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Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique
G Model IJLEO-54081; No. of Pages 4
ARTICLE IN PRESS Optik xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique Kejun Zhong a,b,∗ , Yiqing Gao a,b , Feng Li b , Zhimin Zhang b , Ningning Luo b a b
College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Key Laboratory of Nondestructive Testing (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China
a r t i c l e
i n f o
Article history: Received 22 May 2013 Accepted 16 October 2013 Available online xxx Keywords: Convex/concave microlens array Maskless grayscale lithography Replica molding PDMS
a b s t r a c t This paper presents a facile and effective method to fabricate microlens array in polydimethylsiloxane (PDMS). The microlens array model is fabricated in photoresist via digital maskless grayscale lithography technique and the replica molding technique is used to fabricate PDMS microlens array. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated. The morphological characteristics of the microlens arrays are measured by microscope and 3D profiler. The results indicate that the profiles of the PDMS microlens arrays are clear and distinct. This method provides a simple and low-cost approach to prepare large area, concave or convex with arbitrary shape microlens array, which has potential application in many optoelectronic devices. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Microlens array has extensive applications in optical communication, optical interconnection, digital displays, sensing applications and data storage. The integrated microlens array also provides interesting applications such as enhancing the illumination brightness and simplifying light-guide module construction. Several fabrication techniques have been applied to microlens array fabrication processes, such as photoresist melting, UVlithography, electron beam or proton beam lithography, laser writing, grayscale lithography, nanoimprinting lithography, which have been described detailed in literatures [1–6]. However, most of above processes are only employed to fabricate convex or concave microlens array and cannot be employed to fabricate the two forms microlens array at the same system. Crystal, glass and polymers are conventional materials utilized to fabricate microlens array. Polydimethylsiloxane (PDMS) is a promising polymer material for micro-optical, it has attractive properties, such as low surface energy, thermal curing property, and chemical stability, easy replica molding, non-toxic and biocompatible, and has been widely used to fabricate MEMS, microfluidic devices, biosensors and biochips [7–9]. Optical clarity and low attenuation are another potential characteristic of PDMS. Some researchers have taken PDMS as the final material to fabricate optical components such
∗ Corresponding author at: College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. Tel.: +86 079183953468; fax: +86 079183953468. E-mail address: [email protected] (K. Zhong).
as waveguides, passband filters, microlens and diffraction gratings [10–12]. In this paper, we present a simple and effective method to fabricate convex and concave PDMS microlens array. The method combines the digital grayscale maskless lithography technique and the replica molding technique. The digital grayscale maskless lithography technique is a promising technique for fabrication 3D micro-structures and has been developed in resent years [13,14]. The technique combines the advantages of a programmable digital micromirror device (DMD) system and a photolithographic projection system. The DMD is a good candidate as pattern generator for its high brightness, high resolution, etc. By controlling the amount of time a micro-mirror reflects UV light onto the photoresist compared to the total time of exposure, the DMD generates a grayscale, which is replaces conventional grayscale mask and can control the exposure dose precisely. Comparing to other fabrication techniques of microlens array, the digital grayscale maskless lithography technique has many advantages such as maskless, low-cost, one-step fabricate concave or convex microlens array. In addition, replica molding technique is a cost-effective method in the large scale production stage due to its excellent reproducibility and productivity. Herein we combine the advantages of the digital grayscale maskless lithography technique and the replica molding technique. The principle and setup of the digital maskless grayscale lithography technique are described in Section 2. The technology process of fabrication microlens array with PDMS as the final optical material is presented in Section 3. The results are given in Section 4, and conclusions are demonstrated finally.
0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.10.082
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
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Fig. 4. PDMS microlens array fabrication process flowchart. (a) Photoresist molding (b) PDMS casting and curing (c) PDMS microlens array.
Fig. 1. Schematic diagram of the digital maskless grayscale lithography system.
2. Digital maskless grayscale lithography system The schematic diagram of the system is shown in Fig. 1. The system includes a UV light source, which filtered at a wavelength of 365 nm, the DMD, a projection lens, moveable stage, computer and control system. The DMD comprises thousands of individually addressable micro mirrors which can be tilted to an on-or off-state, in which incident light is reflected toward the projection plane or to a light dump, respectively. Rapid changes between these states lead to
gray values of intermediate light intensity by means of pulse width modulation. The DMD is directly controlled by the computer and displays the designed mask image. The mask image from the DMD is demagnify and projected on the photoresist. Physical mask is not required and the image exposed onto the photoresist can be reconfigured almost instantly by just loading a different electronic image file. 3. Experimental 3.1. Fabrication microlens array model in photoresist Firstly, the microlens array model was fabricated in photoresist. Grayscale mask need designed before experiment. The
Fig. 2. The relationship between the developed depth and gray levels.
Fig. 3. (a) Mask image of spherical microlens array with the rectangular aperture (gray level is from 40 to 230). (b) Mask image of spherical microlens array with the hexagonal aperture (gray level is from 230 to 40).
Fig. 5. (a) Magnifying image of the PDMS convex microlens array with rectangular aperture. (b) 3D profile of the microlens. (c) Cross-section profile of the microlens.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
G Model IJLEO-54081; No. of Pages 4
ARTICLE IN PRESS K. Zhong et al. / Optik xxx (2014) xxx–xxx
relationship between the developed depth and gray level of the DMD was investigated. The positive photoresist BP-28 was spin coated at the substrate. And then, the wafer was prebaked at 90 ◦ C for 20 min and exposed on the experimental system at 50 s. Finally, the wafer was developed in 0.5% NaOH solution for 10 s. Fig. 2 shows the relationship between the developed depth and the gray level of the DMD, which implies that the developed depth can be linearly controlled by the gray level of the DMD mask. Fig. 3(a) is mask image of the spherical microlens array with the rectangular aperture drew by image process software. The gray level is from 40 to 230. Fig. 3(b) is mask image of the spherical microlens array with the hexagonal aperture. The gray level is from 230 to 40. The radius of the spherical is 40 m, the distance between arbitrary two proximate microlens is 10 m. At the same conditions described above, two microlens array models, concave microlens array with the rectangular aperture and convex microlens array with the hexagonal aperture, were developed in photoresist by experiment. 3.2. Fabrication microlens array in PDMS by replica molding technique The replica molding technique was used to fabricate PDMS microlens array. The microlens array in photoresist was used as the model. The manufacturing process of convex microlens array is sketched in Fig. 4(a) is the concave microlens array in photoresist. The PDMS made by combining a 10:1 ratio of PDMS prepolymer
3
and PDMS curing agent, was cased onto the concave photoresist array and heated at 80 ◦ C on a hotplate for about 15 min, as shown in Fig. 4(b). Finally, the PDMS convex microlens array was peeled off from the photoresist array carefully after the PDMS had been cured completely, as in Fig. 4(c). The PDMS concave microlens array can also be obtained as the same sketch. 4. Results The microscope image of PDMS convex microlens array is shown in Fig. 5(a). It displays the edges of the microlenses are clear and complete. 3D profiler is employed to test the surface profile of the PDMS microlens array. Fig. 5(b) and (c) are 3D and 2D profile of the microlens. The two figures show that the microlenses have smooth profile. The height of the microlens is about 4.5 m, and the radius is about 40 m. Fig. 6(a–c) is a microscope image of the PDMS concave microlens array with the hexagonal aperture, the 3D and the 2D profiles, respectively. These images manifest that the edges of the microlenses are clear and completed and have smooth and uniform profile also. 5. Conclusions The digital grayscale maskless lithography technique can be used to fabricate concave or convex with arbitrary shape microlens array, and has many advantages such as maskless, low-cost, onestep fabricate. PDMS has desirable properties such as easy replica molding and ease of bonding to seal patterned structures. It is practical using the digital grayscale maskless lithography technique to fabricate microlens array model in photoresist and furthermore to replicate microlens array in PDMS. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated by the approach. The experiment results show that the surface profiles of the microlens are clear and distinguishable. Due to many merits such as low surface energy, thermal curing property, chemical stability, non-toxic, biocompatible and optical clear, PDMS microlens array have potentially applications in diffusers and scanners. In addition, PDMS is soft so that the soft microlens array can also serve as curved microlens array integrated with other photoelectric devices to afford large field view. Acknowledgements This work is supported by funding from the Chinese National Natural Science Foundation (Grant No. 61072131 and Grant No. 61261026) and from Aeronautical Science Foundation of China (Grant No. CASC201105). References
Fig. 6. (a) Magnifying image of the PDMS concave microlens array with hexagonal aperture. (b) 3D profile of the microlens. (c) Cross-section profile of the microlens.
[1] F.T. O’Neill, J.T. Sheridan, Photoresist reflow method of microlens production. Part I: Background and experiments, Optik 113 (2002) 391–404. [2] C.-P. Lin, H. Yang, C.-K. Chao, A new microlens array fabrication method using UV proximity printing, J. Micromech. Microeng. 13 (2003) 748–757. [3] F. Yongqi, N.K. Bryan, Semiconductor microlenses fabricated by one-step focused ion beam direct writing, IEEE Trans. Semicond. Manuf. 15 (2002) 229–231. [4] A. Zˇ ukauskas, M. Malinauskas, C. Reinhardt, B.N. Chichkov, R. Gadonas, Closely packed hexagonal conical microlens array fabricated by direct laser photopolymerization, Appl. Opt. 51 (2012) 4995–5003. [5] M.-H. Wu, C. Park, G.M. Whitesides, Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photo-lithography, Langmuir 18 (2002) 9312–9318. [6] M.C. Cheng, L.K. Chen, C.K. Sung, Fabrication of controllable profile microlens array by nanoimprinting process, in: 7th IEEE International Conference on NEMS, 2012, pp. 542–546.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
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[7] A.S. Nezhad, M. Ghanbari, C.G. Agudelo, M. Packirisamy, R.B. Bhat, A. Geitmann, PDMS microcantilever-based flow sensor integration for lab-on-a-chip, IEEE Sens. J. 13 (2013) 601–609. [8] F. Liu, K.B. Tan, P. Malar, S.K. Bikkarolla, J.A. van Kan, Fabrication of nickel molds using proton beam writing for micro/nano fluidic devices, Microelectron. Eng. 102 (2013) 36–39. [9] Y. Hosseini, P. Zellner, M. Agah, A single-mask process for 3-D microstructure fabrication in PDMS, J. Microelectromech. Syst. 22 (2012) 356–362. [10] R. Huszank, S.Z. Szilasi, I. Rajta, A. Csik, Fabrication of optical devices in poly(dimethylsiloxane) by proton microbeam, Opt. Commun. 283 (2010) 176–180.
[11] F. Li, S. Chen, H. Luo, Y. Zhou, J. Lai, Y. Gao, Fabrication and characterization of polydimethylsiloxane concave microlens array, Opt. Laser Technol. 44 (2012) 1054–1059. [12] M. Fleger, A. Neyer, PDMS microfluidic chip with integrated waveguides for optical detection, Microelectron. Eng. 83 (2006) 1291–1293. [13] K. Totsu, K. Fujishiro, S. Tanaka, M. Esashi, Fabrication of three-dimensional microstructure using maskless gray-scale lithography, Sens. Actuators A 130/131 (2006) 387–392. [14] A. Rammohan, P.K. Dwivedi, R. Martinez-Duarte, H. Katepalli, M.J. Madou, A. Sharma, One-step maskless grayscale lithography for the fabrication of 3dimensional structures in SU-8, Sens. Actuators B 153 (2011) 125–134.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
ARTICLE IN PRESS Optik xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique Kejun Zhong a,b,∗ , Yiqing Gao a,b , Feng Li b , Zhimin Zhang b , Ningning Luo b a b
College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Key Laboratory of Nondestructive Testing (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China
a r t i c l e
i n f o
Article history: Received 22 May 2013 Accepted 16 October 2013 Available online xxx Keywords: Convex/concave microlens array Maskless grayscale lithography Replica molding PDMS
a b s t r a c t This paper presents a facile and effective method to fabricate microlens array in polydimethylsiloxane (PDMS). The microlens array model is fabricated in photoresist via digital maskless grayscale lithography technique and the replica molding technique is used to fabricate PDMS microlens array. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated. The morphological characteristics of the microlens arrays are measured by microscope and 3D profiler. The results indicate that the profiles of the PDMS microlens arrays are clear and distinct. This method provides a simple and low-cost approach to prepare large area, concave or convex with arbitrary shape microlens array, which has potential application in many optoelectronic devices. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Microlens array has extensive applications in optical communication, optical interconnection, digital displays, sensing applications and data storage. The integrated microlens array also provides interesting applications such as enhancing the illumination brightness and simplifying light-guide module construction. Several fabrication techniques have been applied to microlens array fabrication processes, such as photoresist melting, UVlithography, electron beam or proton beam lithography, laser writing, grayscale lithography, nanoimprinting lithography, which have been described detailed in literatures [1–6]. However, most of above processes are only employed to fabricate convex or concave microlens array and cannot be employed to fabricate the two forms microlens array at the same system. Crystal, glass and polymers are conventional materials utilized to fabricate microlens array. Polydimethylsiloxane (PDMS) is a promising polymer material for micro-optical, it has attractive properties, such as low surface energy, thermal curing property, and chemical stability, easy replica molding, non-toxic and biocompatible, and has been widely used to fabricate MEMS, microfluidic devices, biosensors and biochips [7–9]. Optical clarity and low attenuation are another potential characteristic of PDMS. Some researchers have taken PDMS as the final material to fabricate optical components such
∗ Corresponding author at: College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. Tel.: +86 079183953468; fax: +86 079183953468. E-mail address: [email protected] (K. Zhong).
as waveguides, passband filters, microlens and diffraction gratings [10–12]. In this paper, we present a simple and effective method to fabricate convex and concave PDMS microlens array. The method combines the digital grayscale maskless lithography technique and the replica molding technique. The digital grayscale maskless lithography technique is a promising technique for fabrication 3D micro-structures and has been developed in resent years [13,14]. The technique combines the advantages of a programmable digital micromirror device (DMD) system and a photolithographic projection system. The DMD is a good candidate as pattern generator for its high brightness, high resolution, etc. By controlling the amount of time a micro-mirror reflects UV light onto the photoresist compared to the total time of exposure, the DMD generates a grayscale, which is replaces conventional grayscale mask and can control the exposure dose precisely. Comparing to other fabrication techniques of microlens array, the digital grayscale maskless lithography technique has many advantages such as maskless, low-cost, one-step fabricate concave or convex microlens array. In addition, replica molding technique is a cost-effective method in the large scale production stage due to its excellent reproducibility and productivity. Herein we combine the advantages of the digital grayscale maskless lithography technique and the replica molding technique. The principle and setup of the digital maskless grayscale lithography technique are described in Section 2. The technology process of fabrication microlens array with PDMS as the final optical material is presented in Section 3. The results are given in Section 4, and conclusions are demonstrated finally.
0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.10.082
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
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ARTICLE IN PRESS K. Zhong et al. / Optik xxx (2014) xxx–xxx
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Fig. 4. PDMS microlens array fabrication process flowchart. (a) Photoresist molding (b) PDMS casting and curing (c) PDMS microlens array.
Fig. 1. Schematic diagram of the digital maskless grayscale lithography system.
2. Digital maskless grayscale lithography system The schematic diagram of the system is shown in Fig. 1. The system includes a UV light source, which filtered at a wavelength of 365 nm, the DMD, a projection lens, moveable stage, computer and control system. The DMD comprises thousands of individually addressable micro mirrors which can be tilted to an on-or off-state, in which incident light is reflected toward the projection plane or to a light dump, respectively. Rapid changes between these states lead to
gray values of intermediate light intensity by means of pulse width modulation. The DMD is directly controlled by the computer and displays the designed mask image. The mask image from the DMD is demagnify and projected on the photoresist. Physical mask is not required and the image exposed onto the photoresist can be reconfigured almost instantly by just loading a different electronic image file. 3. Experimental 3.1. Fabrication microlens array model in photoresist Firstly, the microlens array model was fabricated in photoresist. Grayscale mask need designed before experiment. The
Fig. 2. The relationship between the developed depth and gray levels.
Fig. 3. (a) Mask image of spherical microlens array with the rectangular aperture (gray level is from 40 to 230). (b) Mask image of spherical microlens array with the hexagonal aperture (gray level is from 230 to 40).
Fig. 5. (a) Magnifying image of the PDMS convex microlens array with rectangular aperture. (b) 3D profile of the microlens. (c) Cross-section profile of the microlens.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
G Model IJLEO-54081; No. of Pages 4
ARTICLE IN PRESS K. Zhong et al. / Optik xxx (2014) xxx–xxx
relationship between the developed depth and gray level of the DMD was investigated. The positive photoresist BP-28 was spin coated at the substrate. And then, the wafer was prebaked at 90 ◦ C for 20 min and exposed on the experimental system at 50 s. Finally, the wafer was developed in 0.5% NaOH solution for 10 s. Fig. 2 shows the relationship between the developed depth and the gray level of the DMD, which implies that the developed depth can be linearly controlled by the gray level of the DMD mask. Fig. 3(a) is mask image of the spherical microlens array with the rectangular aperture drew by image process software. The gray level is from 40 to 230. Fig. 3(b) is mask image of the spherical microlens array with the hexagonal aperture. The gray level is from 230 to 40. The radius of the spherical is 40 m, the distance between arbitrary two proximate microlens is 10 m. At the same conditions described above, two microlens array models, concave microlens array with the rectangular aperture and convex microlens array with the hexagonal aperture, were developed in photoresist by experiment. 3.2. Fabrication microlens array in PDMS by replica molding technique The replica molding technique was used to fabricate PDMS microlens array. The microlens array in photoresist was used as the model. The manufacturing process of convex microlens array is sketched in Fig. 4(a) is the concave microlens array in photoresist. The PDMS made by combining a 10:1 ratio of PDMS prepolymer
3
and PDMS curing agent, was cased onto the concave photoresist array and heated at 80 ◦ C on a hotplate for about 15 min, as shown in Fig. 4(b). Finally, the PDMS convex microlens array was peeled off from the photoresist array carefully after the PDMS had been cured completely, as in Fig. 4(c). The PDMS concave microlens array can also be obtained as the same sketch. 4. Results The microscope image of PDMS convex microlens array is shown in Fig. 5(a). It displays the edges of the microlenses are clear and complete. 3D profiler is employed to test the surface profile of the PDMS microlens array. Fig. 5(b) and (c) are 3D and 2D profile of the microlens. The two figures show that the microlenses have smooth profile. The height of the microlens is about 4.5 m, and the radius is about 40 m. Fig. 6(a–c) is a microscope image of the PDMS concave microlens array with the hexagonal aperture, the 3D and the 2D profiles, respectively. These images manifest that the edges of the microlenses are clear and completed and have smooth and uniform profile also. 5. Conclusions The digital grayscale maskless lithography technique can be used to fabricate concave or convex with arbitrary shape microlens array, and has many advantages such as maskless, low-cost, onestep fabricate. PDMS has desirable properties such as easy replica molding and ease of bonding to seal patterned structures. It is practical using the digital grayscale maskless lithography technique to fabricate microlens array model in photoresist and furthermore to replicate microlens array in PDMS. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated by the approach. The experiment results show that the surface profiles of the microlens are clear and distinguishable. Due to many merits such as low surface energy, thermal curing property, chemical stability, non-toxic, biocompatible and optical clear, PDMS microlens array have potentially applications in diffusers and scanners. In addition, PDMS is soft so that the soft microlens array can also serve as curved microlens array integrated with other photoelectric devices to afford large field view. Acknowledgements This work is supported by funding from the Chinese National Natural Science Foundation (Grant No. 61072131 and Grant No. 61261026) and from Aeronautical Science Foundation of China (Grant No. CASC201105). References
Fig. 6. (a) Magnifying image of the PDMS concave microlens array with hexagonal aperture. (b) 3D profile of the microlens. (c) Cross-section profile of the microlens.
[1] F.T. O’Neill, J.T. Sheridan, Photoresist reflow method of microlens production. Part I: Background and experiments, Optik 113 (2002) 391–404. [2] C.-P. Lin, H. Yang, C.-K. Chao, A new microlens array fabrication method using UV proximity printing, J. Micromech. Microeng. 13 (2003) 748–757. [3] F. Yongqi, N.K. Bryan, Semiconductor microlenses fabricated by one-step focused ion beam direct writing, IEEE Trans. Semicond. Manuf. 15 (2002) 229–231. [4] A. Zˇ ukauskas, M. Malinauskas, C. Reinhardt, B.N. Chichkov, R. Gadonas, Closely packed hexagonal conical microlens array fabricated by direct laser photopolymerization, Appl. Opt. 51 (2012) 4995–5003. [5] M.-H. Wu, C. Park, G.M. Whitesides, Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photo-lithography, Langmuir 18 (2002) 9312–9318. [6] M.C. Cheng, L.K. Chen, C.K. Sung, Fabrication of controllable profile microlens array by nanoimprinting process, in: 7th IEEE International Conference on NEMS, 2012, pp. 542–546.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082
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ARTICLE IN PRESS K. Zhong et al. / Optik xxx (2014) xxx–xxx
[7] A.S. Nezhad, M. Ghanbari, C.G. Agudelo, M. Packirisamy, R.B. Bhat, A. Geitmann, PDMS microcantilever-based flow sensor integration for lab-on-a-chip, IEEE Sens. J. 13 (2013) 601–609. [8] F. Liu, K.B. Tan, P. Malar, S.K. Bikkarolla, J.A. van Kan, Fabrication of nickel molds using proton beam writing for micro/nano fluidic devices, Microelectron. Eng. 102 (2013) 36–39. [9] Y. Hosseini, P. Zellner, M. Agah, A single-mask process for 3-D microstructure fabrication in PDMS, J. Microelectromech. Syst. 22 (2012) 356–362. [10] R. Huszank, S.Z. Szilasi, I. Rajta, A. Csik, Fabrication of optical devices in poly(dimethylsiloxane) by proton microbeam, Opt. Commun. 283 (2010) 176–180.
[11] F. Li, S. Chen, H. Luo, Y. Zhou, J. Lai, Y. Gao, Fabrication and characterization of polydimethylsiloxane concave microlens array, Opt. Laser Technol. 44 (2012) 1054–1059. [12] M. Fleger, A. Neyer, PDMS microfluidic chip with integrated waveguides for optical detection, Microelectron. Eng. 83 (2006) 1291–1293. [13] K. Totsu, K. Fujishiro, S. Tanaka, M. Esashi, Fabrication of three-dimensional microstructure using maskless gray-scale lithography, Sens. Actuators A 130/131 (2006) 387–392. [14] A. Rammohan, P.K. Dwivedi, R. Martinez-Duarte, H. Katepalli, M.J. Madou, A. Sharma, One-step maskless grayscale lithography for the fabrication of 3dimensional structures in SU-8, Sens. Actuators B 153 (2011) 125–134.
Please cite this article in press as: K. Zhong, et al., Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.10.082