Fabrication of a Microlens Array with Controlled Curvature by

Apr 28, 2017 - ... the softbake temperature of 60 °C, was placed on a translation stage and moved relative to a CCD camera mounted with an objective ...
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Fabrication of a Microlens Array with Controlled Curvature by Thermally Curving Photosensitive Gel Film beneath Microholes Dawei Zhang,† Qiao Xu,† Chaolong Fang,† Kaimin Wang,† Xu Wang,‡ Songlin Zhuang,† and Bo Dai*,† †

Engineering Research Center of Optical Instrument and System, the Ministry of Education, Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093, China ‡ The Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K. ABSTRACT: A rapid method is developed for fabricating low-cost and high-numerical-aperture photosensitive-gel microlens arrays (MLAs) with well-controlled curvatures. An UV-curable photosensitive-gel film beneath the microholes of a silicon mold can be flexibly deformed by thermally manipulating the surface tension of the photosensitive gel and the pressure difference across the air−photosensitive-gel interface. The concave interface is then solidified through UV curing, forming a MLA with a concave curvature. MLAs with a focal length ranging from 51.4 to 71.9 μm and a numerical aperture (NA) of 0.49 were fabricated. The photocured MLA has high mechanical and thermal strength and is suitable as a master mold for the further production of convex MLAs. The fabricated microlenses have uniform shapes and smooth surfaces. In a demonstration of imaging and focusing performance, clear and uniform images and focused light spots were observed using concave and convex MLAs. KEYWORDS: microlens array, micromolding, surface tension, microstructure fabrication, optical performance

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on the surface is developed first. MLAs can then be produced by using the mold based on different mechanisms. Fabrication of a MLA with convex curvature has been demonstrated by exploiting gas pressure to press a plastic film onto a heated microhole array mold in a closed chamber.19 Gas pressure is isotropic and provides a uniform embossing pressure over the whole film, but the process requires a complex apparatus. In addition, the electrohydrodynamic deformation technique, through the use of an electrically conductive mold, is an attractive approach for fabricating MLAs with concave curvatures. The curvature of polymer microlenses can be controlled by applying a high voltage to a parallel electrode pair consisting of a conductive substrate and a conductive mold when the mold is inserted into liquid prepolymer for trapping liquid or placed over the prepolymer with a proper air gap.22,23 However, the electrohydrodynamic process requires a prepolymer with a high dielectric constant for efficient deformation with relatively low operating voltage. Moreover, the formation of a concave MLA is achieved by a lateral flow of liquid prepolymer through a parallel gap between a substrate and a microhole array mold, which is driven by surface hydrophilicity and liquid edge pinning.24 The sag height of the microlenses is inversely proportional to the thickness of the gap, but an extremely small gap leads to a flat bottom because the

INTRODUCTION Microlens arrays (MLAs) are crucial optical components because of their broad range of applications, including enhancement of light coupling efficiency in light-emitting devices,1,2 photovoltaic cells and microfluidic devices,3−6 three-dimensional imaging,7 single-molecule bioimaging,8 and artificial compound eyes.9 Various techniques have been put forward for the fabrication of MLAs of aspheric profiles. The microremoval process, such as laser ablation and ion-beam milling, is a straightforward way to engrave a customized three-dimensional pattern onto a material and is thus a powerful technique to produce MLAs of superior aspheric surface shapes.10,11 Nevertheless, the microremoval process cannot easily achieve high surface smoothness owing to cascaded material removal. In addition to the microremoval process, techniques such as inkjet printing,12,13 polymer swelling,14−17 and thermal reflow18,19 have been applied to fabricate MLAs. These techniques have a reasonable manufacturing cost and can guarantee the smoothness of the surface. The curvature of the microlenses can be precisely controlled but is associated with the entrance pupil. In other words, affecting the pupil of the microlenses is unavoidable when setting a specific focal length.20,21 Fabricating MLAs with the assistance of a mold attracts significant attention because these techniques are cost-effective and capable of producing microlenses with a smooth surface and controllable curvature without influencing other microlens properties. In these techniques, a mold with a microhole array © XXXX American Chemical Society

Received: January 17, 2017 Accepted: April 28, 2017 Published: April 28, 2017 A

DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the fabrication process. (b) The SEM image of the concave MLA obtained with the fabrication conditions of a 1500 r/min spin coating rate and a 50 °C softbake. film adhered to the solidified photosensitive gel after demolding. Ten percent sodium hydroxide (NaOH) was used to dissolve the aluminum film. Finally, the photosensitive gel was rinsed with deionized water. Furthermore, since the cured photosensitive gel presents high mechanical and thermal strength, the fabricated concave MLA can be used as a master to further produce a convex MLA by means of the micromolding process. First, liquid-state polydimethylsiloxane (PDMS) was prepared by mixing silicone elastomer and a curing agent (Sylgard 184, Dow Corning) at a weight ratio of 10:1. The PDMS liquid was then properly poured onto the master with thickness of 1 mm. The micromolding process was conducted in a vacuum oven, where microbubbles could be removed from the PDMS and the PDMS could completely fill onto the concave MLA master. After 4 h of heating at 80 °C, the PDMS became elastic solid (refractive index of cured PDMS: 1.40325). The solidified PDMS could be easily peeled from the master without any damage to either the master or the replicate. The structure of the concave MLA was well duplicated to the PDMS, forming a convex MLA. In the experiment, the morphology of the fabricated microlens array was characterized by an XL30 ESEM-FEG scanning electron microscope (SEM, FEI) and Talysurf CCI-Lite noncontact 3D profiler (Taylor Hobson).

substrate of poor wettability cannot exert a surface driving force large enough to overcome the viscous resistance to allow prepolymer passage throughout the substrate beneath the microholes. Hence, the sag height-to-diameter ratio (aspect ratio) or curvature of the microlenses is limited, which has a detrimental effect on the numerical aperture (NA) and controllable range of the focal length. The present paper reports a rapid and economical technique for fabricating concave MLAs by harnessing the surface tension of a photosensitive gel film and the pressure difference across the interface of air and the photosensitive gel beneath microholes of a silicon mold. The curvature of the microlenses can be well controlled by setting a proper temperature. A wide tuning range of aspect ratios from 0.21 to 0.48 can be achieved within the temperature change from 30 to 80 °C. A theoretical model is introduced to explain the principle of microlens formation. Furthermore, because of the high mechanical and thermal strength of the photocured photosensitive gel (elongation at break: 50%, coefficient of thermal expansion: 5 × 10−5/°C, operating temperature: − 50 to 150 °C), the concave MLA is employed as a master to produce convex MLAs. The optical performance of the MLAs is evaluated in the demonstration of image projection and light spot distribution.

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RESULTS AND DISCUSSION Features of the Fabricated Concave Microlens Array. Figure 1b shows a SEM image of the fabricated concave MLA. In the demonstration, the spin coating rate is 1500 r/min, and the temperature of the softbake is 50 °C. The thickness of the photosensitive-gel MLA is 62 μm. The total area of the MLA is 1.8 × 1.8 cm2. Good uniformity is achieved for a large area, and the surface is extremely smooth. Each microlens is 50 μm in diameter (D), which is determined by the microhole size. The sag height, h, of the microlens is 15 μm. Based on optical theory,26,27 focal length ( f) and NA can be estimated as follows.

EXPERIMENTAL METHOD

Figure 1 illustrates the process for producing the MLA with a concave curvature. A UV curable liquid-state photosensitive gel (acrylate, viscosity: 12000 mPa·s, density: 1.03 g/cm3, refractive index: 1.487, type 3662, Aroh Alona) was first spun over a sheet of glass. After a softbake for 10 min, a silicon mold with an array of 250 × 250 microholes, which was fabricated by photolithography and a deep reactive ion etching process, was placed on the photosensitive gel. The silicon mold was coated with a 10 nm polymethylphenylsiloxane oil film as a lubricant layer for demolding and a 100 nm aluminum film as an insulating layer to insulate the photosensitive gel from the oil. Each microhole on the silicon mold has 50 μm diameter and 100 μm depth. The softbake temperature was maintained when placing the silicon mold. The surface of the photosensitive gel beneath the microholes became concave owing to the surface tension. The photosensitive gel was then exposed to ultraviolet (UV) light. UV light at 365 nm has the intensity of 800 mJ/ cm2. After 5 min of UV exposure, the photosensitive gel was solidified, and the concave shape was fixed. Owing to the existence of the polymethylphenylsiloxane oil, the separation of the silicon mold from the solidified photosensitive gel could be easily achieved. The aluminum

f=

h2 + D2 /4 2h(n − 1)

NA = D/2f where n is the refractive index of the photosensitive gel. The focal length is 58.2 μm, and NA is 0.43. Formation Mechanism of the Concave Microlens Array. The concave MLA is fabricated by using a silicon mold, B

DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Influence of the temperature during the softbake on the formation of the MLA. The SEM images of the MLA obtained with the spin coating rate of 1500 r/min and a (a) 40 °C, (b) 60 °C, and (c) 80 °C softbake. (d) The cross-sectional profiles of the MLAs obtained with the different softbake temperatures. (e) The influence of the softbake temperature on the surface tension and the sag height of the microlens.

Figure 3. Influence of the spin-coating rate on the formation of the MLA. The SEM images of the MLA obtained with the condition of 80 °C softbake and the spin-coating rate of (a) 4000 r/min, (b) 3000 r/min, and (c) 2000 r/min. (d) The cross-sectional profiles of the MLAs obtained with the spincoating rate of 4000 r/min (top), 3000 r/min (middle), and 2000 r/min (bottom). (e) The influence of the spin-coating rate on the thickness of the photosensitive gel film and the sag height of the microlens. Circles: the thickness of the film after spin coating. Diamond: the thickness of the film after the silicon mold is placed on the film.

on which there is a microhole array. Each microhole functions as a capillary. When the silicon mold is placed on the liquid-state photosensitive gel, thermal convection occurs between the

room-temperature air captured in the microholes and the softbaked photosensitive gel. The air inside the microhole expands, and the pressure across the interface of the air and the C

DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Test of the imaging performance of the fabricated concave MLA. (b) The arrayed images of letter cluster “A” on the false focal plane of the concave MLA. (c) The magnified image of a part of the arrayed images.

The temperature of the softbake has a great influence on the curvature of the microlens. A high aspect ratio can be achieved by setting a properly high temperature. Nevertheless, a largecurvature microlens cannot be produced if the spin-coating rate in the photosensitive gel coating process is too high. Figure 3a−c shows the MLAs obtained when the temperature in the softbake is 80 °C, and the spin coating rate is 4000 r/min, 3000 r/min, and 2000 r/min, respectively. The cross-sectional profile is illustrated in Figure 3d. It is obvious that the high spin-coating rate leads to flat bottoms of the microlenses. To produce a concave interface, the surface tension force must overcome the viscous resistance to ensure the deformation and fluidity of the photosensitive gel. Furthermore, the viscous resistance of the photosensitive gel is related to the thickness of the film, which is mainly determined by the spin-coating rate in the coating process, as shown in Figure 3e. If the spin-coating rate is high, the film becomes thin, strengthening the viscous resistance because of the increase of the shear gradient. Once the surface tension force cannot overcome the viscous resistance, the deformation of the interface becomes hard, and consequently a flat bottom is formed. It is worth noting that placing the silicon mold on the photosensitive gel causes the reduction of the thickness of the film because the gravity of the silicon mold squeezes the photosensitive gel from the edge of the mold. Thus, in the fabrication process, the gravity of the silicon mold should be considered for a proper film thickness. The sag height of the microlenses fabricated with different spin-coating rates is depicted in Figure 3e. With the increasing spin-coating rate, the sag height decreases, indicating that the surface tension force is weak in contrast to the viscous resistance, and the deformation of the air−liquid interface becomes difficult. In addition, to avoid a nonuniform coating, the spin-coating rate cannot be too low. Thus, in the experiment, a spin-coating rate of approximately 1500 r/min is set to guarantee the uniformity of the film coating and ensure an appropriate thickness for the formation of a large curvature. Optical Properties. To measure the optical performance of the concave and convex MLAs, imaging and focusing experiments were carried out. The experimental setup for imaging is shown in Figure 4a. A white-light light-emitting diode (LED) was located behind a mask, which had a transparent pattern of the letter “A”. The concave MLA, which was obtained with the spincoating rate of 1500 r/min and the softbake temperature of 60 °C, was placed on a translation stage and moved relative to a CCD camera mounted with an objective lens. An array of false and size-reduced images was captured by the CCD on the false focal plane of the concave MLA, as shown in Figure 4b. The “A”

photosensitive gel becomes different. Owing to the existence of the pressure, the liquid surface of the photosensitive gel beneath the microholes turns into a concave shape, which can be explained by the Young−Laplace equation

R = 2σ /ΔP where R is the radius of the curve; σ is the surface tension; and ΔP is the pressure difference across the photosensitive gel and air. In addition, if the softbake temperature of the photosensitive gel, T, changes, the condition of the thermal convection varies, and the pressure difference is affected. In addition, the change in the softbake temperature influences the surface tension according to the Eotvos Ramsay−Shield relation. Thus, the surface of the photosensitive gel beneath the microholes can be described as follows. R = 2k(TC − T − 6)/ΔP(T )V 2/3

where k is the Eotvos−Ramsay coefficient; TC is the critical temperature of the photosensitive gel; and V is the molar volume. Thereby, the softbake can not only impel the formation of the curve but also manipulate the curvature. With increasing softbake temperature, the thermal expansion of the air is aggravated, and the viscous resistance of the photosensitive gel becomes weak, strengthening the pressure difference across the interface. Furthermore, a high temperature in the softbake leads to a low surface tension of the photosensitive gel. Therefore, the high softbake temperature leads to a large curvature of the air−liquid interface. The variation of the curvature corresponding to the temperature change is monotonous. Figure 2a−c shows the MLAs fabricated under the conditions when the spin-coating rate is 1500 r/min and the temperature in the softbake is 80, 60, and 40 °C, respectively. Figure 2d illustrates the cross-sectional profiles of the MLAs. Obviously, with decreasing temperature, the curvature becomes small, and the sag height decreases. The experimental results justify the theoretical discussion. The influence of the temperature on the surface tension of the photosensitive gel and the sag height of the microlens is depicted in Figure 2e. The surface tension decreases with increasing temperature. The sag height varies from 10.5 to 23.5 μm with the change in temperature from 30 to 80 °C. A wide range of focal length from 71.9 to 51.4 μm is achieved. When the temperature is 80 °C, the sag height is 23.5 μm, corresponding to the aspect ratio of 0.47. According to optical theory, the NA is 0.49, which is high for a microlens. This indicates that the MLA can achieve a high light collection efficiency. D

DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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letters were clear and uniform, which meant that the MLA had a good imaging property. Figure 5a shows the experimental setup for measuring the optical properties of the convex MLA. The mask was removed to

Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bo Dai: 0000-0002-0029-792X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research work is supported by The National Key Research and Development Program of China (2016YFD0500603); National Natural Science Foundation of China (61378060, 61601292); Chenguang Project of Shanghai Municipal Education Commission (14CG45); Innovation Program of Shanghai Municipal Education Commission (15ZZ071).

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Figure 5. (a) Test of the focusing performance of the fabricated convex MLA. (b) The fabricated PDMS convex MLA. (c) The images of focused light spots. (d) The intensity distribution of the light spots focused by the convex MLA.

cause the light to pass through a convex MLA directly. The concave MLA on the translation stage was replaced by a convex MLA, which was obtained by casting the concave MLA. The SEM image of the convex MLA is shown in Figure 5b. On the false focal plane of the MLA, the focused light spots were capture by the CCD camera. The image of the focused light spots and the intensity distribution are shown in Figure 5c and 5d, respectively. The light spots in an orderly fashion have the same peak intensity, which hints at the uniformity of the microlenses.

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CONCLUSION In conclusion, a simple and low-cost method has been proposed and demonstrated to produce MLAs by harnessing the surface tension of the photosensitive gel and the pressure difference across the air−photosensitive gel interface beneath the microholes of the silicon mold. By controlling the softbake temperature before molding, the curvature of the microlenses can be flexibly manipulated based on the theory of capillarity and the Eotvos Ramsay−Shield relation so that the focal length variation from 51.4 to 71.9 μm can be achieved. Furthermore, the method can effectively produce an MLA with a high aspect ratio and high NA because of the significant deformation of the air− liquid interface. Since the photocured-photosensitive-gel concave MLA has high mechanical and thermal strength, the produced concave MLA can be used as a master mold to further duplicate convex MLAs. In the proposed method, no complex apparatus or expensive material is required, and there is no timeconsuming fabrication process. Thus, the method could be a new efficient approach to developing various micro-optical components. E

DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b00766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX