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Fabrication of seamless roller mold with 3D micropatterns using inner curved surface photolithography
Microelectronic Engineering 150 (2016) 19–25
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Opinion paper
Fabrication of seamless roller mold with 3D micropatterns using inner curved surface photolithography Sung-Wen Tsai, Po-Yu Chen, Shr-Ren Huang, Yung-Chun Lee ⁎ Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 7 May 2015 Received in revised form 2 September 2015 Accepted 19 October 2015 Available online 20 October 2015 Keywords: Seamless roller mold Curved surface photolithography Roller imprinting
a b s t r a c t Continuous roller imprinting is one of the most promising means of fabricating micro/nano structures over a large area. However, the fabrication of roller molds with seamless and three-dimensional (3D) complex patterns poses a significant challenge. Accordingly, this paper presents an innovative approach for fabricating a seamless hollow roller master mold patterned with discrete or continuous 3D structures. The major steps in the proposed fabrication method include an inner pneumatic rotary photoresist (PR) coating process followed by a step-androtate lithography process or a continuous rotation lithography process. Having fabricated the master mold, the microstructure patterns are transferred to a roller by means of a polydimethyl-siloxane (PDMS) casting technique. The effectiveness of the proposed method is demonstrated by patterning a PDMS-casted roller mold with wavy microstructures and then using the roller mold to fabricate a diffusion optical film. The experimental results show that the diffusion film has a haze of 97.1% and a total transmittance of 91.6%. © 2015 Published by Elsevier B.V.
1. Introduction Roller imprinting is an attractive micro/nano-fabrication method with many important advantages, including a high throughput, a low cost, and the potential for large-area replication. Existing roller imprinting methods can be broadly classified as either hot embossing or UV curing [1–5]. Both methods are well developed and widely used for the mass production of micro/nano-structures for application in the micro-optics, electro-optics, and flexible electronics fields. The roller molds used in roller imprinting processes are generally fabricated using thin mold wrapping [6–8], soft mold casting [9–11], or modified LIGA [12–13] techniques. In all three methods, the process starts with the fabrication of a thin planar mold containing the required microstructure pattern. Typically, the mold is fabricated from either a metallic material (e.g., nickel) or a polymer material such as polycarbonate (PC) or polyurethaneacrylate (PUA). Having patterned the mold, it is wrapped around and bonded to a metal cylinder in order to form the final roller mold. However, thin mold wrapping methods have a serious limitation, namely the roller mold contains a seam which produces a discontinuity in the patterned structure. Various methods have been proposed for manufacturing seamless roller molds, including direct machining using diamond-tip cutters [14–17] or direct laser ablation [18]. Both methods form the required microstructure patterns directly on the surface of the metal roller, and therefore avoid the seam problem described above. However, the diamond cutter machining method is limited to the fabrication of ⁎ Corresponding author. E-mail address: [email protected] (Y.-C. Lee).
http://dx.doi.org/10.1016/j.mee.2015.10.008 0167-9317/© 2015 Published by Elsevier B.V.
simple 2D patterns and is vulnerable to the long-term wearing or breakage of the tool during the machining process. Moreover, in the laser machining process, the machined surface quality is poor and the machining process is long and expensive. Accordingly, the present study proposes a new approach for fabricating seamless hollow roller molds containing 3D microstructures by means of a curved surface photolithography technique. The fabrication process consists of an inner pneumatic PR rotary coating method followed by a step-and-rotate lithography technique (to form discrete 3D features on the mold surface) or a continuous rotation lithography technique (to form continuous 3D features). Having fabricated the hollow mold with needed 3D surface features on its inner surface, a polydimethyl-siloxane (PDMS) coated roller mold containing discrete or continuous 3D microstructures is formed using a PDMS casting process. Importantly, PDMS has a low surface energy, and thus the patterned roller is easily removed from the hollow master mold without breakage following the casting procedure [19]. The proposed method is demonstrated by patterning a roller mold with continuous 3D microstructures and then using the roller to fabricate a diffusion optical film by means of a continuous roller imprinting process. 2. Fabrication of seamless hollow roller master mold and PDMScasted imprinting roller Fig. 1 presents a schematic overview of the fabrication method proposed in this study. In performing inner curved surface photolithography, achieving a uniform PR layer thickness on the hollow cylindrical roller surface represents a significant challenge. In this study, this problem is resolved by means of an inner pneumatic
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Fig. 1. Schematic overview of fabrication processes for hollow master mold and PDMS-casted roller mold for roller imprinting.
rotary coating method, in which the air pressure and distance between the roller and the air ring, as well as the roller rotation speed and air ring translation speed, are carefully controlled (see Fig. 1(a)). In fabricating the roller mold, positive PR (AZ4903, ECHO chemical Corp., Tokyo, Japan) with a viscosity of 1550 cps was deposited on the roller surface using an air pressure of 2.4 kg/cm2, a distance of 2 mm between the roller surface and the air-ring, a rotation speed of 160 rpm, and a translation speed of 1 mm/s. The thickness of the coated PR layer was measured using a laser confocal microscope (VK-9710, Keyence Company, Osaka, Japan) along both axial and circumferential directions on the inner surface of the hollow tube. The measured PR thickness is shown in Fig. 2, in which the horizontal axis represents the axial distance and the error bars represent thickness variation along circumferential direction. It was found the coated PR is quite uniform in its layer thickness with an average value of 31.4 μm. PR layers with different thicknesses can also be obtained by carefully adjusting several important parameters such as PR's viscosity, air gap distance, air pressure, rotating and translating speeds, and so forth. Currently, PR layers with a thickness from 10 to 100 μm can be easily and successfully coated with similar thickness uniformity shown in Fig. 2. The PR coating was soft baked in an over at a temperature of 90 °C for 10 min and was then patterned using an inner curved surface UV exposure process (see Fig. 1(b)). In patterning the PR layer, a flexible mask was mounted in a holder, carefully aligned with the longitudinal axis of the roller, and then placed in close proximity (~150 μm) to the roller surface. The pattern was transferred to the roller surface through a narrow slot-like window in the mask in accordance with the proximity mode lithography exposure method presented by the current group in previous studies [20–22]. In practice, the width of the line patterned on the PR coating is controlled by both the width of the exposure slot
and the mask pattern design and should be much smaller than the radius of curvature of the roller to ensure that the patterning process is effectively performed on a flat plane. The length of the patterned area,
Fig. 2. Thickness variation of PR layer coated on hollow roller (note that the mean PR film thickness is 31.4 ± 1 μm).
S.-W. Tsai et al. / Microelectronic Engineering 150 (2016) 19–25
Fig. 3. Photograph of inner curved surface exposure system used for step-and-rotate and continuous rotation exposure modes.
however, can extend over the entire length of the mask provided that the mask is properly aligned with the longitudinal axis of the roller. Fig. 3 presents a photograph of the inner curved surface photolithography system used in the present study to pattern the hollow roller master mold. As shown, the main components include a precision roller mounting and drive belt rotation system fitted with a high-resolution servo-motor (HC-KFS43, Mitsubishi Electric Corporation, Tokyo, Japan). The exposure process was performed using a 6″ UV light source (AG1000-6N, M&R Nano Technology Company. Ltd., Taoyuan, Taiwan) with a peak intensity of 26.3 mW/cm 2 at wavelengths in the range of 365–405 nm. As described in Section 1, the roller was patterned using either a step-and-rotate lithography technique to form discrete 3D features on the mold surface or a continuous rotation lithography technique to form continuous 3D features. Following the exposure process, the hollow roller was immersed in an AZ400K (ECHO chemical Corp., Tokyo, Japan) developer solution (see Fig. 1(c)) and then hard baked in an oven at a temperature of 90 °C for 20 min to form a hollow master mold with patterned 3D surface features on its inner surface. A stainless steel cylinder is then placed in the center of the hollow master mold to form an annular space between them with a gap of few millimeters. Liquid PDMS (Sylgard 184, Dow Corning, USA) is first put in a vacuum chamber for degasing and then poured into this annular space to fill the gap completely. After thermally curing the PDMS at 80 °C for 2 h, the profiles of microstructures on the inner surface of the hollow master mold were transferred to the PDMS layer, and the PDMS layer is coated on a metal roller. After immersion in a PR stripper, the AZ4903 PR layer is dissolved so that the PDMS/metal roller mold (see Fig. 1(d)) can be finally released from the hollow master mold and used for continuous roller imprinting process.
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Fig. 5. Schematic illustration of step-and-rotate UV exposure process.
3. Experimental results 3.1. Seamless patterns fabricated using step-and-rotate UV exposure mode Fig. 4(a) presents an optical microscope (OM) image of the flexible mask used in the step-and-rotate UV exposure process. As shown, the patterned area has a width and length of 436 μm and 100 mm, respectively. In addition, the hole diameter is equal to 34 μm and the centerto-center pitch distance is 54 μm. In the present study, the hollow roller master mold had a length of 140 mm and an internal diameter of 50 mm. Furthermore, the stepand-rotate UV exposure process [23] was performed using an incremental rotation step of 1°, as shown in Fig. 5. Thus, the mask width was specified as 436 μm (i.e., 50*π/360 mm) in order to ensure a seamless pattern on the roller surface (see Fig. 4(b)). It is noted that while the photomask used in the present study was patterned with a micro-hole array, the mask can in fact be patterned with any form of array, including hexagon arrays, square arrays, and so on. When using the proximity mode lithography exposure technique, the exposure depth of the PR layer is not easily predicted in advance since the light passing through the mask and subsequently incident on the PR surface is subject to diffraction effects. Consequently, a series of exposure tests were performed with different exposure times. Fig. 6(a) shows the relationship between the exposure time and the depth of the 3D microstructures patterned on the PR layer. It is seen that the feature depth increases from around 4.8 μm to approximately 15.8 μm as the exposure time is increased from 10 to 40 s. Fig. 6(b) presents a 3D image of the pattern fabricated on an imprinting roller produced using a master mold patterned with an exposure time of 30 s. It is seen that the microstructure has a height of 12.87 μm (see Fig. 6(c)).
Fig. 4. Optical microscope images of: (a) flexible mask with hole diameter of 34-μm; (b) seamless pattern on hollow roller master mold.
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Fig. 6. (a) Variation of patterned feature depth with exposure time; (b) 3D topography of microstructure on imprinting roller; (c) 2D profile of microstructure on imprinting roller (note that the results presented in (b) and (c) were obtained using a 3D confocal microscope).
3.2. Seamless patterns fabricated using continuous rotation UV exposure mode Fig. 7(a) presents a photograph of the flexible mask used to pattern continuous wavy microstructures on the inner surface of the master roller. As shown, the triangular features have a base length of 86 μm, a height of 70 μm, and a center-to-center pitch distance of 100 μm. The width of the patterned area is again equal to 436 μm and the length is 100 mm. Note that the hollow roller master mold had a length of 100 mm and an internal diameter of 110 mm was used in this work to fabricate larger master mold. During the exposure process, the hollow roller was rotated at a constant speed of 60 rpm for 3 h. Note that the exposure time was deliberately specified as 3 h in order to ensure that the total UV exposure dose exceeded the threshold light intensity (TLI) of the PR layer. Following the exposure process, the microstructure pattern was transferred to a stainless steel roller with a length of 140 mm and a diameter of 100 mm by means of a PDMS casting technique. A photograph of the PDMS-casted roller old is shown in Fig. 7(b). It should be mentioned that the outer diameter of the stainless steel roller is 100 mm and the inner diameter of the hollow master mold is 110 mm so that around 5 mm is reserved for the thickness of PDMS casting. The 3D surface profile on the surface of cast-PDMS layer of the imprinting roller was then measured using a laser confocal microscope. The results showed that the triangular structures had a mean height of 9.83 μm, a base length of 88.9 μm and a center-to-center pitch distance of 100 μm (see Fig. 7(c)).
3.3. Continuous roller imprinting test using PDMS-casted imprinting roller The patterned roller shown in Fig. 7(b) was mounted in a UVtype roller imprinting system in order to carry out the continuous roll-to-roll fabrication of a diffusion film. Fig. 8 shows schematically the UV-cured roller imprinting system for continuously imprinting of microstructures on a flexible substrate using the PDMS-
casted roller mold. The replication process was performed using a polyethylene terephthalate (PET) film with a width of 150 mm and a thickness of 100 μm. Moreover, the roller speed was specified as 0.28 m/min. In performing the imprinting process, a commercial UV curable resin (HICO Technology Co., Ltd.) was mixed with 25 wt.% of diffusion beads (DIFR-3, Eternal, Taiwan, mean particle size: 2 μm) and was coated on the PET substrate using a knife coating system. In addition, a web tension force of 1.45 kg/cm was exerted on the substrate to force the UV-resin layer into the cavities of the wavy microstructures on the roller surface. Finally, a curing process was performed using a high-intensity UV lamp (500 w/cm2) with a central wavelength of 365 nm. The width of the wavy microstructure area on the imprinted film was equal to 100 mm while the length of the film in the rolling direction was equal to 500 mm. However, it is noted that the length of the film along the rolling direction can in fact be infinite since the imprinting process is continuous (see Fig. 9(a)). Fig. 9(b) presents a top view of the replicated wavy microstructures on the PET film. It is seen that the microstructures are free of defects. In other words, the success of the continuous replication process is confirmed. The 3-D profile and 2-D cross-sectional profile of the replicated wavy microstructures are shown in Fig. 9(c) and (d), respectively. The microstructures are seen to have a mean height of 9.37 μm. It is noted that this height is slightly lower than that of the original features on the imprinting roller (i.e., 9.83 μm). The slight reduction in height is thought to be due to a volume shrinkage effect of the UV resin during the curing process. The optical haze, total transmittance and diffuse transmittance of the patterned PET film were measured using a haze meter (Nippon Denshoku NDH-5000W) in accordance with the ASTM D1003 standard. The film was found to have a haze of 97.1%, a total transmittance of 91.6%, and a diffuse transmittance of 88.9%. In other words, the diffusion properties of the patterned film are superior to those of the particle-diffusing bottom diffuser presented in [24]. (i.e., the haze value of 92.6%.)
S.-W. Tsai et al. / Microelectronic Engineering 150 (2016) 19–25
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Fig. 7. (a) Optical microscope image of flexible mask containing triangle patterns with base length of 86 μm, height of 70 μm and center-to-center pitch distance of 100 μm; (b) photograph of patterned metallic imprinting roller; (c) measurement results for wavy microstructures of imprinting roller.
4. Conclusion This paper has presented an innovative technique for fabricating imprinting roller molds with seamless patterns by means of an inner curved surface photolithography technique and a PDMS casting method. In the proposed approach, the inner surface of a hollow roller is coated with a thin PR layer and is then exposed using a proximity mode lithography exposure technique. Two different exposure modes are performed, namely step-and-rotate or continuous rotation, in order to produce discrete or continuous 3D microstructures, respectively. For both exposure modes, the height of the patterned microstructures is determined by a careful control of the exposure time. A PDMS casting technique is then applied to produce the final imprinting roller mold from the hollow roller master mold. The validity of the proposed approach has been demonstrated by performing the continuous rollto-roll imprinting of a PET diffusion film using a metallic roller containing continuous wavy microstructures with a depth of 9.83 μm and a pitch of 100 μm. The results have shown that the wavy microstructures are accurately reproduced on the surface of the diffusion film. Moreover,
Fig. 8. Schematic diagram of the UV-cured roller imprinting system for continuously imprinting of microstructures on a PET substrate with the PDMS-casted roller mold.
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Fig. 9. (a) Photograph of imprinted film with seamless pattern; (b) optical microscope image of wavy microstructures on PET film; (c) 3D topography of wavy microstructure; (d) 2D profile of wavy microstructure.
it has been shown that the imprinted diffusion film has a haze of 97.1% and a total transmittance of 91.6%. In summary, the inner curved surface photolithography method proposed in this study has significant potential for the seamless patterning and fabrication of various real-world 3D microstructure patterns, including continuous wavy, dot arrays, microlens arrays, pillar arrays, and so forth. Acknowledgments This study was supported by the National Science Council (NSC) of Taiwan under Project No. MOST 104-2119-M-006-011 and MOST 1043113-E-006-002. References [1] S.H. Ahn, L.J. Guo, Adv. Mater. 20 (2008) 2044. [2] S.H. Ahn, L.J. Guo, ACS Nano 3 (2009) 2304.
[3] C.H. Chuang, S.W. Tsai, J.F. Lin, C.P. Chen, Jpn. J. Appl. Phys. 50 (2011) (06GK011-06GK01-5). [4] L.P. Yeo, S.H. Ng, H.M. Xia, Z.P. Wang, V.S. Thang, Z.W. Zhong, N.F. de Rooij, J. Micromech. Microeng. 20 (2010) 015017 (10 pp.). [5] T.H. Chou, K.Y. Cheng, C.W. Hsieh, Y. Takaya, J. Micromech. Microeng. 22 (2012) 045009 (7 pp.). [6] T. Makela, T. Haatainen, P. Majander, J. Ahopelto, V. Lambertini, Jpn. J. Appl. Phys. 47 (2008) 5142. [7] C.Y. Chang, S.Y. Yang, J.L. Sheh, Microsyst. Technol. 12 (2006) 754. [8] C.J. Ting, F.Y. Chang, C.F. Chen, C.P. Chou, J. Micromech. Microeng. 18 (2008) (075001(9)). [9] S.J. Liu, Y.C. Chang, J. Micromech. Microeng. 17 (2007) 172. [10] S.Y. Yang, F.S. Cheng, S.W. Xu, P.H. Huang, T.C. Huang, Microelectron. Eng. 85 (2008) 603. [11] C.Y. Chang, S.Y. Yang, M.H. Chu, Microelectron. Eng. 84 (2007) 355. [12] C. Marques, Y.M. Desta, J. Rogers, M.C. Murphy, K. Kelly, J. Microelectromech. Syst. 6 (1997) 329. [13] N. Ishizawa, K. Idei, T. Kimura, D. Noda, T. Hattori, Microsyst. Technol. 14 (2008) 1381–1388. [14] W. Kurnia, M. Yoshino, J. Micromech. Microeng. 19 (2009) 125028 (11 pp.). [15] M.W. Wang, C.C. Tseng, Opt. Express 17 (2009) 4718. [16] H.H. Lin, C.H. Lee, M.H. Lu, Opt. Express 17 (2009) 12397. [17] J.H. Hsu, C.H. Lee, R. Chen, Opt. Express 19 (2011) 13257.
S.-W. Tsai et al. / Microelectronic Engineering 150 (2016) 19–25 [18] W. Wang, X. Mei, G. Jiang, Int. J. Adv. Manuf. Technol. 41 (2009) 504. [19] H. Lan, Y. Ding, in: M. Wang (Ed.), Nanoimprint Lithography Lithography, InTech, ISBN: 978-953-307-064-3, 2010, http://dx.doi.org/10.5772/8189 (Available from: http://www.intechopen.com/books/lithography/nanoimprint-lithography). [20] C.P. Lin, H. Yang, C.K. Chao, J. Micromech. Microeng. 13 (2003) 748.
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H. Yang, C.K. Chao, T.H. Lin, C.P. Lin, Microsyst. Technol. 12 (2005) 82. T.H. Lin, H. Yang, C.K. Chao, S.Y. Hung, Microsyst. Technol. 15 (2009) 1255. Y.C. Lee, H.W. Chen, F.B. Hsiao, J. Microelectromech. Syst. 21 (2012) 316. H.P. Kuo, M.Y. Chuang, C.C. Lin, Powder Technol. 192 (2009) 116.
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Opinion paper
Fabrication of seamless roller mold with 3D micropatterns using inner curved surface photolithography Sung-Wen Tsai, Po-Yu Chen, Shr-Ren Huang, Yung-Chun Lee ⁎ Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 7 May 2015 Received in revised form 2 September 2015 Accepted 19 October 2015 Available online 20 October 2015 Keywords: Seamless roller mold Curved surface photolithography Roller imprinting
a b s t r a c t Continuous roller imprinting is one of the most promising means of fabricating micro/nano structures over a large area. However, the fabrication of roller molds with seamless and three-dimensional (3D) complex patterns poses a significant challenge. Accordingly, this paper presents an innovative approach for fabricating a seamless hollow roller master mold patterned with discrete or continuous 3D structures. The major steps in the proposed fabrication method include an inner pneumatic rotary photoresist (PR) coating process followed by a step-androtate lithography process or a continuous rotation lithography process. Having fabricated the master mold, the microstructure patterns are transferred to a roller by means of a polydimethyl-siloxane (PDMS) casting technique. The effectiveness of the proposed method is demonstrated by patterning a PDMS-casted roller mold with wavy microstructures and then using the roller mold to fabricate a diffusion optical film. The experimental results show that the diffusion film has a haze of 97.1% and a total transmittance of 91.6%. © 2015 Published by Elsevier B.V.
1. Introduction Roller imprinting is an attractive micro/nano-fabrication method with many important advantages, including a high throughput, a low cost, and the potential for large-area replication. Existing roller imprinting methods can be broadly classified as either hot embossing or UV curing [1–5]. Both methods are well developed and widely used for the mass production of micro/nano-structures for application in the micro-optics, electro-optics, and flexible electronics fields. The roller molds used in roller imprinting processes are generally fabricated using thin mold wrapping [6–8], soft mold casting [9–11], or modified LIGA [12–13] techniques. In all three methods, the process starts with the fabrication of a thin planar mold containing the required microstructure pattern. Typically, the mold is fabricated from either a metallic material (e.g., nickel) or a polymer material such as polycarbonate (PC) or polyurethaneacrylate (PUA). Having patterned the mold, it is wrapped around and bonded to a metal cylinder in order to form the final roller mold. However, thin mold wrapping methods have a serious limitation, namely the roller mold contains a seam which produces a discontinuity in the patterned structure. Various methods have been proposed for manufacturing seamless roller molds, including direct machining using diamond-tip cutters [14–17] or direct laser ablation [18]. Both methods form the required microstructure patterns directly on the surface of the metal roller, and therefore avoid the seam problem described above. However, the diamond cutter machining method is limited to the fabrication of ⁎ Corresponding author. E-mail address: [email protected] (Y.-C. Lee).
http://dx.doi.org/10.1016/j.mee.2015.10.008 0167-9317/© 2015 Published by Elsevier B.V.
simple 2D patterns and is vulnerable to the long-term wearing or breakage of the tool during the machining process. Moreover, in the laser machining process, the machined surface quality is poor and the machining process is long and expensive. Accordingly, the present study proposes a new approach for fabricating seamless hollow roller molds containing 3D microstructures by means of a curved surface photolithography technique. The fabrication process consists of an inner pneumatic PR rotary coating method followed by a step-and-rotate lithography technique (to form discrete 3D features on the mold surface) or a continuous rotation lithography technique (to form continuous 3D features). Having fabricated the hollow mold with needed 3D surface features on its inner surface, a polydimethyl-siloxane (PDMS) coated roller mold containing discrete or continuous 3D microstructures is formed using a PDMS casting process. Importantly, PDMS has a low surface energy, and thus the patterned roller is easily removed from the hollow master mold without breakage following the casting procedure [19]. The proposed method is demonstrated by patterning a roller mold with continuous 3D microstructures and then using the roller to fabricate a diffusion optical film by means of a continuous roller imprinting process. 2. Fabrication of seamless hollow roller master mold and PDMScasted imprinting roller Fig. 1 presents a schematic overview of the fabrication method proposed in this study. In performing inner curved surface photolithography, achieving a uniform PR layer thickness on the hollow cylindrical roller surface represents a significant challenge. In this study, this problem is resolved by means of an inner pneumatic
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Fig. 1. Schematic overview of fabrication processes for hollow master mold and PDMS-casted roller mold for roller imprinting.
rotary coating method, in which the air pressure and distance between the roller and the air ring, as well as the roller rotation speed and air ring translation speed, are carefully controlled (see Fig. 1(a)). In fabricating the roller mold, positive PR (AZ4903, ECHO chemical Corp., Tokyo, Japan) with a viscosity of 1550 cps was deposited on the roller surface using an air pressure of 2.4 kg/cm2, a distance of 2 mm between the roller surface and the air-ring, a rotation speed of 160 rpm, and a translation speed of 1 mm/s. The thickness of the coated PR layer was measured using a laser confocal microscope (VK-9710, Keyence Company, Osaka, Japan) along both axial and circumferential directions on the inner surface of the hollow tube. The measured PR thickness is shown in Fig. 2, in which the horizontal axis represents the axial distance and the error bars represent thickness variation along circumferential direction. It was found the coated PR is quite uniform in its layer thickness with an average value of 31.4 μm. PR layers with different thicknesses can also be obtained by carefully adjusting several important parameters such as PR's viscosity, air gap distance, air pressure, rotating and translating speeds, and so forth. Currently, PR layers with a thickness from 10 to 100 μm can be easily and successfully coated with similar thickness uniformity shown in Fig. 2. The PR coating was soft baked in an over at a temperature of 90 °C for 10 min and was then patterned using an inner curved surface UV exposure process (see Fig. 1(b)). In patterning the PR layer, a flexible mask was mounted in a holder, carefully aligned with the longitudinal axis of the roller, and then placed in close proximity (~150 μm) to the roller surface. The pattern was transferred to the roller surface through a narrow slot-like window in the mask in accordance with the proximity mode lithography exposure method presented by the current group in previous studies [20–22]. In practice, the width of the line patterned on the PR coating is controlled by both the width of the exposure slot
and the mask pattern design and should be much smaller than the radius of curvature of the roller to ensure that the patterning process is effectively performed on a flat plane. The length of the patterned area,
Fig. 2. Thickness variation of PR layer coated on hollow roller (note that the mean PR film thickness is 31.4 ± 1 μm).
S.-W. Tsai et al. / Microelectronic Engineering 150 (2016) 19–25
Fig. 3. Photograph of inner curved surface exposure system used for step-and-rotate and continuous rotation exposure modes.
however, can extend over the entire length of the mask provided that the mask is properly aligned with the longitudinal axis of the roller. Fig. 3 presents a photograph of the inner curved surface photolithography system used in the present study to pattern the hollow roller master mold. As shown, the main components include a precision roller mounting and drive belt rotation system fitted with a high-resolution servo-motor (HC-KFS43, Mitsubishi Electric Corporation, Tokyo, Japan). The exposure process was performed using a 6″ UV light source (AG1000-6N, M&R Nano Technology Company. Ltd., Taoyuan, Taiwan) with a peak intensity of 26.3 mW/cm 2 at wavelengths in the range of 365–405 nm. As described in Section 1, the roller was patterned using either a step-and-rotate lithography technique to form discrete 3D features on the mold surface or a continuous rotation lithography technique to form continuous 3D features. Following the exposure process, the hollow roller was immersed in an AZ400K (ECHO chemical Corp., Tokyo, Japan) developer solution (see Fig. 1(c)) and then hard baked in an oven at a temperature of 90 °C for 20 min to form a hollow master mold with patterned 3D surface features on its inner surface. A stainless steel cylinder is then placed in the center of the hollow master mold to form an annular space between them with a gap of few millimeters. Liquid PDMS (Sylgard 184, Dow Corning, USA) is first put in a vacuum chamber for degasing and then poured into this annular space to fill the gap completely. After thermally curing the PDMS at 80 °C for 2 h, the profiles of microstructures on the inner surface of the hollow master mold were transferred to the PDMS layer, and the PDMS layer is coated on a metal roller. After immersion in a PR stripper, the AZ4903 PR layer is dissolved so that the PDMS/metal roller mold (see Fig. 1(d)) can be finally released from the hollow master mold and used for continuous roller imprinting process.
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Fig. 5. Schematic illustration of step-and-rotate UV exposure process.
3. Experimental results 3.1. Seamless patterns fabricated using step-and-rotate UV exposure mode Fig. 4(a) presents an optical microscope (OM) image of the flexible mask used in the step-and-rotate UV exposure process. As shown, the patterned area has a width and length of 436 μm and 100 mm, respectively. In addition, the hole diameter is equal to 34 μm and the centerto-center pitch distance is 54 μm. In the present study, the hollow roller master mold had a length of 140 mm and an internal diameter of 50 mm. Furthermore, the stepand-rotate UV exposure process [23] was performed using an incremental rotation step of 1°, as shown in Fig. 5. Thus, the mask width was specified as 436 μm (i.e., 50*π/360 mm) in order to ensure a seamless pattern on the roller surface (see Fig. 4(b)). It is noted that while the photomask used in the present study was patterned with a micro-hole array, the mask can in fact be patterned with any form of array, including hexagon arrays, square arrays, and so on. When using the proximity mode lithography exposure technique, the exposure depth of the PR layer is not easily predicted in advance since the light passing through the mask and subsequently incident on the PR surface is subject to diffraction effects. Consequently, a series of exposure tests were performed with different exposure times. Fig. 6(a) shows the relationship between the exposure time and the depth of the 3D microstructures patterned on the PR layer. It is seen that the feature depth increases from around 4.8 μm to approximately 15.8 μm as the exposure time is increased from 10 to 40 s. Fig. 6(b) presents a 3D image of the pattern fabricated on an imprinting roller produced using a master mold patterned with an exposure time of 30 s. It is seen that the microstructure has a height of 12.87 μm (see Fig. 6(c)).
Fig. 4. Optical microscope images of: (a) flexible mask with hole diameter of 34-μm; (b) seamless pattern on hollow roller master mold.
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Fig. 6. (a) Variation of patterned feature depth with exposure time; (b) 3D topography of microstructure on imprinting roller; (c) 2D profile of microstructure on imprinting roller (note that the results presented in (b) and (c) were obtained using a 3D confocal microscope).
3.2. Seamless patterns fabricated using continuous rotation UV exposure mode Fig. 7(a) presents a photograph of the flexible mask used to pattern continuous wavy microstructures on the inner surface of the master roller. As shown, the triangular features have a base length of 86 μm, a height of 70 μm, and a center-to-center pitch distance of 100 μm. The width of the patterned area is again equal to 436 μm and the length is 100 mm. Note that the hollow roller master mold had a length of 100 mm and an internal diameter of 110 mm was used in this work to fabricate larger master mold. During the exposure process, the hollow roller was rotated at a constant speed of 60 rpm for 3 h. Note that the exposure time was deliberately specified as 3 h in order to ensure that the total UV exposure dose exceeded the threshold light intensity (TLI) of the PR layer. Following the exposure process, the microstructure pattern was transferred to a stainless steel roller with a length of 140 mm and a diameter of 100 mm by means of a PDMS casting technique. A photograph of the PDMS-casted roller old is shown in Fig. 7(b). It should be mentioned that the outer diameter of the stainless steel roller is 100 mm and the inner diameter of the hollow master mold is 110 mm so that around 5 mm is reserved for the thickness of PDMS casting. The 3D surface profile on the surface of cast-PDMS layer of the imprinting roller was then measured using a laser confocal microscope. The results showed that the triangular structures had a mean height of 9.83 μm, a base length of 88.9 μm and a center-to-center pitch distance of 100 μm (see Fig. 7(c)).
3.3. Continuous roller imprinting test using PDMS-casted imprinting roller The patterned roller shown in Fig. 7(b) was mounted in a UVtype roller imprinting system in order to carry out the continuous roll-to-roll fabrication of a diffusion film. Fig. 8 shows schematically the UV-cured roller imprinting system for continuously imprinting of microstructures on a flexible substrate using the PDMS-
casted roller mold. The replication process was performed using a polyethylene terephthalate (PET) film with a width of 150 mm and a thickness of 100 μm. Moreover, the roller speed was specified as 0.28 m/min. In performing the imprinting process, a commercial UV curable resin (HICO Technology Co., Ltd.) was mixed with 25 wt.% of diffusion beads (DIFR-3, Eternal, Taiwan, mean particle size: 2 μm) and was coated on the PET substrate using a knife coating system. In addition, a web tension force of 1.45 kg/cm was exerted on the substrate to force the UV-resin layer into the cavities of the wavy microstructures on the roller surface. Finally, a curing process was performed using a high-intensity UV lamp (500 w/cm2) with a central wavelength of 365 nm. The width of the wavy microstructure area on the imprinted film was equal to 100 mm while the length of the film in the rolling direction was equal to 500 mm. However, it is noted that the length of the film along the rolling direction can in fact be infinite since the imprinting process is continuous (see Fig. 9(a)). Fig. 9(b) presents a top view of the replicated wavy microstructures on the PET film. It is seen that the microstructures are free of defects. In other words, the success of the continuous replication process is confirmed. The 3-D profile and 2-D cross-sectional profile of the replicated wavy microstructures are shown in Fig. 9(c) and (d), respectively. The microstructures are seen to have a mean height of 9.37 μm. It is noted that this height is slightly lower than that of the original features on the imprinting roller (i.e., 9.83 μm). The slight reduction in height is thought to be due to a volume shrinkage effect of the UV resin during the curing process. The optical haze, total transmittance and diffuse transmittance of the patterned PET film were measured using a haze meter (Nippon Denshoku NDH-5000W) in accordance with the ASTM D1003 standard. The film was found to have a haze of 97.1%, a total transmittance of 91.6%, and a diffuse transmittance of 88.9%. In other words, the diffusion properties of the patterned film are superior to those of the particle-diffusing bottom diffuser presented in [24]. (i.e., the haze value of 92.6%.)
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Fig. 7. (a) Optical microscope image of flexible mask containing triangle patterns with base length of 86 μm, height of 70 μm and center-to-center pitch distance of 100 μm; (b) photograph of patterned metallic imprinting roller; (c) measurement results for wavy microstructures of imprinting roller.
4. Conclusion This paper has presented an innovative technique for fabricating imprinting roller molds with seamless patterns by means of an inner curved surface photolithography technique and a PDMS casting method. In the proposed approach, the inner surface of a hollow roller is coated with a thin PR layer and is then exposed using a proximity mode lithography exposure technique. Two different exposure modes are performed, namely step-and-rotate or continuous rotation, in order to produce discrete or continuous 3D microstructures, respectively. For both exposure modes, the height of the patterned microstructures is determined by a careful control of the exposure time. A PDMS casting technique is then applied to produce the final imprinting roller mold from the hollow roller master mold. The validity of the proposed approach has been demonstrated by performing the continuous rollto-roll imprinting of a PET diffusion film using a metallic roller containing continuous wavy microstructures with a depth of 9.83 μm and a pitch of 100 μm. The results have shown that the wavy microstructures are accurately reproduced on the surface of the diffusion film. Moreover,
Fig. 8. Schematic diagram of the UV-cured roller imprinting system for continuously imprinting of microstructures on a PET substrate with the PDMS-casted roller mold.
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Fig. 9. (a) Photograph of imprinted film with seamless pattern; (b) optical microscope image of wavy microstructures on PET film; (c) 3D topography of wavy microstructure; (d) 2D profile of wavy microstructure.
it has been shown that the imprinted diffusion film has a haze of 97.1% and a total transmittance of 91.6%. In summary, the inner curved surface photolithography method proposed in this study has significant potential for the seamless patterning and fabrication of various real-world 3D microstructure patterns, including continuous wavy, dot arrays, microlens arrays, pillar arrays, and so forth. Acknowledgments This study was supported by the National Science Council (NSC) of Taiwan under Project No. MOST 104-2119-M-006-011 and MOST 1043113-E-006-002. References [1] S.H. Ahn, L.J. Guo, Adv. Mater. 20 (2008) 2044. [2] S.H. Ahn, L.J. Guo, ACS Nano 3 (2009) 2304.
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