtained on polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) surfaces by a replica molding process. The MLAs’ morphology and three-dimensional (3D) profiles are measured by a scanning electron microscopy (SEM) and confocal laser scanning microscope (CLSM). Additionally, optical simulation and experiment results show that the replicated convex MLAs have excellent focusing ability.
2. Fabrication process
Theoretically, the fabrication procedure of the large-scale square-shaped MLAs is basically the same with the small-scale ones. However, in the actual fabrication process there are some factors, including the flatness of the sample, continuous operation ability of the mechanical shutter and the irradiance uniformity of fs-laser and so on, which restrict the feasibility to fabricate large-scale MLAs. Therefore, the crafts and parameters are improved to fabricate it. The optimizational crafts and parameters will be discussed in Section 4.
Here we show the process of fabricating MLAs by fs-laser wet etching method. As shown in Fig. 1, the fabrication process can be divided into a four-step process, including fs-laser exposure, wet etching, replica molding and cleaning [22]. Firstly, fs-laser pulses (800 nm, 50 fs, 1 KHz) with laser power (P) of 5mW were focused onto the surface of silica glass by an objective lens (NA = 0.5), generating a square-arranged laser-modified spots array [Fig. 1(a)]. The laser exposure craters were generated point-by-point. For each point, the exposure time could be controlled by a fast mechanical shutter and the laser power could be tuned through a variable density filter. Subsequently, the post-preparative sample was treated with 3% hydrofluoric (HF) acid solution at room temperature. The square-arranged craters were transformed into square-shaped concave structures after the wet etching process [Fig. 1(b)]. During the etching process, it is worth to point out that an ultrasonic bath is used to guarantee the conformity of the microlenses and high speed of the etching process. The ultrasonic bath is conductive to removing the products and bubbles produced during the etching process. Then, the convex MLAs could be replicated by pouring liquid PDMS onto the square-shaped concave structures and keeping it at the temperature of 80 °C for about 60 minutes [Fig. 1(c)]. Finally, we could get square-shaped convex MLAs on the surface of solidified PDMS by removing the mold [Fig. 1(d)]. Additionally, owing to thermoplastic effect of PMMA, convex MLAs could be also fabricated by pressing PMMA chip onto the mold at the temperature of 120 °C for several minutes.
Fig. 1 Schematic illustration of fabrication process of MLAs: (a) laser irradiate caters on silica glass; (b) HF wet etching process; (c) and (d) show the replication process of convex MLAs on PMMA or PDMS.
3. Results
3.1Formation process of the concave structures
The formation process of the concave structures could be observed through an optical microscope (OM) equipped with a CCD camera. The square-arranged craters array with interspaces of 20 μm was irradiated by 300 fs-laser pulses and the energy of single pulse was 5 μJ. Figure 2 shows the formation process of the concave structures. The crater’s SEM image is shown in Fig. 2(a). We can clearly see that there are some nanostructures in the laser-irradiated zone. These nanostructures are believed to be caused by the fs-laser’s nonlinear breaking process, and would accelerate the etching velocity around these areas [24]. Figure 2(b) shows the SEM image of concave microstructure after HF etching, which has perfect spherical morphology and can be treated as a microlens. At the start of wet etching process, the velocity of the etching in the laser-affected zone is higher than other areas. Actually, the material in the irradiation areas has occurred a configuration deformation and generated some new substances called Lewis base, which is more active in the chemical reaction with HF [25]. Due to the high etching velocity in the irradiation areas, the diameter and depth of the craters would increase quickly at the beginning, forming circular-shaped concave microstructures, which can be served as the microlenses. With the etching carrying on, the modified material around the craters was gradually consumed and eventually the etching velocity would decrease to the intrinsic etching speed of glass. Hence, the diameter of the concave structure would increase, while the depth of it would maintain a constant owing to the same etching velocity between the bottom of the concave structure and the surface of silica glass. With the etching process continuing, the diameter of the concave structure extends gradually until the edge of the adjacent concave structures have wholly connected with each other, as shown in Figs. 2(c) and 2(d). The area of the fabricated square-shaped MLA is 1 × 1 cm2 [Fig. 2(f)]. And more than 260,000 high quality concave microlenses can be fabricated efficiently in less than 25 hours, which is much more efficient than the other methods. Figure 2(e) shows the excellent precision and uniformity of the MLA.
Fig. 2 The formation process of MLAs (a) The SEM image of a single laser irradiated crater; (b) The morphology of the micro concave structure after a 30-minutes wet etching; (c) and (d) show the square-shaped craters array forming square-shaped microlens array; (e) The optical microscope (OM) image of fabricated concave MLAs with excellent precision and uniformity; (f) The digital photo of the fabricated MLA.
3.2Spherical morphology of the large square-shaped concave structures
Figures 3(a) and 3(b) show the SEM images of the square-shaped MLAs captured from different angles and different magnifications, which excellently reveal the MLAs’ perfect surface quality and uniformity. Figures 3(c) and 3(d) present the three-dimensional (3D) morphologies and cross-sectional profiles of the concave structures which were observed by a CLSM. The inserted figure in Fig. 3(d) shows the SEM cross-sectional profiles of the concave structures. The side length of the square-shaped concave microlens is 19.967 μm, which is very close to the interspaces (20 μm) that we set at the beginning, and the depth of the microlens is 2.352 μm. The curvature radius, R, of the square-shaped concave MLAs can be figured out by a simple equation: R = (h2 + r2)/2h (1), where h is the depth of the microlens, r is a half of the side length of the squared-shaped microlens. After the calculation, the result of the curvature radius of the concave microlens is 43.56 μm. The focal length of the microlens array, f, can be calculated by the equation: f = R/(n-1) (2), where n is the value of the refractive index of the silica glass at wavelength of 633 nm. Considering the approximation of n, 1.45, the focal length of microlens array equals 96.8 μm.
Fig. 3 The characterization of square-shaped concave MLAs. (a) and (b) SEM images of square-shaped concave MLAs at different angles and different magnifications; (c) and (d) show the 3D morphologies and cross-sectional profiles of square-shaped concave MLAs measured by CLSM.
3.3Results of the replicated ultra large square-shaped convex microlens
The squared-shaped concave microlens array is used as a mold to replicate the convex microlenses on other materials, such as PDMS and PMMA. Figures 4(a) and 4(b) present the SEM image of the replicated convex square-shaped microlens on PMMA. The image shows that the replicated convex square-shaped microlenses have a perfect surface quality and uniformity. Figures 4(c) and 4(b) present the 3D profile of the replicated convex MLAs. As shown in Fig. 4(e), the length and the sag-height of the replicated square microlens is 19.933 μm and 2.332 μm, respectively. To be compared with the concave MLAs mold, the inconsistency of the size is less than 1%. A large area (1 × 1 cm2) of square-shaped convex MLAs, containing more than 260,000 microlenses, was simply and rapidly replicated on PMMA. The replication time of the convex MLA is less than 20 minutes. In this way, we can achieve the efficient replication of high quality large area MLAs on PMMA chips.
Fig. 4 The characterization of replicated square-shaped convex MLAs. (a) and (b) show the SEM images of convex MLAs at different magnifications; (c) and (d) show the 3D morphologies of convex MLAs; (e) the cross-sectional profiles of convex MLAs.
Figure 5(a) shows the irradiance distribution of flat random light irradiating on the screen by using the Tracepro software. Figure 5(b) shows the simulation results of the light intensity distribution on the focal plane after the same flat light irradiating through a square-shaped spherical MLA. To be compared with the direct irradiation, the maximum irradiance intensity increases almost 4.5 times, which indicates that the MLAs have the ability to improve the sensitivity of image sensors, enhance their signal to noise ratio (SNR) and improve their imaging capability by converging the energy of light onto the photosensitive areas. In Fig. 5(c), a square array of bright spots with same size and brightness is observed by a OM system on the focal plane of MLAs, which is highly consistent with the simulation result.
Fig. 5 (a) and (b) show the results of the optical simulation experiment of the square-shaped spherical MLAs by using Tracepro; (c) the bright focal spots observed by OM; (d) the letter “A” generated through replicated convex MLAs. The insertion represents the magnified images.
The imaging ability of the square-shaped MLAs is also tested. A mask with letter “A” was inserted between a tungsten light source and the MLAs. And the images were captured by the CCD camera equipped on the OM system. As shown in Fig. 5(d), the images are very clear and have a uniform brightness, which shows the MLAs’ excellent imaging quality.
4. Discussions
It has been mentioned in section 2 that the fabrication procedure of large-scale square-shaped convex MLAs has some restrictions: flatness of the sample, continuous operation ability of the mechanical shutter and the irradiance uniformity of fs-laser and so on, which need to be lifted by optimizing crafts and parameters.
In the experiment, the flatness of sample will mainly influence the uniformity of the MLAs. Figures 6(a) and 6(b) show that the diameter and depth of laser irradiated carters will change with the distance between sample surface and the focal plane of objective lens. In Fig. 6, zero in x-axis represents that the sample surface is just exactly on the focal plane of the objective lens. The negative number represents the distance decrease and the positive number represents the distance increase. The y-axis represents the depth and diameter of the fs-laser irradiation crater in Figs. 6(a) and 6(b), respectively. When the surface of the sample coincides with the focal plane the depth and diameter of the crater are about 2.6 μm and 5 μm, respectively. With the sample surface deviating from the focal plane, both the depth and diameter increase until the deviation becomes larger than 15 μm. Actually, the depth and diameter of craters at −25 μm and 20 μm are so small that the sample surface has been considered to be out of the fs-laser irradiation area. According to the etching theory about Lewis base mentioned above in section 3, the inconformity of the depth and diameter of laser irradiation craters will greatly influence the uniformity of MLAs after the HF etching process. Crater with bigger diameter and depth forms microlens with bigger diameter and depth after etching. It indicates that to achieve high uniformity in the whole irradiation area the sample surface’s deviation must be controlled within several micrometers.
Fig. 6 Relationship between the sample deviation and the profile of laser irradiated craters. (a) and (b) show the depth and diameter of the craters depend on the focal plane deviation, respectively.
In laser irradiation process, the mechanical shutter needs to switches on every spot. To fabricate a large-scale MLA, the mechanical shutter would switch thousands of times at a high frequency. To avoid the damage of excessive temperature, a recirculation water system to cool the mechanical shutter was utilized. Besides, the stability of fs-laser’s output power will also influence the uniformity of MLAs. To solve this problem, we could increase the irradiation time. In general, the longer the time of laser irradiation is, the more the number of laser pulses is and the better uniform it will keep. However, blindly increasing the time of laser irradiation of each spot will cause a significant increase of fabrication time. Considering the efficiency and uniformity, the optimized time of laser irradiation on each spot was set to 0.3 s. In the etching process, the concentration of HF is quite important. High concentration will shorten the etching time, but influence the uniformity of the MLAs. As shown in Fig. 7(b), 5% HF would result in an irregular MLA. Eventually, in order to obtain a better uniformity the concentration of the HF was optimized to be 3%.
Fig. 7 (a) the OM images of craters array before HF wet etching; (b) the OM images of craters array after etching in 5% HF for 20 minutes.
5. Conclusion
The fabrication method we present in this paper has the ability to fabricate ultra large area high quality concave MLAs and convex MLAs. Within 25 hours, a 512 × 512 square-shaped concave microlens array with a area of 1 × 1cm2 mold on silica glass was successfully manufactured. The size of every unit is 20μm × 20μm. Moreover, the replicated convex MLAs on PMMA and PDMS can be obtained in tens of minutes by a replication technique. To achieve high uniformity, several problems need to be resolved, including flatness of the sample, continuous operation ability of the mechanical shutter and the irradiance uniformity of femtosecond laser as well as the concentration of HF solution. By optimizing crafts and parameters, we fabricated large-scale microlens array with perfect spherical morphology, high fill factor, high uniformity and high imaging performance.
Acknowledgments
This work is supported by the National Science Foundation of China under the Grant Nos. 51335008, 61275008 and 61176133, the Special-funded programme on national key scientific instruments and equipment development of China under the Grant No. 2012YQ12004706.
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