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High-speed imprinting on plastic optical fibers using cylindrical mold with hybrid microstructures
Microelectronic Engineering xxx (2013) xxx–xxx
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High-speed imprinting on plastic optical fibers using cylindrical mold with hybrid microstructures Harutaka Mekaru a,b,⇑, Akihiro Ohtomo a,c, Hideki Takagi a,b, Mitsunori Kokubo a,c, Hiroshi Goto a,c a
Macro Bio-Electromechanical-Autonomous-Nano-Systems (BEANS) Center, BEANS Project, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan Research Center for Ubiquitous MEMS and Micro Engineering, National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan c Toshiba Machine Co., Ltd., 2068-3 Ooka, Numazu, Shizuoka 410-8510, Japan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Roller nanoimprint Reel-to-reel process Plastic optical fiber Cylindrical mold E-textile Weaving guide MEMS
a b s t r a c t We are developing flexible and large-size e-textiles by weaving in a smart fiber. On the surface of the fiber, micro-electro-mechanical-systems (MEMS) structures, electronic circuit patterns, and guide structures for positioning during the weaving process are fabricated. In order to manufacture smart fibers in large quantities, we developed a cylindrical mold with hybrid-layered structures consisting of 260-lmwide macro patterns and several 10-lm-wide patterns fabricated by precise cutting of substrate employing forming tools and 3-D photolithography, and using a micro-patterned flexible photomask. Initially, the polished surface of the cylinder is covered with an electroless-plated Ni–P alloy; and convex structures with rectangular cross-sections and arc shaped convex structures are cut. Next, the surface of the cylinder is coated with a positive-tone photoresist by a dipping process. Ultraviolet (UV) lights are then irradiated on the photoresist through a flexible micro-patterned photomask wrapped around the cylinder; and then the micro-patterns on the photomask are thus transferred onto the cylindrical surface. Next, following a Cu electroplating, hybrid-layered patterns are made that comprise 260-lm-wide macro and several 10-lm-wide microstructures. In the final step, the photoresist is then removed. And moreover, in order to realize high-speed imprinting, the mechanical stiffness of a reel-to-reel thermal imprint system is improved, and the press-force control method is changed. The imprint system is equipped with a completed cylindrical mold; and using the system, a plastic optical fiber (POF) comprising a 240-lmdiameter PMMA core with a 5-lm-thick fluoride clad is imprinted for a demonstration. The cylindrical mold heated up to 50 °C with an infrared lamp, is pushed into the surface of the POF to a depth of 4.5–6.7 lm. As a result, a continuous imprinting on the POF is achieved at an imprint speed of 20 m/min. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction It is important to realize that low impact on the environment is achievable by the industry departing from its dependence on the ever increasing size of Si wafers, and by circumventing the constraints imposed by the micro-electro-mechanical-systems (MEMS) manufacturing equipments that require a vacuum environment for their operations. We are now developing large-size MEMS devices, and technologies to make them. As one of the techniques for overcoming the size limitations of MEMS devices, we chose to fabricate MEMS structure on fibrous substrates instead of using the Si chips; and we are now witnessing the regular fibers evolve into ‘‘smart fibers’’ with built-in functions such as sensors ⇑ Corresponding author at: Macro Bio-Electromechanical-Autonomous-NanoSystems (BEANS) Center, BEANS Project, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan. Tel.: +81 29 861 7020. E-mail address: [email protected] (H. Mekaru).
and actuators as shown in Fig. 1. For the next step, we are developing flexible and large-size e-textiles by weaving-in of a smart fiber. Since an e-textile device is flexible like clothes we wear, it is considered quite fitting for covering the curved as well as continuous surface of human body parts. Such a characteristic becomes helpful in the development of in-the-living-body healthy monitor which would integrate ultrasonic probes with flexible displays. On the surface of the fiber, MEMS structures, electronic circuit patterns, and guide structures for positioning during the weaving process are fabricated. In order to manufacture smart fibers in large quantities, we have explored the applicability of hot embossing and nanoimprinting processes capable of providing high throughput that can qualify as a commercially viable process for the fabrication of microstructures directly on the cylindrical fibers. Among the various nanoimprint technologies, the roller imprint technology [1] using a cylindrical mold is a suitable way to continuously fabricate micro-/nano-structures on the surface of films and fibrous substrates without putting any limits on the length of the
0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.036
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Fig. 1. Illustration of MEMS structures and weaving guides on the surface of a smart fiber.
substrate [2]. Especially when a molding material happens to be a large-size film, a roll-to-roll nanoimprinting [3] where a pattern is continuously being transferred by a cylindrical mold onto a film that is conveyed from a sending roll to a winding roll, is seen as an ultimate mass-production method of micro-/nanostructures. Since ultraviolet (UV) roll-to-roll nanoimprinting (without causing any thermal expansion or contraction) can transfer micro-/nanostructures with high accuracy [4], the process has been used to fabricate optical devices for displays [5] and anti-reflective films [6], as well as has been used in flexible electronics [7]. On the other hand, thermal roll-to-roll nanoimprinting has contributed to the development of chemical/bio-chips like microfluidics [8] and labon-a-chips [9] with comparatively large-size patterns because it offers a wide-range of molding materials to select from. Furthermore, in another work a hot-embossing was employed on a polyster fiber instead of films, where 50-lm-wide diffraction grating was transferred on the fiber at a feeding speed of some tens mm/s [10]. Moreover, many trials that attach a new function to imprinted patterns were also made by combining with other technologies. Nagato, et al. succeeded in continuous manufacturing of multi-layered nanostructures of PMMA and Cr, by combining technologies such as thermal imprinting, vapor deposition, and thermal bonding [11]. Mäkeäl, et al. employed gravure technique in a room-temperature nanoimprinting and achieved 500-nm-wide diffraction grating pattern on conductive polyaniline-dodecylbenzenesufonic acid on a polypropylene substrate [12]. Moreover, Jiang, et al. dropped a semiconductor polymer ink into microcavities between the convex patterns on the cylindrical mold surface, and the ink patterns were transplanted to a polyrthylene terephthalate and polypropylen substrates by a roller-reversal imprinting [13]. In our technique, various patterns are transferred on a fibrous substrate by a reel-to-reel thermal imprint method. And next, we plan to manufacture a smart fiber after metallization by inkjet printing to fill up the cavities of the imprinted patterns with various materials. Since cantilevers and electric circuits are currently assumed as main components in MEMS structures, we presume that the pattern width would be tens of lm. On the other hand, since a weaving guide has to fix several-hundred-lm diameter woofs, a comparatively large structure of the order of hundreds of microns wide was required. Such structures because of the differences in their pattern sizes, would naturally differ in their imprint conditions [14]. In a previous work, by taking special precaution that a fibrous substrate be not damaged, we processed MEMS structures by roller imprinting with sliding planar molds [15–17]. On the other hand, weaving guides had been processed where the convex mold patterns of several 100-lm width were pushed into the surface of the fibrous substrates, where the fibrous substrates were partially damaged [18–20]. However, if such structures were fabricated by some other imprint methods, the result would have been of low throughput and imprint technologies could not have met the criteria for high-volume manufacturing.
Therefore, in this experiment, MEMS structures and weaving guides are processed on the surface of a cylindrical mold, and we tried to imprint both patterns simultaneously by a single shot of roller-imprinting on a fibrous substrate. Using this seamless cylindrical mold, a continuous and high-speed imprinting technology on a fibrous substrate was reported in this paper.
2. Cylindrical mold with hybrid-layered microstructures Fig. 1 shows an example of a smart fiber that we designed. A weaving guide and a deep hollow which patterns MEMS structures are arranged in 1 mm of pitches on a 250-lm-diameter plastic optical fiber (POF). A circular cross-sectional shaped hollow with a curvature radius of 290 lm was formed on the POF so that another POF with the same diameter could be inserted into the hollow as warps. Moreover, in order to insert the conducting material into the cavity of a concave imprinted pattern, obtained by inkjet printing, a flat surface rather than a cylindrical surface would be the right shape for holding the dropped material. Therefore, a rectangular cross-sectional-shaped groove with a width of 260 lm was fabricated on the POF, and a several 10-lm-wide MEMS patterns were transferred on the flat bottom surface of the rectangular groove of the POF. In order to process such structures on POFs by a single shot of the roller thermal imprinting, we developed a seamless cylindrical mold 100 mm in diameter and 30 mm in width as shown in Fig. 2(a). Fig. 3 shows a process flow to fabricate the cylindrical mold. The polished surface of the cylinder was covered with electroless-plated Ni–P, and convex structures with rectangular cross-sections and arc shaped convex structures were cut with a forming tool. After Cu was sputter-deposited, the surface of the cylinder was coated with a positive-tone photoresist by a dipping process. In order to get a uniform thickness of photoresist film coated on the steel cylinder, obtained by a dipping process, the photoresist was diluted with a solvent to minimize its viscosity. And moreover, the photoresist film of the surface was dried by rotating the cylinder in order to prevent any dripping. UV lights were then irradiated on the photoresist through a micro-patterned flexible photomask wrapped around the cylinder. The flexible mask was set up to make physical contact with the photoresist’s surface while maintaining a moderate tension. Then, the cylindrical surface was divided into 20 (or more) areas, and an alignment process was carried out while visually monitoring each area at the time of UV exposure. We chose this division number because no variation in pattern width caused by inclined exposure on the curved surface was observed. In this way, the micro-patterns from the mask were transferred onto the cylinder surface. Next, following a Cu electroplating, hybrid-layered patterns comprising 260lm-wide macro and several 10-lm-wide microstructures were made; and the photoresist was then removed. T. Hung, et al. had also reported on a fabrication method of a cylindrical mold using
Please cite this article in press as: H. Mekaru et al., Microelectron. Eng. (2013), http://dx.doi.org/10.1016/j.mee.2013.03.036
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Fig. 2. (a) Photograph of a cylindrical mold with convex hybrid-layered microstructures and optical micrographs of mold patterns at: (b) center and (c) edge of the surface of the cylindrical mold.
1. Ni-P electroless-plating
Steel cylinder
Amorphous Ni-P
2. Cutting with forming tool
Macro-sized structures
5. UV exposure 6. Development with flexible photomask
3. Cu sputter-depositing
4. Photoresist coating by dipping Photoresist
Cu
7. Cu electroplating
8. Photoresist removal
UV lights Flexible photomask
Fig. 3. Process flow to fabricate a cylindrical mold with convex hybrid-layered microstructures.
a plane photomask [21]. If there were no uneven structure on the cylindrical surface, a standard 3-D UV lithography using a plane photomask could be used. However, like in our case, where an uneven structure with a width of several 100 lm and a depth of 50 lm does exist on the cylindrical surface, a flexible photomask which can conform to the cylindrical surface is considered apt for the task. In the manufacture process of the cylindrical mold used here, the tasks of electroplating followed by a 3-D photolithography using a flexible photomask were carried out by Optnics precision Co., Ltd., which is a manufacturer of X-ray masks with polyimide membrane. Fig. 2(b) shows an optical micrograph of line/space and dotted patterns on a rectangular convex structure and weaving guides arranged alternately on the cylindrical surface.
Fig. 2(c) shows another optical micrograph to which the edge of the mold pattern area was expanded, and where the rectangular and cylindrical cross-sectional-shaped convex structures can be clearly observed. 3. Re-modelling of reel-to-reel imprint system Previously, using a cylindrical mold with weaving guides, we had developed a reel-to-reel imprint system, and succeeded in a continuous imprinting at a speed of 5 m/min [19–20]. Our new target of thermal imprinting on POF at an imprint speed of 20 m/min was twice the maximum speed of the commercial roll-to-roll systems to do the job. In order to realize this improvement in the
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imprint speed, a load cell unit, which was set above the upper cylindrical mold, was then moved down to its new position under the lower cylindrical mold, which then raised the mechanical rigidity of a press device as shown in Fig. 4. An inserted figure in the figure shows a schematic diagram of an arrangement of the press device showing a side-view of the winding reel station. A guideway was fixed on beside the load cell to reduce any horizontal blur; and then vibration of the transverse direction in regard to the sending direction of a fibrous substrate was restrained. Moreover, a feedback system, that controlled the fluctuation of the distance between the upper and lower cylindrical molds by a load measurement, was changed into a feed-forward control device to imprint at a high speed, while referring to the load change data measured at a low rotation speed. Furthermore, the information was read into the database; and a program that predicted the vertical position of the upper cylindrical mold was also developed, while considering the delay time taken to actually move the position of the cylindrical mold by a pulse motor drive. After re-modelling, the standard deviation of the press force was reduced from 5.86 to 2.80 N. In addition, improvement in the imprint speed of the reel-to-reel imprint system by a feed-forward controlling has been explained in detail in ref. [22].
(a)
(b) Weaving guides
Hybrid-layered patterns 200 µ m Fig. 5. (a) Optical micrograph of imprinted POF, and (b) expanded optical micrograph of weaving guides and hybrid-layered microstructures on the surface of the POF.
4. Imprint results on POFs An imprinting result on a 250-lm-diameter POF CK-10 (Mitsubishi Rayon) are shown in Fig. 5 under conditions of imprint temperature, press force, and feeding speed being 50 °C, 30 N, and 20 m/min. The POF CK-10 consists of 240-lm diameter polymenthylmethacrylate (PMMA) core and 5-lm-thick fluororesin clad. Based on the experimental data in the previous work [14], conditions that could be used for deep imprinting without crushing the POFs, were selected. On the surface of the fiber, the method successfully fabricated microstructures comprising concave rectangle of a maximum depth of 6.7 lm, and an arc structure of a maximum depth of 5.0 lm, as shown in Fig. 5(b). Furthermore, line/space patterns were transferred onto the bottom surface of the concave rectangle grooves (refer to the white arrow in Fig. 2(b)). In order to evaluate the patterns transfer accuracy on POFs from the cylindrical mold, a square dot with its designed width 20 lm was observed. Fig. 6(a) and (b) also show the relative setups of the mold and the POF in the imprint step and an optical
micrograph of concave-dotted mold patterns, respectively. Fig. 6(c) and (d) also show an illustration of the imprint position of the POF on the mold pattern surface and an optical micrograph of a convex dotted pattern, respectively. The thickness of Cu top layer varied around 2–3 lm depending upon the position of observation as measured by a high-precision non-contact depth measuring microscope Hisomet-II DH II (Union Optical). Table 1 shows a transition of average size of square dotted patterns. From the enlarged optical micrographs inserted at the upper-right corner of Fig. 6(b) and (d), the average width of the concave and convex square-dotted patterns on the mold was found to be 13.0 and 25.3 lm, respectively. These values are estimated to be 35% and +26.5% as compared to the ideal width of 20 lm which was a designed value in the photomask. And it was observed that as the widths became large, the error became small. This behavior can result in the shrinkage of the photoresist structures in a 3-D photolithography process, and in the expansion of the Cu structures in an electroplating process. Fig. 7 shows a cross-sectional profile of the square-dotted im-
Load cell unit
(a)
(b) Load cell unit
Upper cylindrical mold
POF
Remodelling Cylindrical molds Lower cylindrical mold
Load cell unit Guideways
Fig. 4. Photographs of press device included upper and lower cylindrical molds in a reel-to-reel imprint system: (a) before, and (b) after re-modelling. Schematic diagram of an arrangement of the press device showing a side-view of the winding reel station was inserted in the figure.
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Fig. 6. (a) Illustration of relative positions between concave-square-dot mold patterns and POF in the imprint process, and (b) optical micrograph of concave-dots patterns on the cylindrical mold. Enlarged optical photograph inserted in the upper- right corner shows concave square dots with a designed pattern width of 20 lm. (c) Illustration of relative position between the convex-square-dot mold patterns and POF in the imprint process, and (d) optical micrograph of convex-dots patterns on the cylindrical mold. Enlarged optical photograph inserted in upper- right corner shows convex square dots with a designed pattern width of 20 lm.
Table 1 Transition of average size of square dotted patterns. Pattern shape
Concave
Item
Width
Depth
Width
Height
Photomask (Designed value) (Electroplating)
20 lm
–
20 lm
–
Mold (Imprint)
13.3 lm
2-3 lm
25.3 lm
2-3 lm
POF
28.4 lm
662 nm
14.2 lm
547 nm
printed pattern measured by a confocal microscope Optelics S130 (Lasertec). Here, after considering the disparity between the size of a mold pattern and its actual design value, we now address the difference between the size of an imprinted pattern and that of the mold pattern. The bottom surface of POFs where dotted patterns were transferred was not deformed into a perfect flat surface. In convex square dotted patterns of Fig. 7(a), the average height of the pattern was 547 nm. Moreover, the average pattern width was 14.2 lm and the error in reference to the concave mold pattern width was +9%. Next, in concave square-dotted patterns of Fig. 7(b), the average value of the pattern depth and width was 662 nm and 28.4 lm, respectively. The imprinted pattern width
Convex
became larger than 12% as compared with the convex mold pattern, and the error in reference to the pattern width became large in comparison to that of the concave mold pattern. In general, when we compare the vertical movement of a convex mold pattern on a plane surface of mold with a convex mold pattern on a convex surface of a rotating cylindrical mold, we can find that during the imprinting and de-molding process the later setup may cause some scooping out of material [23]. Therefore, the patterns which are transferred from the cylindrical mold to a molding material result in the imprinting of relatively large widths. Moreover, the width and depth of the rectangular groove were measured to be 267.5 and 6.78 lm in Fig. 7(a), and 271.1 and 4.58 lm in Fig. 7(b), respec-
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Fig. 7. (a) 3-D image of convex square-dot patterns imprinted on the surface of POF and cross-sectional profile along a line A-A0 of convex square patterns. (b) 3-D image of concave square-dot patterns imprinted on the surface of POF, and cross-sectional profile along a line B-B0 of concave square patterns.
tively. Since the design value of rectangle width was 260 lm, the error of the rectangular groove width estimates as 2.9 and 4.3%, respectively. In the ref. [8], when a PMMA film was processed by a roll-to-roll embossing using a cylindrical mold at a feeding speed of 0.6–0.8 m/min, it can be estimated that an error in the imprinted pattern width was nearly 15% compared to that of the bottom width of a mold pattern. The transfer accuracy of the reel-to-reel imprinting on POFs does not show great difference as compared with the result of embossing on the PMMA film. Although the mechanical stability of the POF may be adversely affected by the concave imprinted patterns, the tension added to the POF at the time of reel-to-reel imprinting was 2–10 N. On the other hand, where the tension added to a warp in a weaving system [24] was 1 N, the likelihood of the imprinted POF breaking during the weaving process would be highly improbable. From these results, although the manufacture process of a seamless cylindrical mold needs to be improved, its practicality was checked, and it was proven that a high-speed of imprinting at 20 m/min was possible without causing any damage to the POFs. Furthermore, we are now investigating the relationship between the feeding speed and the imprinted depth. 5. Summary We manufactured the cylindrical mold with hybrid-layered patterns in order to fabricate weaving guides and MEMS structures on a fibrous substrate by a single shot of imprinting to develop a smart fiber. Circular and rectangular cross-sectional shaped convex structures with a width of 260 lm were processed on the cylindrical mold by precise cutting using a forming tool. Then hybridlayered patterns were formed by a 3-D photolithography using the flexible photomask and Cu electroforming. Moreover, in order
to make high-speed imprinting possible at a feeding speed of 20 m/min, the control system of press force was changed from a feedback method to a feed-forward method in consideration of the delay time, and then the standard deviation of the press force was reduced to half the value obtained from a conventional method using feed-back controlling. The cylindrical mold was included in the reel-to-reel thermal imprint system after re-modelling, and experiment was carried out using the POF CK-10 as a fibrous substrate for a demonstration. As a result, it succeeded in high-speed imprinting of circular shaped weaving guides, and rectangular shaped grooves which contained MEMS patterns on its bottom surface. The pattern widths of the concave and convex square dots on the cylindrical mold increased by 13.0 and 25.3%, respectively, as compared with the design value of 20 lm. Moreover, the error of the convex and concave imprinted pattern widths was 9 and 12%, respectively as compared to that of the width of the mold patterns. Moreover, the average depth of convex and concave imprinted dotted patterns was 547 and 662 nm, respectively. As mentioned above, we re-modelled the reel-to-reel thermal imprint system in order to carry out high-speed imprinting on the fibrous substrate at a feeding speed of 20 m/min. The cylindrical mold was produced in order to carry out continuous transferring of hybridlayered patterns onto the POFs. These results show that the reelto-reel imprint technology using the cylindrical mold is quite a useful tool to manufacture a smart fiber. Acknowledgements The 3-D photolithography and Cu electroplating processes were executed by Masashi Kobayashi and Yoshiyuki Ichinosawa of Optnics Precision Co. Ltd. This work was supported by New Energy and Industrial Technology Development Organization (NEDO).
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
High-speed imprinting on plastic optical fibers using cylindrical mold with hybrid microstructures Harutaka Mekaru a,b,⇑, Akihiro Ohtomo a,c, Hideki Takagi a,b, Mitsunori Kokubo a,c, Hiroshi Goto a,c a
Macro Bio-Electromechanical-Autonomous-Nano-Systems (BEANS) Center, BEANS Project, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan Research Center for Ubiquitous MEMS and Micro Engineering, National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan c Toshiba Machine Co., Ltd., 2068-3 Ooka, Numazu, Shizuoka 410-8510, Japan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Roller nanoimprint Reel-to-reel process Plastic optical fiber Cylindrical mold E-textile Weaving guide MEMS
a b s t r a c t We are developing flexible and large-size e-textiles by weaving in a smart fiber. On the surface of the fiber, micro-electro-mechanical-systems (MEMS) structures, electronic circuit patterns, and guide structures for positioning during the weaving process are fabricated. In order to manufacture smart fibers in large quantities, we developed a cylindrical mold with hybrid-layered structures consisting of 260-lmwide macro patterns and several 10-lm-wide patterns fabricated by precise cutting of substrate employing forming tools and 3-D photolithography, and using a micro-patterned flexible photomask. Initially, the polished surface of the cylinder is covered with an electroless-plated Ni–P alloy; and convex structures with rectangular cross-sections and arc shaped convex structures are cut. Next, the surface of the cylinder is coated with a positive-tone photoresist by a dipping process. Ultraviolet (UV) lights are then irradiated on the photoresist through a flexible micro-patterned photomask wrapped around the cylinder; and then the micro-patterns on the photomask are thus transferred onto the cylindrical surface. Next, following a Cu electroplating, hybrid-layered patterns are made that comprise 260-lm-wide macro and several 10-lm-wide microstructures. In the final step, the photoresist is then removed. And moreover, in order to realize high-speed imprinting, the mechanical stiffness of a reel-to-reel thermal imprint system is improved, and the press-force control method is changed. The imprint system is equipped with a completed cylindrical mold; and using the system, a plastic optical fiber (POF) comprising a 240-lmdiameter PMMA core with a 5-lm-thick fluoride clad is imprinted for a demonstration. The cylindrical mold heated up to 50 °C with an infrared lamp, is pushed into the surface of the POF to a depth of 4.5–6.7 lm. As a result, a continuous imprinting on the POF is achieved at an imprint speed of 20 m/min. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction It is important to realize that low impact on the environment is achievable by the industry departing from its dependence on the ever increasing size of Si wafers, and by circumventing the constraints imposed by the micro-electro-mechanical-systems (MEMS) manufacturing equipments that require a vacuum environment for their operations. We are now developing large-size MEMS devices, and technologies to make them. As one of the techniques for overcoming the size limitations of MEMS devices, we chose to fabricate MEMS structure on fibrous substrates instead of using the Si chips; and we are now witnessing the regular fibers evolve into ‘‘smart fibers’’ with built-in functions such as sensors ⇑ Corresponding author at: Macro Bio-Electromechanical-Autonomous-NanoSystems (BEANS) Center, BEANS Project, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan. Tel.: +81 29 861 7020. E-mail address: [email protected] (H. Mekaru).
and actuators as shown in Fig. 1. For the next step, we are developing flexible and large-size e-textiles by weaving-in of a smart fiber. Since an e-textile device is flexible like clothes we wear, it is considered quite fitting for covering the curved as well as continuous surface of human body parts. Such a characteristic becomes helpful in the development of in-the-living-body healthy monitor which would integrate ultrasonic probes with flexible displays. On the surface of the fiber, MEMS structures, electronic circuit patterns, and guide structures for positioning during the weaving process are fabricated. In order to manufacture smart fibers in large quantities, we have explored the applicability of hot embossing and nanoimprinting processes capable of providing high throughput that can qualify as a commercially viable process for the fabrication of microstructures directly on the cylindrical fibers. Among the various nanoimprint technologies, the roller imprint technology [1] using a cylindrical mold is a suitable way to continuously fabricate micro-/nano-structures on the surface of films and fibrous substrates without putting any limits on the length of the
0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.036
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Fig. 1. Illustration of MEMS structures and weaving guides on the surface of a smart fiber.
substrate [2]. Especially when a molding material happens to be a large-size film, a roll-to-roll nanoimprinting [3] where a pattern is continuously being transferred by a cylindrical mold onto a film that is conveyed from a sending roll to a winding roll, is seen as an ultimate mass-production method of micro-/nanostructures. Since ultraviolet (UV) roll-to-roll nanoimprinting (without causing any thermal expansion or contraction) can transfer micro-/nanostructures with high accuracy [4], the process has been used to fabricate optical devices for displays [5] and anti-reflective films [6], as well as has been used in flexible electronics [7]. On the other hand, thermal roll-to-roll nanoimprinting has contributed to the development of chemical/bio-chips like microfluidics [8] and labon-a-chips [9] with comparatively large-size patterns because it offers a wide-range of molding materials to select from. Furthermore, in another work a hot-embossing was employed on a polyster fiber instead of films, where 50-lm-wide diffraction grating was transferred on the fiber at a feeding speed of some tens mm/s [10]. Moreover, many trials that attach a new function to imprinted patterns were also made by combining with other technologies. Nagato, et al. succeeded in continuous manufacturing of multi-layered nanostructures of PMMA and Cr, by combining technologies such as thermal imprinting, vapor deposition, and thermal bonding [11]. Mäkeäl, et al. employed gravure technique in a room-temperature nanoimprinting and achieved 500-nm-wide diffraction grating pattern on conductive polyaniline-dodecylbenzenesufonic acid on a polypropylene substrate [12]. Moreover, Jiang, et al. dropped a semiconductor polymer ink into microcavities between the convex patterns on the cylindrical mold surface, and the ink patterns were transplanted to a polyrthylene terephthalate and polypropylen substrates by a roller-reversal imprinting [13]. In our technique, various patterns are transferred on a fibrous substrate by a reel-to-reel thermal imprint method. And next, we plan to manufacture a smart fiber after metallization by inkjet printing to fill up the cavities of the imprinted patterns with various materials. Since cantilevers and electric circuits are currently assumed as main components in MEMS structures, we presume that the pattern width would be tens of lm. On the other hand, since a weaving guide has to fix several-hundred-lm diameter woofs, a comparatively large structure of the order of hundreds of microns wide was required. Such structures because of the differences in their pattern sizes, would naturally differ in their imprint conditions [14]. In a previous work, by taking special precaution that a fibrous substrate be not damaged, we processed MEMS structures by roller imprinting with sliding planar molds [15–17]. On the other hand, weaving guides had been processed where the convex mold patterns of several 100-lm width were pushed into the surface of the fibrous substrates, where the fibrous substrates were partially damaged [18–20]. However, if such structures were fabricated by some other imprint methods, the result would have been of low throughput and imprint technologies could not have met the criteria for high-volume manufacturing.
Therefore, in this experiment, MEMS structures and weaving guides are processed on the surface of a cylindrical mold, and we tried to imprint both patterns simultaneously by a single shot of roller-imprinting on a fibrous substrate. Using this seamless cylindrical mold, a continuous and high-speed imprinting technology on a fibrous substrate was reported in this paper.
2. Cylindrical mold with hybrid-layered microstructures Fig. 1 shows an example of a smart fiber that we designed. A weaving guide and a deep hollow which patterns MEMS structures are arranged in 1 mm of pitches on a 250-lm-diameter plastic optical fiber (POF). A circular cross-sectional shaped hollow with a curvature radius of 290 lm was formed on the POF so that another POF with the same diameter could be inserted into the hollow as warps. Moreover, in order to insert the conducting material into the cavity of a concave imprinted pattern, obtained by inkjet printing, a flat surface rather than a cylindrical surface would be the right shape for holding the dropped material. Therefore, a rectangular cross-sectional-shaped groove with a width of 260 lm was fabricated on the POF, and a several 10-lm-wide MEMS patterns were transferred on the flat bottom surface of the rectangular groove of the POF. In order to process such structures on POFs by a single shot of the roller thermal imprinting, we developed a seamless cylindrical mold 100 mm in diameter and 30 mm in width as shown in Fig. 2(a). Fig. 3 shows a process flow to fabricate the cylindrical mold. The polished surface of the cylinder was covered with electroless-plated Ni–P, and convex structures with rectangular cross-sections and arc shaped convex structures were cut with a forming tool. After Cu was sputter-deposited, the surface of the cylinder was coated with a positive-tone photoresist by a dipping process. In order to get a uniform thickness of photoresist film coated on the steel cylinder, obtained by a dipping process, the photoresist was diluted with a solvent to minimize its viscosity. And moreover, the photoresist film of the surface was dried by rotating the cylinder in order to prevent any dripping. UV lights were then irradiated on the photoresist through a micro-patterned flexible photomask wrapped around the cylinder. The flexible mask was set up to make physical contact with the photoresist’s surface while maintaining a moderate tension. Then, the cylindrical surface was divided into 20 (or more) areas, and an alignment process was carried out while visually monitoring each area at the time of UV exposure. We chose this division number because no variation in pattern width caused by inclined exposure on the curved surface was observed. In this way, the micro-patterns from the mask were transferred onto the cylinder surface. Next, following a Cu electroplating, hybrid-layered patterns comprising 260lm-wide macro and several 10-lm-wide microstructures were made; and the photoresist was then removed. T. Hung, et al. had also reported on a fabrication method of a cylindrical mold using
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Fig. 2. (a) Photograph of a cylindrical mold with convex hybrid-layered microstructures and optical micrographs of mold patterns at: (b) center and (c) edge of the surface of the cylindrical mold.
1. Ni-P electroless-plating
Steel cylinder
Amorphous Ni-P
2. Cutting with forming tool
Macro-sized structures
5. UV exposure 6. Development with flexible photomask
3. Cu sputter-depositing
4. Photoresist coating by dipping Photoresist
Cu
7. Cu electroplating
8. Photoresist removal
UV lights Flexible photomask
Fig. 3. Process flow to fabricate a cylindrical mold with convex hybrid-layered microstructures.
a plane photomask [21]. If there were no uneven structure on the cylindrical surface, a standard 3-D UV lithography using a plane photomask could be used. However, like in our case, where an uneven structure with a width of several 100 lm and a depth of 50 lm does exist on the cylindrical surface, a flexible photomask which can conform to the cylindrical surface is considered apt for the task. In the manufacture process of the cylindrical mold used here, the tasks of electroplating followed by a 3-D photolithography using a flexible photomask were carried out by Optnics precision Co., Ltd., which is a manufacturer of X-ray masks with polyimide membrane. Fig. 2(b) shows an optical micrograph of line/space and dotted patterns on a rectangular convex structure and weaving guides arranged alternately on the cylindrical surface.
Fig. 2(c) shows another optical micrograph to which the edge of the mold pattern area was expanded, and where the rectangular and cylindrical cross-sectional-shaped convex structures can be clearly observed. 3. Re-modelling of reel-to-reel imprint system Previously, using a cylindrical mold with weaving guides, we had developed a reel-to-reel imprint system, and succeeded in a continuous imprinting at a speed of 5 m/min [19–20]. Our new target of thermal imprinting on POF at an imprint speed of 20 m/min was twice the maximum speed of the commercial roll-to-roll systems to do the job. In order to realize this improvement in the
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imprint speed, a load cell unit, which was set above the upper cylindrical mold, was then moved down to its new position under the lower cylindrical mold, which then raised the mechanical rigidity of a press device as shown in Fig. 4. An inserted figure in the figure shows a schematic diagram of an arrangement of the press device showing a side-view of the winding reel station. A guideway was fixed on beside the load cell to reduce any horizontal blur; and then vibration of the transverse direction in regard to the sending direction of a fibrous substrate was restrained. Moreover, a feedback system, that controlled the fluctuation of the distance between the upper and lower cylindrical molds by a load measurement, was changed into a feed-forward control device to imprint at a high speed, while referring to the load change data measured at a low rotation speed. Furthermore, the information was read into the database; and a program that predicted the vertical position of the upper cylindrical mold was also developed, while considering the delay time taken to actually move the position of the cylindrical mold by a pulse motor drive. After re-modelling, the standard deviation of the press force was reduced from 5.86 to 2.80 N. In addition, improvement in the imprint speed of the reel-to-reel imprint system by a feed-forward controlling has been explained in detail in ref. [22].
(a)
(b) Weaving guides
Hybrid-layered patterns 200 µ m Fig. 5. (a) Optical micrograph of imprinted POF, and (b) expanded optical micrograph of weaving guides and hybrid-layered microstructures on the surface of the POF.
4. Imprint results on POFs An imprinting result on a 250-lm-diameter POF CK-10 (Mitsubishi Rayon) are shown in Fig. 5 under conditions of imprint temperature, press force, and feeding speed being 50 °C, 30 N, and 20 m/min. The POF CK-10 consists of 240-lm diameter polymenthylmethacrylate (PMMA) core and 5-lm-thick fluororesin clad. Based on the experimental data in the previous work [14], conditions that could be used for deep imprinting without crushing the POFs, were selected. On the surface of the fiber, the method successfully fabricated microstructures comprising concave rectangle of a maximum depth of 6.7 lm, and an arc structure of a maximum depth of 5.0 lm, as shown in Fig. 5(b). Furthermore, line/space patterns were transferred onto the bottom surface of the concave rectangle grooves (refer to the white arrow in Fig. 2(b)). In order to evaluate the patterns transfer accuracy on POFs from the cylindrical mold, a square dot with its designed width 20 lm was observed. Fig. 6(a) and (b) also show the relative setups of the mold and the POF in the imprint step and an optical
micrograph of concave-dotted mold patterns, respectively. Fig. 6(c) and (d) also show an illustration of the imprint position of the POF on the mold pattern surface and an optical micrograph of a convex dotted pattern, respectively. The thickness of Cu top layer varied around 2–3 lm depending upon the position of observation as measured by a high-precision non-contact depth measuring microscope Hisomet-II DH II (Union Optical). Table 1 shows a transition of average size of square dotted patterns. From the enlarged optical micrographs inserted at the upper-right corner of Fig. 6(b) and (d), the average width of the concave and convex square-dotted patterns on the mold was found to be 13.0 and 25.3 lm, respectively. These values are estimated to be 35% and +26.5% as compared to the ideal width of 20 lm which was a designed value in the photomask. And it was observed that as the widths became large, the error became small. This behavior can result in the shrinkage of the photoresist structures in a 3-D photolithography process, and in the expansion of the Cu structures in an electroplating process. Fig. 7 shows a cross-sectional profile of the square-dotted im-
Load cell unit
(a)
(b) Load cell unit
Upper cylindrical mold
POF
Remodelling Cylindrical molds Lower cylindrical mold
Load cell unit Guideways
Fig. 4. Photographs of press device included upper and lower cylindrical molds in a reel-to-reel imprint system: (a) before, and (b) after re-modelling. Schematic diagram of an arrangement of the press device showing a side-view of the winding reel station was inserted in the figure.
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Fig. 6. (a) Illustration of relative positions between concave-square-dot mold patterns and POF in the imprint process, and (b) optical micrograph of concave-dots patterns on the cylindrical mold. Enlarged optical photograph inserted in the upper- right corner shows concave square dots with a designed pattern width of 20 lm. (c) Illustration of relative position between the convex-square-dot mold patterns and POF in the imprint process, and (d) optical micrograph of convex-dots patterns on the cylindrical mold. Enlarged optical photograph inserted in upper- right corner shows convex square dots with a designed pattern width of 20 lm.
Table 1 Transition of average size of square dotted patterns. Pattern shape
Concave
Item
Width
Depth
Width
Height
Photomask (Designed value) (Electroplating)
20 lm
–
20 lm
–
Mold (Imprint)
13.3 lm
2-3 lm
25.3 lm
2-3 lm
POF
28.4 lm
662 nm
14.2 lm
547 nm
printed pattern measured by a confocal microscope Optelics S130 (Lasertec). Here, after considering the disparity between the size of a mold pattern and its actual design value, we now address the difference between the size of an imprinted pattern and that of the mold pattern. The bottom surface of POFs where dotted patterns were transferred was not deformed into a perfect flat surface. In convex square dotted patterns of Fig. 7(a), the average height of the pattern was 547 nm. Moreover, the average pattern width was 14.2 lm and the error in reference to the concave mold pattern width was +9%. Next, in concave square-dotted patterns of Fig. 7(b), the average value of the pattern depth and width was 662 nm and 28.4 lm, respectively. The imprinted pattern width
Convex
became larger than 12% as compared with the convex mold pattern, and the error in reference to the pattern width became large in comparison to that of the concave mold pattern. In general, when we compare the vertical movement of a convex mold pattern on a plane surface of mold with a convex mold pattern on a convex surface of a rotating cylindrical mold, we can find that during the imprinting and de-molding process the later setup may cause some scooping out of material [23]. Therefore, the patterns which are transferred from the cylindrical mold to a molding material result in the imprinting of relatively large widths. Moreover, the width and depth of the rectangular groove were measured to be 267.5 and 6.78 lm in Fig. 7(a), and 271.1 and 4.58 lm in Fig. 7(b), respec-
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Fig. 7. (a) 3-D image of convex square-dot patterns imprinted on the surface of POF and cross-sectional profile along a line A-A0 of convex square patterns. (b) 3-D image of concave square-dot patterns imprinted on the surface of POF, and cross-sectional profile along a line B-B0 of concave square patterns.
tively. Since the design value of rectangle width was 260 lm, the error of the rectangular groove width estimates as 2.9 and 4.3%, respectively. In the ref. [8], when a PMMA film was processed by a roll-to-roll embossing using a cylindrical mold at a feeding speed of 0.6–0.8 m/min, it can be estimated that an error in the imprinted pattern width was nearly 15% compared to that of the bottom width of a mold pattern. The transfer accuracy of the reel-to-reel imprinting on POFs does not show great difference as compared with the result of embossing on the PMMA film. Although the mechanical stability of the POF may be adversely affected by the concave imprinted patterns, the tension added to the POF at the time of reel-to-reel imprinting was 2–10 N. On the other hand, where the tension added to a warp in a weaving system [24] was 1 N, the likelihood of the imprinted POF breaking during the weaving process would be highly improbable. From these results, although the manufacture process of a seamless cylindrical mold needs to be improved, its practicality was checked, and it was proven that a high-speed of imprinting at 20 m/min was possible without causing any damage to the POFs. Furthermore, we are now investigating the relationship between the feeding speed and the imprinted depth. 5. Summary We manufactured the cylindrical mold with hybrid-layered patterns in order to fabricate weaving guides and MEMS structures on a fibrous substrate by a single shot of imprinting to develop a smart fiber. Circular and rectangular cross-sectional shaped convex structures with a width of 260 lm were processed on the cylindrical mold by precise cutting using a forming tool. Then hybridlayered patterns were formed by a 3-D photolithography using the flexible photomask and Cu electroforming. Moreover, in order
to make high-speed imprinting possible at a feeding speed of 20 m/min, the control system of press force was changed from a feedback method to a feed-forward method in consideration of the delay time, and then the standard deviation of the press force was reduced to half the value obtained from a conventional method using feed-back controlling. The cylindrical mold was included in the reel-to-reel thermal imprint system after re-modelling, and experiment was carried out using the POF CK-10 as a fibrous substrate for a demonstration. As a result, it succeeded in high-speed imprinting of circular shaped weaving guides, and rectangular shaped grooves which contained MEMS patterns on its bottom surface. The pattern widths of the concave and convex square dots on the cylindrical mold increased by 13.0 and 25.3%, respectively, as compared with the design value of 20 lm. Moreover, the error of the convex and concave imprinted pattern widths was 9 and 12%, respectively as compared to that of the width of the mold patterns. Moreover, the average depth of convex and concave imprinted dotted patterns was 547 and 662 nm, respectively. As mentioned above, we re-modelled the reel-to-reel thermal imprint system in order to carry out high-speed imprinting on the fibrous substrate at a feeding speed of 20 m/min. The cylindrical mold was produced in order to carry out continuous transferring of hybridlayered patterns onto the POFs. These results show that the reelto-reel imprint technology using the cylindrical mold is quite a useful tool to manufacture a smart fiber. Acknowledgements The 3-D photolithography and Cu electroplating processes were executed by Masashi Kobayashi and Yoshiyuki Ichinosawa of Optnics Precision Co. Ltd. This work was supported by New Energy and Industrial Technology Development Organization (NEDO).
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