- Email: [email protected]
Fabrication of continuous relief micro-optic elements using real-time maskless lithography technique based on DMD
Optics & Laser Technology 56 (2014) 367–371
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Fabrication of continuous relief micro-optic elements using real-time maskless lithography technique based on DMD Kejun Zhong a,b,n, Yiqing Gao a,b, Feng Li b, Ningning Luo b, Weiwei Zhang 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
art ic l e i nf o
a b s t r a c t
Article history: Received 7 April 2013 Received in revised form 16 July 2013 Accepted 30 August 2013 Available online 6 October 2013
A novel method is proposed to fabricate continuous relief micro-optic elements using real-time maskless lithography technique based on digital mirror device (DMD). To evaluate the method, aspheric and spheric micro-lens array was fabricated by following the proposed principle. Firstly distribution of the required exposure dose of lens array was obtained and sliced into a number of contours of equal proportions. Then the contour planes instead of virtual masks were converted into binary image. On the lithography system, the dose accumulated over multiple exposures and the required exposure dose profiles were reconstructed. Finally in the photoresist layer, virtual profiles of lens array were formed, consistent with the original designed elements. The method is feasible and reliable for the fabrication of arbitrary continuous relief micro-optic elements. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Continuous relief micro-lens Aspheric and spheric micro-lens array Maskless lithography
1. Introduction Micro-optic elements with three-dimensional (3D) continuous structures have been used in optical communication, information processing, aerospace applications, biomedicine, and other fields. There are many methods for fabrication micro-optical elements, such as the traditional binary optics technique [1], the direct writing technique by either laser [2–4] or electron-beam [5,6], and the conventional gray-scale masks lithography[7–9], etc. The binary optics technique has disadvantages such as complex machining processes, long cycle, high cost and difficult control of alignment precision. The direct writing technique needs expensive equipment and the exposure time of a single substrate is very long. The conventional gray-scale mask technique requires a gray-scale mask, which is difficult to design and expensive to fabricate. Recently a novel photolithographic method has been developed which is based on digital mirror device (DMD) [10–12]. The DMD replaces the physical mask and can successively display images corresponding to different masks in real-time, which dramatically reduces the running cost and saves time. The process of DMD display of the images is controlled by computer, so there is no alignment error in the technique. Typically, 3D micro-fabrication by the technique is performed either in a layer-by-layer fashion such as in microstereolithography
n Corresponding author at: Key laboratory of Nondestructive Testing (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China. Tel./fax: þ 86 79183953468. E-mail address: [email protected] (K. Zhong).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.08.027
[13–15], or by the scanning method [16,17]. However, the two ways both require precise alignment and repeatability, and time consuming. In this article, we present a new method that can achieve arbitrary continuous relief micro-optic elements using the maskless lithography technique. The method need not repeat calibration, and benefits greater productivity. The experimental system and principle of the method will be described, and experimental fabrication of aspheric micro-lens array and spheric micro-lens with rectangular foundation will be discussed for evaluation of the method. 2. Setup of the maskless lithography system The schematic system of the real-time maskless lithography based on DMD is shown in Fig. 1. The base components are uniform illumination system, DMD, the reduction object lens group, a micro-alignment device, movable stage, computer and a control system. A mercury lamp is used as a light source and filtered at a wavelength of 365 nm. The collimating lens device is used to provide uniform illumination. The DMD made by Texas Instruments is consists of many micromirrors, and each micromirror is independently tilted at 7121. The pattern-making principle of the DMD is that each micromirror is tilted according to the pixel information of the binary image. Each micromirror stands for one pixel of the pattern. The pattern on the DMD can be controlled by the computer, which is equivalent to real-time replacing the mask. When one pixel of the pattern is white, the corresponding micromirror would deflect þ121 and the incident light to be focused on the photoresist surface, thus creating the diminished pattern, through the projection reduction objectlens.
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K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
Firstly, we must obtain the required dose distribution of the design profile. The required dose distribution is related to the design profile and the development rate of the photoresist. Assuming tðx; zÞ is the design profile, as shown in Fig. 2(a), in the case of a positive photoresist, a threshold dose Eth exists, defined as the minimum exposure dose required initiating a photoresist reaction. The cured depth z is a logarithmic function of the exposure dose [18], such as Fig. 2(b), Ec is the exposure dose when the cured depth is the maximum cured depth Z. The dose distribution required for achieving the target thickness profile is determined by the contrast curve. The value of the contrast γ is defined as the linear slope of the contrast curve.
γ¼
1 tðx; zÞ ¼ ðln Ec ln Eth Þ fZ ðln Eðx; zÞ ln Eth Þg
where 0 o tðx; zÞ o Z and Eth o Eðx; zÞ oEc :
ð1Þ
Eq. (2) can be obtained from Eq. (1), and Eq. (2) indicates the relation of the exposure dose and the design profile. Fig. 1. Schematic of DMD-based real-time maskless lithography system.
Fig. 2. Principle of the continuous relief lithography method. (a) Design profile, (b) relation between the cured depth and the exposure dose, (c) required dose distribution for fabricating the design profile, (d) sliced contours of the required dose, (e) reconstructed exposure dose on the surface of photoresist, and (f) resulting profile of photoresist structure.
Eðx; zÞ ¼ exp
tðx; zÞ þ ln Eth Zγ
Fig. 3. Design profile.
While the binary image is black, the corresponding micromirror would deflect 121, the incident light to be reflected dummy direction and cannot create the pattern on the surface of the photoresist. The DMD consists of 1024 768 micromirrors, and each micromirror has sides of 13.68 μm, the toting magnific of the reduce lens is 14. So the minimum pattern on the resin surface is about 1 μm. The XY stage, with a resolution of 100 nm and a travel range 5 cm 5 cm, enables us to achieve the largest exposed area at the substrate approximately 5 5 cm2.
3. Principle of the continuous relief lithography method To fabricate continuous relief micro-optic elements, we proposed the process schematically as illustrated in Fig. 2.
Fig. 4. Relation of cured depth and exposure dose.
ð2Þ
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
According to Eq. (2), the required dose distribution Eðx; zÞ can be calculated, which is required for achieving the desired profile tðx; zÞ, as shown in Fig. 2(c). Then, the required dose distribution must be reconstructed on the surface of the photoresist for fabricating the design profile. The required dose distribution is sliced into a number of dose contours of equal proportions, as Fig. 2(d), these contour planes are used to form binary image and replace virtual layers and stored in a computer. Controlled by the computer, the DMD displays these contour planes from the first frame to the nth frame in real-time, n is the total number of contour planes. When the contour planes are displayed on the DMD, the frame images are formed on the surface of the photoresist synchronously. If the display time of the ith frame is t i , the imaging intensity distribution of the ith frame is I i on the surface of photoresist, then the total exposure dose can be obtained, as n
Etotal ¼ ∑ I i t i
ð3Þ
i¼1
369
The dose accumulated over multiple exposures and the required exposure dose is reconstructed, as Fig. 2(e), then the photoresist undergoes a chemical change to afford the design profile, as in Fig. 2(f). When the source intensity is constant, the exposure dose on the surface of photoresist is determined by the display time of each frame. Assuming that the time is t when the cure depth is the maximum depth Z, the display time of every frame can be calculated as t=n. By adjusting the two parameters t i and I i appropriately, arbitrary exposure dose distribution can be obtained to form the desired photoresist profile also.
4. Results and discussions To evaluate the validity of the proposed method in Section 3, as example, aspheric micro-lens array and spheric micro-lens with rectangular foundation were fabricated. Setting the radius R of the elliptical continuous relief micro-lens as 40 μm and the height h¼4 μm, the profile can be expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx2 þ y2 Þ z2 1 ðx2 þy2 Þ þ 2 ¼ 1; or zðx; yÞ ¼ h 2 R R2 h where 0 r z r h; x2 þ y2 r R
ð4Þ
The profile of the micro-lens is shown in Fig. 3. Fig. 4 is the relation of cured depth and exposure dose, which was obtained on the experimental system descried in Section 2.
Fig. 5. The profile of the calculate dose distribution.
Fig. 6. Mask pattern of the micro-lens.
Fig. 8. Resulting profile of the aspheric micro-lens.
Fig. 7. The magnifying image and 3D image of the aspheric micro-lens.
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K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
Fig. 9. The magnifying image and 3D image of the spheric micro-lens with rectangular foundation.
Fig. 9 is its image under the microscope and a micro-lens's 3D image under the 3D profiler. Fig. 10 is profile of the micro-lens. Figs. 9 and 10 indicate the micro-lenses are smooth. The profile is in good agreement with the design profile also.
5. Conclusion The paper presented a novel method for fabrication of continuous relief micro-optic elements based on DMD real-time maskless lithography, and demonstrated the method is feasible and reliable on experiment. As examples, the fabricated aspheric micro-lens array and spheric micro-lens array with rectangular foundation were quite consistent with original design. The method is suitable for fabricate micro-optic elements with continuous relief structures. It is maskless, real time, no alignment error, time and cost saving.
Fig. 10. Resulting profile of the spheric micro-lens.
The primary process steps were as following: the first time, the substrate was spin coated at 1100 rpm with positive photoresist BP-28. And then, the wafer was prebaked at 90 1C for 20 min, a photoresist layer about 4.3 μm thick was obtained. The wafer was exposed on the experimental system; UV light of constant intensity was 8.4 mW/cm2. Finally, the wafer was developed in 0.35% NaOH solution for 15 s. As shown in Fig. 4, the cured depth is a logarithmic function of the exposure dose, the contrast γ and threshold dose Eth of the photoresist were 0.334 and 16.8 mJ/cm2, respectively. According to Eqs. (2) and (4), the dose distribution of the design profile was obtained. The profile of the calculated dose distribution is showed in Fig. 5. The dose distribution was sliced into 20 layers equally. These contour planes were used to binary image and replace a series of mask pattern of the micro-lens, as in Fig. 6. The display time of each mask pattern was 2 s, the dose accumulated over multiple exposures and the required exposure dose were reconstructed, and the designed micro-lens array was developed in photoresist. Metallographic microscope and 3D profiler were used to measure the micro-lens array. Fig. 7 is the image of the array of aspheric micro-lens under the microscope and a micro-lens's 3D image under the 3D profiler. The micro-lens is clear and smooth. Fig. 8 shows the radius of the micro-lens is about 40 μm and the height is 4 μm. The results meet the design profile very well. Setting the radius r of the spherical crown as 40 μm and the height h¼4 μm also. At the same experimental condition, a spheric continuous relief micro-lens array with rectangular foundation was fabricated through the same process described above.
Acknowledgements This work is supported by funding from the Chinese National Natural Science Foundation (Grant nos. 61072131 and 61261026) and from Aeronautical Science Foundation of China (Grant no. CASC201105). References [1] Margaret B, Stern. Binary optics: A VLSI-based microoptics technology. Microelectronic Engineering 1996;32:369–88. [2] Gale Michael T, Rossi Markus, Pedersen Jörn, Schutz Helmut. Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists. Optical Engineering 1994;33(11):3556–66. [3] Kohoutek Tomas, Hughes Mark A, Orava Jiri, et al. Direct laser writing of relief diffraction gratings into a bulk chalcogenide glass. Journal of the Optical Society of America B 2012;29(10):2779–86. [4] Malinauskas M, Žukauskas A, Purlys V, et al. 3D microoptical elements formed in a photostructurable germanium silicate by direct laser writing. Optical and Lasers Engineering 2012;50:1785–8. [5] Kowalik A, Gora K, Jaroszewicz Z, Kolodziejczyk A. Multi-step electron beam technology for the fabrication of high performance diffractive optical elements. Microelectronic Engineering 2005;77:347–57. [6] Graells Simó, Aćimović Srdjan, Volpe Giorgio, Quidant Romain. Direct growth of optical antennas using E-beam-induced gold deposition. Plasmonics 2010;5 (2):135–9. [7] Suleski Thomas J, O'Shea Donald C. Gray-scale masks for diffractive-optics fabrication: ǀ. Commercial slide imagers. Applied Optics 1995;34(32):7507–17. [8] Levy Uriel, Desiatov Boris, Goykhman Ilya, et al. Design, fabrication, and characterization of circular Dammann gratings based on grayscale lithography. Optics Letters 2010;35(6):880–2. [9] Zhang Jianming, Guo Chuanfei, Wang Yongsheng, et al. Micro-optical elements fabricated by metal-transparent-metallic-oxides grayscale phtomasks. Applied Optics 2012;51(27):6606–11. [10] Gao Yiqing, Shen Tingzheng, Chen Jinsong, Luo Ningning, et al. Research on high-quality projection reduction lithography system based on digital mask technique. Optik 2005;116:303–10.
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
[11] Kessels MV, Nassour C, Grosso P, Heggarty K. Direct write of optical diffractive elements and planar waveguides with a digital micromirror device based UV photoplotter. Optics Communications 2010;283:3089–94. [12] Wataru Iwasaki, Toshihiro Takeshita, Yao Peng, et al. Maskless lithographic fine patterning on deeply etched or slanted surfaces, and grayscale lithography, using newly developed digital mirror device lithography equipment. Japanese Journal of Applied Physics 2012;51(6):06FB05-5. [13] Sun C, Fang N, Wu DM, Zhang X. Projection micro-stereolithography using digital micro-mirror dynamic mask. Sensors and Actuators A 2005;121: 113–20. [14] Gauvin Robert, Chen Ying-Chieh, Woo Lee Jin, et al. Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography. Biomaterials 2012;33:3824–34.
371
[15] Hyun-Wook Kang Jeong Hun, Park Dong-Woo Cho. A pixel based solidification model for projection based stereolithography technology. Sensors and Actuators A 2012;178:223–9. [16] Yang Ren, Chan Kin Foong, Feng Zhiqiang, et al. Design and fabrication of microlens and spatial filter array by self-alignment for maskless lithography systems. Journal of Microlithography, Microfabrication and Microsystems 2003;2(3):210–9. [17] Hur Jun-Gyu. Maskless fabrication of three-dimensional microstructures with high isotropic resolution: practical and theoretical considerations. Applied Optics 2011;50(16):2383–90. [18] Suzuki Kazuaki, Smith Bruce W. Microstereolithography: Science and Technology. Taylor & Francis Group; 2007.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Fabrication of continuous relief micro-optic elements using real-time maskless lithography technique based on DMD Kejun Zhong a,b,n, Yiqing Gao a,b, Feng Li b, Ningning Luo b, Weiwei Zhang 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
art ic l e i nf o
a b s t r a c t
Article history: Received 7 April 2013 Received in revised form 16 July 2013 Accepted 30 August 2013 Available online 6 October 2013
A novel method is proposed to fabricate continuous relief micro-optic elements using real-time maskless lithography technique based on digital mirror device (DMD). To evaluate the method, aspheric and spheric micro-lens array was fabricated by following the proposed principle. Firstly distribution of the required exposure dose of lens array was obtained and sliced into a number of contours of equal proportions. Then the contour planes instead of virtual masks were converted into binary image. On the lithography system, the dose accumulated over multiple exposures and the required exposure dose profiles were reconstructed. Finally in the photoresist layer, virtual profiles of lens array were formed, consistent with the original designed elements. The method is feasible and reliable for the fabrication of arbitrary continuous relief micro-optic elements. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Continuous relief micro-lens Aspheric and spheric micro-lens array Maskless lithography
1. Introduction Micro-optic elements with three-dimensional (3D) continuous structures have been used in optical communication, information processing, aerospace applications, biomedicine, and other fields. There are many methods for fabrication micro-optical elements, such as the traditional binary optics technique [1], the direct writing technique by either laser [2–4] or electron-beam [5,6], and the conventional gray-scale masks lithography[7–9], etc. The binary optics technique has disadvantages such as complex machining processes, long cycle, high cost and difficult control of alignment precision. The direct writing technique needs expensive equipment and the exposure time of a single substrate is very long. The conventional gray-scale mask technique requires a gray-scale mask, which is difficult to design and expensive to fabricate. Recently a novel photolithographic method has been developed which is based on digital mirror device (DMD) [10–12]. The DMD replaces the physical mask and can successively display images corresponding to different masks in real-time, which dramatically reduces the running cost and saves time. The process of DMD display of the images is controlled by computer, so there is no alignment error in the technique. Typically, 3D micro-fabrication by the technique is performed either in a layer-by-layer fashion such as in microstereolithography
n Corresponding author at: Key laboratory of Nondestructive Testing (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China. Tel./fax: þ 86 79183953468. E-mail address: [email protected] (K. Zhong).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.08.027
[13–15], or by the scanning method [16,17]. However, the two ways both require precise alignment and repeatability, and time consuming. In this article, we present a new method that can achieve arbitrary continuous relief micro-optic elements using the maskless lithography technique. The method need not repeat calibration, and benefits greater productivity. The experimental system and principle of the method will be described, and experimental fabrication of aspheric micro-lens array and spheric micro-lens with rectangular foundation will be discussed for evaluation of the method. 2. Setup of the maskless lithography system The schematic system of the real-time maskless lithography based on DMD is shown in Fig. 1. The base components are uniform illumination system, DMD, the reduction object lens group, a micro-alignment device, movable stage, computer and a control system. A mercury lamp is used as a light source and filtered at a wavelength of 365 nm. The collimating lens device is used to provide uniform illumination. The DMD made by Texas Instruments is consists of many micromirrors, and each micromirror is independently tilted at 7121. The pattern-making principle of the DMD is that each micromirror is tilted according to the pixel information of the binary image. Each micromirror stands for one pixel of the pattern. The pattern on the DMD can be controlled by the computer, which is equivalent to real-time replacing the mask. When one pixel of the pattern is white, the corresponding micromirror would deflect þ121 and the incident light to be focused on the photoresist surface, thus creating the diminished pattern, through the projection reduction objectlens.
368
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
Firstly, we must obtain the required dose distribution of the design profile. The required dose distribution is related to the design profile and the development rate of the photoresist. Assuming tðx; zÞ is the design profile, as shown in Fig. 2(a), in the case of a positive photoresist, a threshold dose Eth exists, defined as the minimum exposure dose required initiating a photoresist reaction. The cured depth z is a logarithmic function of the exposure dose [18], such as Fig. 2(b), Ec is the exposure dose when the cured depth is the maximum cured depth Z. The dose distribution required for achieving the target thickness profile is determined by the contrast curve. The value of the contrast γ is defined as the linear slope of the contrast curve.
γ¼
1 tðx; zÞ ¼ ðln Ec ln Eth Þ fZ ðln Eðx; zÞ ln Eth Þg
where 0 o tðx; zÞ o Z and Eth o Eðx; zÞ oEc :
ð1Þ
Eq. (2) can be obtained from Eq. (1), and Eq. (2) indicates the relation of the exposure dose and the design profile. Fig. 1. Schematic of DMD-based real-time maskless lithography system.
Fig. 2. Principle of the continuous relief lithography method. (a) Design profile, (b) relation between the cured depth and the exposure dose, (c) required dose distribution for fabricating the design profile, (d) sliced contours of the required dose, (e) reconstructed exposure dose on the surface of photoresist, and (f) resulting profile of photoresist structure.
Eðx; zÞ ¼ exp
tðx; zÞ þ ln Eth Zγ
Fig. 3. Design profile.
While the binary image is black, the corresponding micromirror would deflect 121, the incident light to be reflected dummy direction and cannot create the pattern on the surface of the photoresist. The DMD consists of 1024 768 micromirrors, and each micromirror has sides of 13.68 μm, the toting magnific of the reduce lens is 14. So the minimum pattern on the resin surface is about 1 μm. The XY stage, with a resolution of 100 nm and a travel range 5 cm 5 cm, enables us to achieve the largest exposed area at the substrate approximately 5 5 cm2.
3. Principle of the continuous relief lithography method To fabricate continuous relief micro-optic elements, we proposed the process schematically as illustrated in Fig. 2.
Fig. 4. Relation of cured depth and exposure dose.
ð2Þ
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
According to Eq. (2), the required dose distribution Eðx; zÞ can be calculated, which is required for achieving the desired profile tðx; zÞ, as shown in Fig. 2(c). Then, the required dose distribution must be reconstructed on the surface of the photoresist for fabricating the design profile. The required dose distribution is sliced into a number of dose contours of equal proportions, as Fig. 2(d), these contour planes are used to form binary image and replace virtual layers and stored in a computer. Controlled by the computer, the DMD displays these contour planes from the first frame to the nth frame in real-time, n is the total number of contour planes. When the contour planes are displayed on the DMD, the frame images are formed on the surface of the photoresist synchronously. If the display time of the ith frame is t i , the imaging intensity distribution of the ith frame is I i on the surface of photoresist, then the total exposure dose can be obtained, as n
Etotal ¼ ∑ I i t i
ð3Þ
i¼1
369
The dose accumulated over multiple exposures and the required exposure dose is reconstructed, as Fig. 2(e), then the photoresist undergoes a chemical change to afford the design profile, as in Fig. 2(f). When the source intensity is constant, the exposure dose on the surface of photoresist is determined by the display time of each frame. Assuming that the time is t when the cure depth is the maximum depth Z, the display time of every frame can be calculated as t=n. By adjusting the two parameters t i and I i appropriately, arbitrary exposure dose distribution can be obtained to form the desired photoresist profile also.
4. Results and discussions To evaluate the validity of the proposed method in Section 3, as example, aspheric micro-lens array and spheric micro-lens with rectangular foundation were fabricated. Setting the radius R of the elliptical continuous relief micro-lens as 40 μm and the height h¼4 μm, the profile can be expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx2 þ y2 Þ z2 1 ðx2 þy2 Þ þ 2 ¼ 1; or zðx; yÞ ¼ h 2 R R2 h where 0 r z r h; x2 þ y2 r R
ð4Þ
The profile of the micro-lens is shown in Fig. 3. Fig. 4 is the relation of cured depth and exposure dose, which was obtained on the experimental system descried in Section 2.
Fig. 5. The profile of the calculate dose distribution.
Fig. 6. Mask pattern of the micro-lens.
Fig. 8. Resulting profile of the aspheric micro-lens.
Fig. 7. The magnifying image and 3D image of the aspheric micro-lens.
370
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
Fig. 9. The magnifying image and 3D image of the spheric micro-lens with rectangular foundation.
Fig. 9 is its image under the microscope and a micro-lens's 3D image under the 3D profiler. Fig. 10 is profile of the micro-lens. Figs. 9 and 10 indicate the micro-lenses are smooth. The profile is in good agreement with the design profile also.
5. Conclusion The paper presented a novel method for fabrication of continuous relief micro-optic elements based on DMD real-time maskless lithography, and demonstrated the method is feasible and reliable on experiment. As examples, the fabricated aspheric micro-lens array and spheric micro-lens array with rectangular foundation were quite consistent with original design. The method is suitable for fabricate micro-optic elements with continuous relief structures. It is maskless, real time, no alignment error, time and cost saving.
Fig. 10. Resulting profile of the spheric micro-lens.
The primary process steps were as following: the first time, the substrate was spin coated at 1100 rpm with positive photoresist BP-28. And then, the wafer was prebaked at 90 1C for 20 min, a photoresist layer about 4.3 μm thick was obtained. The wafer was exposed on the experimental system; UV light of constant intensity was 8.4 mW/cm2. Finally, the wafer was developed in 0.35% NaOH solution for 15 s. As shown in Fig. 4, the cured depth is a logarithmic function of the exposure dose, the contrast γ and threshold dose Eth of the photoresist were 0.334 and 16.8 mJ/cm2, respectively. According to Eqs. (2) and (4), the dose distribution of the design profile was obtained. The profile of the calculated dose distribution is showed in Fig. 5. The dose distribution was sliced into 20 layers equally. These contour planes were used to binary image and replace a series of mask pattern of the micro-lens, as in Fig. 6. The display time of each mask pattern was 2 s, the dose accumulated over multiple exposures and the required exposure dose were reconstructed, and the designed micro-lens array was developed in photoresist. Metallographic microscope and 3D profiler were used to measure the micro-lens array. Fig. 7 is the image of the array of aspheric micro-lens under the microscope and a micro-lens's 3D image under the 3D profiler. The micro-lens is clear and smooth. Fig. 8 shows the radius of the micro-lens is about 40 μm and the height is 4 μm. The results meet the design profile very well. Setting the radius r of the spherical crown as 40 μm and the height h¼4 μm also. At the same experimental condition, a spheric continuous relief micro-lens array with rectangular foundation was fabricated through the same process described above.
Acknowledgements This work is supported by funding from the Chinese National Natural Science Foundation (Grant nos. 61072131 and 61261026) and from Aeronautical Science Foundation of China (Grant no. CASC201105). References [1] Margaret B, Stern. Binary optics: A VLSI-based microoptics technology. Microelectronic Engineering 1996;32:369–88. [2] Gale Michael T, Rossi Markus, Pedersen Jörn, Schutz Helmut. Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists. Optical Engineering 1994;33(11):3556–66. [3] Kohoutek Tomas, Hughes Mark A, Orava Jiri, et al. Direct laser writing of relief diffraction gratings into a bulk chalcogenide glass. Journal of the Optical Society of America B 2012;29(10):2779–86. [4] Malinauskas M, Žukauskas A, Purlys V, et al. 3D microoptical elements formed in a photostructurable germanium silicate by direct laser writing. Optical and Lasers Engineering 2012;50:1785–8. [5] Kowalik A, Gora K, Jaroszewicz Z, Kolodziejczyk A. Multi-step electron beam technology for the fabrication of high performance diffractive optical elements. Microelectronic Engineering 2005;77:347–57. [6] Graells Simó, Aćimović Srdjan, Volpe Giorgio, Quidant Romain. Direct growth of optical antennas using E-beam-induced gold deposition. Plasmonics 2010;5 (2):135–9. [7] Suleski Thomas J, O'Shea Donald C. Gray-scale masks for diffractive-optics fabrication: ǀ. Commercial slide imagers. Applied Optics 1995;34(32):7507–17. [8] Levy Uriel, Desiatov Boris, Goykhman Ilya, et al. Design, fabrication, and characterization of circular Dammann gratings based on grayscale lithography. Optics Letters 2010;35(6):880–2. [9] Zhang Jianming, Guo Chuanfei, Wang Yongsheng, et al. Micro-optical elements fabricated by metal-transparent-metallic-oxides grayscale phtomasks. Applied Optics 2012;51(27):6606–11. [10] Gao Yiqing, Shen Tingzheng, Chen Jinsong, Luo Ningning, et al. Research on high-quality projection reduction lithography system based on digital mask technique. Optik 2005;116:303–10.
K. Zhong et al. / Optics & Laser Technology 56 (2014) 367–371
[11] Kessels MV, Nassour C, Grosso P, Heggarty K. Direct write of optical diffractive elements and planar waveguides with a digital micromirror device based UV photoplotter. Optics Communications 2010;283:3089–94. [12] Wataru Iwasaki, Toshihiro Takeshita, Yao Peng, et al. Maskless lithographic fine patterning on deeply etched or slanted surfaces, and grayscale lithography, using newly developed digital mirror device lithography equipment. Japanese Journal of Applied Physics 2012;51(6):06FB05-5. [13] Sun C, Fang N, Wu DM, Zhang X. Projection micro-stereolithography using digital micro-mirror dynamic mask. Sensors and Actuators A 2005;121: 113–20. [14] Gauvin Robert, Chen Ying-Chieh, Woo Lee Jin, et al. Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography. Biomaterials 2012;33:3824–34.
371
[15] Hyun-Wook Kang Jeong Hun, Park Dong-Woo Cho. A pixel based solidification model for projection based stereolithography technology. Sensors and Actuators A 2012;178:223–9. [16] Yang Ren, Chan Kin Foong, Feng Zhiqiang, et al. Design and fabrication of microlens and spatial filter array by self-alignment for maskless lithography systems. Journal of Microlithography, Microfabrication and Microsystems 2003;2(3):210–9. [17] Hur Jun-Gyu. Maskless fabrication of three-dimensional microstructures with high isotropic resolution: practical and theoretical considerations. Applied Optics 2011;50(16):2383–90. [18] Suzuki Kazuaki, Smith Bruce W. Microstereolithography: Science and Technology. Taylor & Francis Group; 2007.