Recent developments and design challenges in continuous roller micro- and nanoimprinting
(Color online) Schematic of the conventional imprinting process. Most imprinting techniques were originally developed as batch processes where patterns are fabricated on discrete substrates one at a time as shown. Batch mode imprinting is a contact lithography process in which a rigid mold is pressed into a resist film deposited on a discrete substrate in order to transfer the negative micro- or nano-scale profile of the mold into the resist. However, because the mold is typically very expensive to fabricate, only a small area is patterned and thus only a discrete substrate is required.
(Color online) Schematic of a coventional 2-roller thermal imprint process for thermoplastic polymer webs. Typically the web is preheated to just below the glass transition temperature (Tg) of the web, while the roller mold is heated well above Tg to enable the polymer to flow into the mold cavities. Pressure is usually applied to the web either by pressing the roller mold against a conformal backing roller (as shown) or by rigidly fixing the two rollers to control the gap width between them to be slightly narrower than the thickness of the web feed. In the 2-roller setup, heating, imprinting and demolding processes are all integrated into the scanning action of the roller.
Early roller imprint tool schematic produced by Chou. Designed for patterning of thermoplastic polymers by raising the temperature of the roll heater above the glass transition thereof. The hinge is loaded with a dead weight to apply the required pressure to promote filling of the mold cavities. Reprinted with permission from H. Tan, A. Gilbertson, and S. Y. Chou, J. Vac. Sci. Technol. B. 16, 3926 (1998). Copyright © 1998 by American Vacuum Society.
(a) AFM profile of PMMA gratings, 190 nm period, 40 nm depth imprinted with a flat mold where the roller shown in Fig. 2 acts only as a pressure transfer mechanism. (b) AFM profile of PMMA imprinted by a nickel compact disk master mold wrapped around the aforementioned roller. Each track is 700 nm wide. Reprinted with permission from H. Tan, A. Gilbertson, and S. Y. Chou, J. Vac. Sci. Technol. B. 16, 3926 (1998). Copyright © 1998 by American Vacuum Society.
(Color online) Schematic of a typical continuous UV roller imprinting setup with UV lamp exposure unit. A dispensing system is utilized to deposit a UV curable liquid resin either as a pattern of drops or as a continuous film (shown). Following deposition of the resin, a variety of thickness control measures can be employed, such as a doctor blade (shown). Multiple pressure rollers are often used to ensure uniform spreading of the resin and filling of the roller mold cavities prior to UV exposure. A demolding roller is used to peel the cured patterns off the roller mold. There are many possible variations to this setup. However, this schematic shows some of the most commonly used elements.
(Color online) (a) Schematic showing one-dimensional viscoelastic flow of a thick substrate material into an infinitely long channel of width W and height H. The roller mold rotates with velocity V as the channel makes contact with the substrate and fills over time tfill . The substrate and roller mold remain in contact over the contact width L under applied pressure P(t). The substrate material has shear viscosity η(T). (b) Schematic showing one-dimensional viscoelastic flow of a thin resin film with initial and final thicknesses ho and hf , respectively. The roller mold applies a force F(t) that squeezes the resin into an infinitely long channel over time tfill .
Conventional hydraulic 2-roller thermal imprinting machine. A sheet mold is mounted on the top metallic roller shown. The large roller beneath is the backing or pressure roller. This system is capable of applying a compressive force of up to 50 kN at elevated temperature to pattern thermoplastic webs. Reprinted with permission from N. Ishizawa, K. Idei, T. Kimura, D. Noda, and T. Hattori, Microsyst. Technol. 14, 1381 (2008). Copyright © 2008 by Springer Science+Business Media B.V.
(Color online) (a) Single screw plastic extruder with coat-hanger die for fabricating polycarbonate webs (background). The residual heat from the film extrusion is used to pattern the web by passing it through a conventional 2-roller imprinting setup with a draw reel (foreground). (b) Schematic drawing of the hybrid extrusion roller-imprint setup for fabricating microlens arrays on polycarbonate webs. Reprinted with permission from L. T. Jiang, T. C. Huang, C. R. Chiu, C. Y. Chang, and S. Y. Yang, Opt. Express 15, 12088 (2007). Copyright © 2007 by OSA.
SEM image of a microlens array fabricated on a polycarbonate web by Jiang using a hybrid extrusion roller-imprinting apparatus. The average diameter of each microlens is ∼210 μm with sag height of ∼12 μm. These micro-lenses were fabricated by partial filling in of a corresponding array of micro-holes etched into a copper roller. Reprinted with permission from L. T. Jiang, T. C. Huang, C. R. Chiu, C. Y. Chang, and S. Y. Yang, Opt. Express 15, 12088 (2007). Copyright © 2007 by OSA.
(Color online) (a) Schematic of an improved thermal roller imprinting setup designed by Fagan. This setup utilizes a conveyor ribbon mold with a dummy roller that extends the mold ribbon away from contact with the web in order to allow the mold to cool. An induction heater is placed between the roller mold and the dummy roller to directly heat the mold ribbon to near the melting point of the PET web (255 °C). (b) AFM scan showing the replicated result in PET from a nickel electroformed master consisting of a 250 nm linewidth, 375 nm period and 200 nm height grating. Some deformations are observable. Reprinted with permission from M. D. Fagan, B. H. Kim, and D. Yao, Adv. Polym. Technol. 28, 246 (2009). Copyright © 2010 by Wiley Periodicals, Inc.
(a) Double-sided thermal roller imprinting apparatus designed by Mäkelä. Nickel sheet molds were wrapped on both the top and bottom rollers which were heated to 105 °C in order to pattern the cellulose acetate web on both sides. (b) AFM scan showing 1 μm diameter dots ranging from 110 – 130 nm height on the top side of the web. The scan shows there may have been some viscoelastic recovery after demolding, causing neighboring dots to meld. Reprinted with permission from T. Mäkelä, T. Haatainen, P. Majander, J. Ahopelto, and V. Lambertini, Jpn. J. Appl. Phys. 47, 5142 (2008). Copyright © 2008 by The Japan Society of Applied Physics.
(Color online) (a) Schematic showing iterative, or multilayer roller imprinting by bonding of PMMA near the Tg . This is done by introducing a tertiary bonding roller which bonds a prepatterned PMMA film to the backside of the film emitted from a conventional two-roller thermal imprinting setup. (b) SEM image showing five-layer stack of patterned PMMA films with 800 nm pitch, enclosed nanogap structures. As each layer is bonded to the stack, the gaps formed in the earlier bonding operations deform and shrink. Reprinted with permission from K. Nagato, S. Sugimoto, T. Hamaguchi, and M. Nakao, Microelectron. Eng. 87, 1543 (2010). Copyright © 2009 by Elsevier B.V.
Profilometry scans of an imprinted 100 μm wide microfluidic channel in PMMA at different temperatures. Note the large pile-up of displaced polymer on the trailing edge of the channel. The asymmetry of the pile-up is due to the lateral scanning and demolding action of the roller mold. The microscope image on the right shows the scan direction of the roller mold. For large features and short imprinting times, the displaced polymer is too viscous to level off or travel far from the point of displacement. Among other outcomes, this phenomena can affect the imprint fidelity of other nearby features. Reprinted with permission from S. H. Ng and Z. F. Wang, Microsyst. Technol. 15, 1149 (2009). Copyright © 2008 by Springer Science+Business Media B.V.
(Color online) (a) SEM image showing an anti-reflective conical cylinder array fabricated from a proprietary UV curable resin (Mitsubishi 7700) on a flexible PET substrate. The cones are each of 200 nm diameter, 350 nm height and 400 nm pitch. (b) Reflectance and transmittance spectra over the visible spectrum for the fabricated anti-reflective film. The reflectance maxima is 2.45% at 700 nm. Reprinted with permission from C. J. Ting, F. Y. Chang, C. F. Chen, and C. P. Chou, J. Micromech. Microeng. 18, 075001 (2008). Copyright © 2008 by IOP Publishing Ltd.
(Color online) (a) Photograph of 700 nm period, 300 nm linewidth epoxysilicone grating pattern imprinted on a PET strip by UV roller imprinting and showing bright light diffraction. (b) The smallest features fabricated by Guo, 100 nm period, 70 nm linewidth epoxysilicone grating shown under SEM. (c) SEM cross-section of 200 nm, 70 nm linewidth epoxysilicone grating. Reprinted with permission from S. H. Ahn and L. J. Guo, Adv. Mater. 20, 2044 (2008). Copyright © 2008 by Wiley-VCH Verlag.
(Color online) (a) Schematic of UV roller imprinting setup for flexible substrates utilizing a three-roller resin coating system with reservoir to control the thickness of the resin film and a tensioned ETFE belt mold supported by two rollers. A long belt mold helps to maximize the resin spreading time, which assists with trapped gas dissolution, and expands the potential UV curing area for increased throughput. (b) UV roll-to-flat schematic for rigid substrates. (c) Photograph of six-inch UV roller imprinting apparatus. (d) 4 in. wide, 12 in. long, epoxysilicone grating pattern on flexible PET substrate fabricated by UV roller imprinting. The grating dimensions are 300 nm linewidth, 600 nm height and 700 nm pitch. Reprinted with permission from S. H. Ahn and L. J. Guo, ACS Nano 3, 2304 (2009). Copyright © 2009 by American Chemical Society.
(a) SEM image of a pyramid array with 50 μm pitch and 24 μm height fabricated by UV roller imprinting in urethane acrylate photopolymer. CNC diamond machining, followed by nickel electroforming, was used to produce the mold by which these unique pyrimidal structures were fabricated. Reprinted with permission from S. Ahn, M. Choi, H. Bae, J. Lim, H. Myung, H. Kim, and S. Kang, Jpn. J. Appl. Phys. 46, 5478 (2007). Copyright © 2007 by The Japan Society of Applied Physics. (b) SEM image of UV roller imprinted nanopillars with diameter of 100 nm, height 35 nm, and pitch 150 nm. Reprinted with permission from S. Ahn, J. Cha, H. Myung, S. Kim, and S. Kang, Appl. Phys. Lett. 89, 213101 (2006). Copyright © 2006 by American Institute of Physics.
(a) SEM cross-section of a 70 nm linewidth, 70 nm height, 140 nm pitch grating master mold in silicon. Inset shows the polyurethane acrylate (PUA) replicated mold. The PUA rigiflex mold exhibits rounded corners and a line height of only 90 nm, indicating curing shrinkage. (b) SEM cross-section image of a polystyrene film with a grating replicated from the aforementioned PUA rigiflex mold (shown beneath). The thermal imprint process against the polystyrene film faithfully replicated the aforementioned rounded corners and 90 nm line height of the PUA grating. Reprinted with permission from S. Seo, T. Kim, and H. H. Lee, Microelectron. Eng. 84, 567 (2007). Copyright © 2006 by Elsevier B.V.
Scanning force microscopy (SFM) images of PS-b-PEO self-assembled films at different film thickness on patterned PBT substrates (pitch: ∼130 nm, amplitude: ∼15 nm). Film thickness of (a) 23 nm and (b) 29 nm. A hierarchically structured surface was obtained which, with additional processing steps, can be used to fabricate sheet molds for continuous roller imprinting. Reprinted with permission from S. Park, D. H. Lee, and T. P. Russell, Adv. Mater. 22, 1882 (2010). Copyright © 2010 by WILEY-VCH Verlag.
(Color online) Stepped rotating photolithography apparatus shown with rotation stage, roller and photomask plate. The rotation stage is used to place each region to be exposed directly under the photomask. Each region on the photomask is exposed onto the resist coated roller in a serial process with a UV-LED light source. Reprinted with permission from T. C. Huang, J. T. Wu, S. Y. Yang, P. H. Huang, and S. H. Chang, Microelectron. Eng. 86, 615 (2009). Copyright © 2009 by Elsevier B.V.
(a) Process schematic showing fabrication of a roll mold by e-beam writing and development. Taniguchi and Aratani did not subsequently etch the developed resist patterns into the roll itself, most likely because of the inadequate resolution of wet etch processes. (b) AFM scan showing 312 nm lines replicated in UV curable resin deposited on a PET substrate. These lines were fabricated directly from corresponding PMMA features on the roll mold. Reproduced with permission. Reprinted with permission from J. Taniguchi and M. Aratani, J. Vac. Sci. Technol. B. 27, 2841 (2009). Copyright © 2009 by American Vacuum Society.
(Color online) AFM scans of a ∼1 μm wide trench formed by two-step inclination ablation of an iron film sputtered onto a fused silica cylinder. The laser scan speed was 0.16 mm s−1 at 10.7 nJ and the inclination angle was 75°. (a) First inclination ablation showing sidewalls with differing draft angles. (b) Final trench profile after the second inclination ablation. The sidewalls now have approximately the same draft angle. Reprinted with permission from W. Wang, X. Mei, and G. Jiang, Int. J. Adv. Manuf. Technol. 41, 504 (2009). Copyright © 2008 by Springer Science+Business Media B.V.
(Color online) Procedure for fabricating a soft PDMS roller with an array of microlens cavities. (a) A polycarbonate sheet mold containing an array of microlenses is fabricated from a rigid master. This sheet mold is then wrapped around a silicone or aluminum alloy cylinder with a depression around its circumference and PDMS prepolymer is poured into the hollow shell cavity as shown. (b) After curing of the prepolymer, the polycarbonate film is peeled off the cured PDMS, leaving a soft PDMS roller with a micro-lens cavity array. Reprinted with permission from C. Y. Chang, S. Y. Yang, and M. H. Chu, Microelectron. Eng. 84, 355 (2007). Copyright © 2007 by Elsevier B.V.
Recent achievements in UV roller imprinting, with materials employed and applications. In the first row, the highest reported sustainable imprint patterning throughput (not winding speed) is given. Second row, the largest handling size measured by the width of the continuous substrate. In the Third row, the highest reported feature resolution to date.
Recent achievements in thermal roller imprinting, with materials employed and applications. In the first row, the highest reported sustainable imprint patterning throughput (not winding speed) is given. Second row, the largest handling size measured by the width of the continuous substrate. In the Third row, the highest reported feature resolution to date.
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