Fabrication of Microlens Arrays with Controlled Curvature by

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Fabrication of Microlens Arrays with Controlled Curvature by Micromolding Water Condensing Based Porous Films for Deep Ultraviolet LEDs Yang Peng, Xing Guo, Renli Liang, Yun Mou, Hao Cheng, Mingxiang Chen, and Sheng Liu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00692 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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Fabrication of Microlens Arrays with Controlled Curvature by Micromolding Water Condensing Based Porous Films for Deep Ultraviolet LEDs Yang Peng,a Xing Guo,b Renli Liang,c Yun Mou,a Hao Cheng,a Mingxiang Chen, a,d,* and Sheng Liu e a

School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b

National Engineering Technology Research Center for LED on Si Substrate, Nanchang University, Nanchang 330047, China c

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China d

State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China e

School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China

ABSTRACT Microlens arrays (MLAs) have attracted wide attention due to their crucial applications in optics, optoelectronics, and bio-chemistry. In this paper, we present a simple and green approach for the economical fabrication of MLAs with controlled curvature based on water condensing. By controlling the input current and working time of initiative cooling and the viscoelasticity of UV-curable polymer, uniform porous films with adjustable morphology were

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prepared. The MLAs with the aspect ratio (AR) of 1.41, 1.01, and 0.69 were fabricated by micromolding the porous film templates. Furthermore, the fluoropolymer encapsulations with the MLAs were applied for the packaging of deep ultraviolet light-emitting diodes (DUV-LEDs). Consequently, the light output powers of DUV-LEDs are enhanced by 7.1%, 10.2%, and 15.4%, respectively, by using these MLAs at the driving current of 350 mA.

Keywords: microlens arrays (MLAs), water condensing, porous films, fluoropolymer encapsulation, deep ultraviolet light-emitting diodes (DUV-LEDs), light output power

Microlens arrays (MLAs) have been widely applied in many fields such as optical sensors, 3D displays, light-emitting diodes (LEDs), and photovoltaic cells.1–6 For instance, MLAs had been fabricated to improve the light extraction and far-field pattern radiation of LEDs. Currently, various methods have been developed to fabricate MLAs, including lithography,7 photoresist reflow,8 silicon template based micromolding,9–11 direct laser writing,12,13 and inkjet imprinting.14,15 However, most methods involve at least one photolithographic step, which suffers from some disadvantages such as high-cost, time-consuming, and heavy pollution. Although direct laser writing can fabricate MLAs with high precision, it is expensive and inefficient for large-area fabrication.16 The appeal of inkjet printing lies in it being a low-cost, maskless, and large-scale manufacturing. But this approach is limited by inhomogeneous morphology and large feature size.17 As an attractive alternative, breath figure (BF) micromolding has been employed as a versatile method for the fabrication of MLAs because of its low-cost, time-saving, and easily implementable process. For the BF micromolding, water droplets can spontaneously condense on solution surface during the evaporation cooling of solvents, and then the water droplets are


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treated as templates until BF porous film is formed after the complete evaporation of solvents and water droplets.18–21 After that, convex MLAs are achieved by micromolding BF porous films, which have been applied in organic light-emitting diodes (OLEDs),22,23 triboelectric nanogenerators (TENG),24 diffuser films,25 and antireflection coatings.26 However, the temperature of solution is difficultly controlled during the solvent evaporation, which determines the morphology of porous films.27 In addition, the common evaporation solvents for BF method are toxic solvents, such as trichloromethane and carbon disulfide. These drawbacks limit the application of BF micromolding for the fabrication of MLAs. Although the different dimension of MLAs can be achieved by controlling the feature sizes of BF templates, the curvature of MLAs is not easily tuned by using the BF templates. Therefore, it still is a challenge to fabricate MLAs with controlled curvature in a facile, low-cost, and eco-friendly approach. It is well known that MLAs can reduce total internal reflection (TIR) loss and enhance light extraction from LEDs due to their scattering effect on emission light.28–30 Nowadays, Aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV-LEDs) have drawn considerable attentions in disinfection, air and water purification, and biochemical inspection.31,32 Unlike conventional mercury-based UV lamps, DUV-LEDs are robust, long lifetime, mercury-free, and wavelength tunability.33 However, for the emission wavelengths between 250 to 300 nm, the light extraction efficiency (LEE) of DUV-LEDs is still low, which is insufficient for most practical applications.34–36 Obviously, the TIR at flat chip-air or encapsulation-air interface is one of the main factors limiting the LEE due to the large refractive index difference.37,38 Although MLAs can be directly fabricated on the sapphire for DUV-LEDs, the fabrication process requires etching technique, which affects the photoelectric characteristics of LED chip.39 And it is a big challenge to fabricate MLAs with small feature size on the


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sapphire. In view of these disadvantages, MLAs fabricated on encapsulation layer is another effective alternative. Considering that the conventional encapsulation materials (silicone or epoxy) are prone to UV aging and destruction, C-F-based amorphous fluoropolymers with high UV transparency and excellent UV durable are suitable to fabricate MLAs for DUV-LEDs.40-42 Herein, we present a facile and green water condensing approach for the economical fabrication of MLAs with controlled curvature. First, porous films are prepared by using UV assisted and initiative cooling based water condensing method. The morphology and dimension of pore arrays are adjusted by controlling the input current and working time of initiative cooling and the viscoelasticity of UV-curable polymer. Then, the uniform porous films with different morphologies are treated as negative templates for straightforward micromolding of MLAs with well-controlled curvature on fluoropolymer encapsulations. Finally, these MLAs are applied for the packaging of DUV-LEDs and their optical performances are investigated.

EXPERIMENTAL SECTION Fabrication Process of MLAs. MLAs were fabricated by water condensing, combined with UV curing and micromolding, as illustrated in Figure 1(a). First, a UV-curable polymer (NOA61, Norland, USA) was spin-coated on a glass plate with the sizes of 20×20 mm2, followed by UV pre-exposure under 385 nm UV-LED with a power density of 1.5 W/cm2 for different time. The distance between the polymer and the UV-LED is 50 mm. The polymer film has a viscosity of 0.3 Pas and a thickness of 30 µm. By controlling the UV pre-curing degree, the viscoelastic state of polymer could be well adjusted, which determines the morphology of subsequent pore structure. After that, a directly initiative cooling method by using a thermo electric cooler (TEC, type TEC1-12708, sizes 40×40 mm2, maximum temperature difference ∆Tmax=68°C) was


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applied to realize the cool surface of polymer. The surface temperature of polymer decreases when the input current of TEC increases. Notably, the ambient conditions of polymer also affect the water condensing process. In order to ensure the water vapor nucleation on the polymer surface, the temperature and relative humidity (RH) of ambient were controlled at 15°C and 75%, respectively, by using a constant temperature and humidity box. As the temperature of polymer surface sufficiently lower than that of ambient, small and disordered water droplets were spontaneously condensed on the polymer surface owing to water vapor nucleating, and then grown and self-assembled into closely packed array, as shown in Figure S1 (Supporting Information). By controlling the input current and working time of TEC, uniform water droplet arrays were treated as templates until porous film was formed, followed by solidifying and droplets evaporation under 385 nm UV-LED with a power density of 5 W/cm2 for 2 min. At last, an amorphous fluoropolymer (S-type, CYTOP series, Asahi Glass, Japan) was spin-coated on the porous film template and heated at 120°C in a vacuum chamber for 1.5 h. A MLA was formed on the fluoropolymer surface after peeling off the porous film. The fluoropolymer with the end functional group of -CF3 was selected as a matrix material for MLA fabrication due to its robustness against UV light and high UV transparency, which is greatly suitable for DUV-LED packaging. The fluoropolymer film was easily peeled off from template after solidification due to its low surface energy. DUV-LEDs Packaged Using MLAs. The packaging process of DUV-LEDs by MLAs is illuminated in Figure S2 (Supporting Information). First, a flip-chip DUV-LED (1×1 mm2, peak wavelength of 280 nm) was eutectic bonded on an AlN ceramic substrate (3.2×3.2 mm2), followed by dispensing the fluoropolymer encapsulation on the top surface of flip-chip and ceramic substrate with a height of 0.5 mm. Then, the prepared porous film was placed on the


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fluoropolymer at 120°C in a vacuum chamber for 2.5 h, and the fluoropolymer encapsulation with MLA was achieved after solidifying and demolding. Finally, DUV-LED was packaged by MLA for the reduction of reflection loss. Characterization. The porous films and MLAs were characterized by a scanning electron microscope (SEM, Nova NanoSEM 450, FEI, Holland) and examined by a laser scanning confocal microscopy (LSCM, VK-X200K, KEYENCE). The contact angles of MLAs were examined by a contact angle measuring instrument (DSA30, KRUSS, Germany). The optical performances of DUV-LEDs were measured by a photoelectric analysis system (ATA-1000, EVERFINE, China) in an integrating sphere with a diameter of 30 cm at room temperature.

RESULTS AND DISCUSSION Preparation of Porous Films with Adjustable Morphology. The morphology of prepared porous film is determined by the initiative cooling based water condensing process, which can be adjusted by controlling condensing conditions such as the input current and working time of TEC and the viscoelasticity of UV-curable polymer. The input current of TEC determines the temperature of TEC cooling surface. Figure 2 presents the measured surface temperature of TEC with different input currents at an ambient temperature of 15°C. When the current increases from 0.1 A to 1.2 A, the surface temperature reduces from 14.7°C to 1.7°C, and the temperature distribution is uniform on the TEC surface. At the current of 0.6 A, the cooling surface of TEC achieves a temperature of 6.7°C. These different temperatures at the TEC surface cause the various temperature difference between the polymer film and the ambient, which affects the water droplets condensing onto the polymer surface.


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Figure 3 shows the prepared porous films at different TEC currents with the working time of 1.5 min. When the current increases from 0.6 A to 0.9 A, randomly distributed pore arrays form on the polymer surfaces and the average pore width increases from 0.8 µm to 2.7 µm, and the average pore interval decreases from 2.8 µm to 2.1 µm. The increased pore width and decreased pore interval is attributed to the enlarged temperature difference between the polymer surfaces and the ambient from 8.3°C to 11.4°C, which can significantly increase the radius of water droplets in the growth process.27 The uniformity of pore arrays in this situation is poor since the nucleation of water droplets on the polymer surface is random and then the self-assembling of water droplets is not completely occurred at short condensing time before the polymer solidifying. Thus, it is an effective way to achieve a small and uniform pore array by increasing the working time of TEC at the current of 0.6 A. The initiative cooling process with the various working time of TEC at the current of 0.6 A was carried out to prepared the porous film. As shown in Figure 4, when the working time increases from 1.5 min to 3.0 min, the average pore width increases from 0.8 µm to 1.3 µm, and the dimension distribution of pore array becomes uniform then bad. As the working time of 2.5 min, a uniform porous film is achieved and the corresponding pore width and depth are 1.1 µm and 1.6 µm, respectively, as shown in Figure 1(b). With the increase of TEC working time, the hold time of temperature difference get longer, which leads to the sufficient self-assembly process of water droplets on the polymer surface. As the working time reaches an optimal value, a uniform water droplets array floats on the liquid polymer surface through a subtle balance of gravity, buoyancy, and capillary forces. In addition, some water droplets coalesce and then leave large pores on the polymer surface at longer working time. It can be concluded that the TEC input


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current of 0.6 A with the working time of 2.5 min is an optimized condensing condition to form a small and uniform porous film. As the controlling of TEC parameters is only limit to adjust the surface morphology of porous film, UV pre-curing was carried out to control the viscoelasticity of polymer, which determines the structure morphology of prepared pores. Since the UV-curable polymer surface is viscous liquid-like, the condensed water droplets without sufficient support can easily penetrate the surface to form pore structure. When the UV pre-curing time is increased, the polymer surface changes from viscous liquid to viscoelastic state, which leads to the reduction of penetrating depth for water droplets on the polymer surface, as illuminated in Figure 5(a). Whereas the polymer surface becomes solid-like with longer pre-curing time, the water droplets can not penetrate the surface. As shown in Figure 5(b), the width of pores is increased and the depth of pores is reduced, resulting in the wider and shallower pore structure. With the increase of precuring time from 10 s to 30 s, the width of pores increases from 1.2 µm to 1.7 µm and the depth of pores decreases from 1.53 µm to 1.3 µm. However, the surface morphology of porous film prone to un-uniform when the pre-curing time larger than 40 s, as presented in Figure S3. It is believed that the morphology of porous film can be further adjusted by controlling the abovementioned condensing conditions, simultaneously. Fabrication of MLAs with Controlled Curvature by Micromolding Porous films. The prepared porous films with different structure morphologies were treated as templates to fabricate the MLAs via micromolding process. Figure 6 shows three fabricated MLAs, MLA 1, MLA 2, and MLA 3, by micromolding the three porous film templates with different UV precuring time at the optimized condensing condition. The lens profiles of these three MLAs were measured by the LSCM. The average height of lens for the MLA 1, MLA 2, and MLA 3 are 1.58


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µm, 1.46 µm, and 1.22 µm, respectively. The average bottom-width of lens for the MLA 1, MLA 2, and MLA 3 are 1.12 µm, 1.44 µm, and 1.77 µm, respectively. The aspect ratio (AR, height/bottom-width) of the MLA 1, MLA 2, and MLA 3 are 1.41, 1.01, and 0.69, respectively, which indicates that the fabricated MLAs achieve the morphologies with controlled curvature and the curvature tends to decrease from the MLA 1 to the MLA 3. In this proposed MLAs fabrication approach, no expensive equipments and poisonous solvents are used and the process is time-saving, which will be a new effective method to fabricate MLAs for various applications. The wettability of the fabricated MLAs was investigated by measuring the water contact angles of the fluoropolymer films with smooth surface and MLAs. The average contact angles of the MLA 1, MLA 2, and MLA 3 are increased from 105° to 142°, 139°, and 133°, respectively, compared with that of smooth surface, as shown in Figure 7. The MLAs increase the contact angles of fluoropolymer films about 35.2%, 32.4%, and 26.7%, respectively, which indicates that the fluoropolymer films with MLAs achieve an excellent hydrophobic for the realizing of selfcleaning property. Optical Performances of DUV-LEDs Packaged Using MLAs. The emission spectra of DUVLEDs packaged by the fluoropolymers with smooth surface and MLAs at the current of 350 mA are presented in Figure 8(a). The DUV-LEDs have a peak wavelength of 280 nm and a fullwidth-at-half-maximum (FWHM) of 10 nm, which is same for the different fluoropolymer structures. Importantly, the emission intensities of DUV-LEDs packaged by the fluoropolymers with MLAs are higher than that of DUV-LED packaged by the fluoropolymer with smooth surface. In addition, the light output powers of DUV-LEDs were measured at the current from 50 mA to 500 mA. As the current increases from 50 mA to 500 mA, the light output powers of DUV-LEDs packaged by the fluoropolymers with MLA 1, MLA 2, and MLA 3 are always


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higher than that of DUV-LED packaged by the fluoropolymer with smooth surface, as shown in Figure 8(b). At the current of 350 mA, the MLA 1, MLA 2, and MLA 3 achieve the light output powers of 9.23 mW, 9.5 mW, and 9.95 mW, respectively, which are 7.1%, 10.2%, and 15.4% higher than that of the smooth surface of 8.62 mW. The light extraction enhancement is attributed to the reduction of TIR loss at the fluoropolymer-air interface by the scattering effect from MLA, which increases the critical angle (θ>48.2°) and more light can escape out. Furthermore, the MLA 3 with the minimum AR yields the highest light output power, which can be attributed to its significant diffuse light extraction in the large angle direction.43

CONCLUSIONS In summary, a facile, green, and low-cost approach was proposed and demonstrated to fabricate MLAs with controlled curvature by micromolding water condensing based porous films. The morphology of porous film was adjusted by controlling the initiative cooling based condensing conditions. As the TEC current of 0.6 A and the working time of 2.5 min, a uniform porous film with the pore width of 1.1 µm and the pore depth of 1.6 µm was achieved. When the UV precuring time increases from 10 s to 30 s, the width of pores increases from 1.2 µm to 1.7 µm and the depth of pores decreases from 1.53 µm to 1.3 µm. By micromolding the porous film templates, the MLAs with different aspect ratios, 1.41, 1.01, and 0.69, were achieved. Furthermore, DUV-LEDs were packaged by the fluoropolymers with these MLAs. Consequently, the light output powers of these packaging structures are enhanced by 7.1%, 10.2%, and 15.4%, respectively, compared with that of the flat fluoropolymer structure at the driving current of 350 mA. This study reveals a novel fabrication approach for MLAs, and further provides a feasible and efficient method for enhancing the light extraction from DUV-LED devices.


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ASSOCIATED CONTENT Supporting Information Schematic of water condensing process by directly initiative cooling; schematic of DUV-LED packaging process by using fluoropolymer encapsulation with MLA; SEM images of prepared porous films with long UV pre-curing time. AUTHOR INFORMATION Corresponding Author *E-mail (M. Chen): [email protected]. Tel: +86 027 87542604. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank Analytical and Testing Center of Huazhong University of Science and Technology for the support in SEM measurement. The research is financially supported by National Key Research and Development Program of China (2016YFB0400804); National Natural Science Foundation of China (U1501241, 51775219, 51705230); Fundamental Research Funds for the Central Universities (2016JCTD112). REFERENCES (1) Dong, L.; Agarwal, A. K.; Beebe, D. J.; Jiang, H. Adaptive Liquid Microlenses Activated by Stimuli-Responsive Hydrogels. Nature 2006, 442, 551–554.


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(26) Galeotti, F.; Trespidi, F.; Timò, G.; Pasini, M. Broadband and Crack-Free Antireflection Coatings by Self-Assembled Moth Eye Patterns. ACS Appl. Mater. Interfaces 2014, 6, 5827–5834. (27) Zhang, A.; Bai, H.; Li, L. Breath Figure: A Nature-Inspired Preparation Method for Ordered Porous Films. Chem. Rev. 2015, 115, 9801–9868. (28) Sun, Y.; Forrest, S. R. Organic Light Emitting Devices with Enhanced Outcoupling Via Microlenses Fabricated by Imprint Lithography. J. Appl. Phys. 2006, 100, 073106. (29) Ee, Y.-K.; Arif, R. A.; Tansu, N.; Kumnorkaew, P.; Gilchrist, J. F. Enhancement of Light Extraction Efficiency of InGaN Quantum Wells Light Emitting Diodes Using SiO2/Polystyrene Microlens Arrays. Appl. Phys. Lett. 2007, 91, 221107. (30) Chu, J.; Lei, X.; Wu, J.; Peng, Y.; Liu, S.; Yang, Q.; Zheng, H. Enhanced Light Extraction Efficiency of Chip-on Board Light-Emitting Diodes Through Micro-Lens Array Fabricated by Ion Wind. Opt. Laser Technol. 2017, 89, 92–96. (31) Taniyasu, Y.; Kasu, M.; Makimoto, T. An Aluminium Nitride Light-Emitting Diode with a Wavelength of 210 Nanometres. Nature 2006, 441, 325–328. (32) Hu, H.; Zhou, S.; Liu, X.; Gao, Y.; Gui, C.; Liu, S. Effects of GaN/AlGaN/Sputtered AlN Nucleation Layers on Performance of GaN-Based Ultraviolet Light-Emitting Diodes. Sci. Rep. 2017, 7, 44627. (33) Hirayama, H.; Maeda, N.; Fujikawa, S.; Toyoda, S.; Kamata, N. Recent Progress and Future Prospects of AlGaN-Based High-Efficiency Deep-Ultraviolet Light-emitting Diodes. Jpn. J. Appl. Phys. 2014, 53, 100209.


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AlGaN-Based LED Multichip Array with Hemispherical Encapsulated Structures Using a Selected Resin Through a Detailed Feasibility Study. Jpn. J. Appl. Phys. 2016, 55, 082101. (42) Peng, Y.; Li, R.; Cheng, H.; Chen, Z.; Li, H.; Chen, M. Facile Preparation of Patterned Phosphor-in-Glass with Excellent Luminous Properties Through Screen-Printing for HighPower White Light-Emitting Diodes. J. Alloys Compd. 2017, 693, 279–284. (43) Li, X.-H.; Song, R.; Ee, Y.-K.; Kumnorkaew, P.; Gilchrist, J. F.; Tansu, N. Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes with Colloidal Microlens Arrays with Various Aspect Ratios. IEEE Photonics J. 2011, 3, 489–499.


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Figure 1. (a) Schematic fabrication process of MLAs: I. UV-curable polymer was pre-cured by UV pre-exposure; II. Water droplets array was condensed on the polymer surface based on water condensing process; III. Porous film was solidified after UV exposure and droplets evaporation; IV. MLA was formed on the fluoropolymer surface after micromolding and solidifying by using porous film as template. (b) Top and cross-sectional SEM images of prepared porous film under the TEC current of 0.6 A and the working time of 2.5 min without UV pre-curing.


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Figure 2. Temperature of TEC cooling surface at different input currents. The inset shows the infrared image of TEC at a current of 0.6 A.


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Figure 3. SEM images of prepared porous films by adjusting the TEC current of (a) 0.6 A with the surface temperature of 6.7°C, (b) 0.7 A with the surface temperature of 5.8°C, (c) 0.8 A with the surface temperature of 4.6°C, and (d) 0.9 A with the surface temperature of 3.6°C for the working time of 1.5 min without UV pre-curing.


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Figure 4. SEM images of prepared porous films by adjusting the working time of (a) 1.5 min, (b) 2.0 min, (c) 2.5 min, and (d) 3.0 min at the TEC current of 0.6 A without UV pre-curing.


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Figure 5. (a) Schematic illustration of the condensing situations of water droplets on the UVcurable polymer surface with UV pre-curing time increase. (b) Structural parameters of the pores of prepared porous films with different UV pre-curing time at the TEC current of 0.6 A and the working time of 2.5 min.


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Figure 6. SEM images of fabricated MLAs at 30°-tilted oblique view. The inset shows the LSCM images of fabricated MLAs. (a) MLA 1 fabricated by porous template without UV precuring, (b) MLA 2 fabricated by porous template with UV pre-curing time of 20 s, (c) MLA 3 fabricated by porous template with UV pre-curing time of 30 s, and (d) lens profiles of these three MLAs measured by LSCM.


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Figure 7. Average contact angles of fluoropolymer films with smooth surface, MLA 1, MLA 2, and MLA 3.


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Figure 8. (a) Emission spectra of DUV-LEDs packaged by different fluoropolymer structures at the current of 350 mA. The inset shows the pictures of DUV-LEDs under operation. (b) Light output powers of DUV-LEDs packaged by different fluoropolymer structures as a function of forward current. The inset presents the schematic illustration of light transmission in fluoropolymer structures with smooth surface and MLA surface.


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Table of contents (TOC) graphic


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Fabrication of Microlens Arrays with Controlled Curvature by

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