Rapid and conformal coating of polymer resins by airbrushing for continuous and high-speed roll-to-roll nanopatterning: parametric quality controls and extended applications
© Korea Nano Technology Research Society 2017
Received: 7 March 2017
Accepted: 13 April 2017
Published: 1 May 2017
We present a facile and scalable coating method based on controlled airbrushing, which is suitable for conformal resin coating in continuous roll-to-roll (R2R) nanoimprint lithography (NIL) process. By controlling the concentration of UV-curable polymeric resin with mixing the volatile solvent and its airbrushing time, the coated resin film thickness can be readily tuned. After R2R NIL using a flexible nanoscale line pattern (nanograting) mold is conducted upon the airbrushed resin film, a large-area uniform nanograting pattern is fabricated with controlled residual layer thickness (RLT) based on the initial film thickness. We investigate the faithful airbrushing condition that can reliably create the uniform thin films as well as various nanopatterns with controlled morphologies. Using more diluted resin and shorter airbrushing time can reduce the RLTs favourably for many applications, yet is apt to induce the nanoscale pores and discontinued lines. We also discuss how to further improve the quality and scalability of resin airbrushing and its potential applications particularly requiring high-speed and conformal coating on highly topographic and flexible surfaces.
Roll-to-roll (R2R) nanoimprint lithography (NIL) enables large-area nanopatterning either on a rigid or flexible substrate by the continuous imprinting of the nanoscale pattern upon a polymeric resin film based on the pressurized rolling of a mold-bearing roll [1–4]. A wide choice of substrate materials, large processable area, and high speed of R2R NIL provide a unique methodology for the scalable and practical fabrication of many flexible and large-area applications ranging from optical and photonic components such as wire-grid polarizers [5, 6], metastructured optical filters , and antireflection films  through energy devices  to plasmonic sensors . A successful R2R NIL is a combination of a well-prepared flexible mold that is large enough to wrap around the roll, a well-formulated resin that is suitable for high-speed R2R NIL with quick curing and smooth demolding, and a reliable resin coating method that can afford continuous R2R processing principle. While these three main aspects have been addressed in detail elsewhere , it still remains a challenge to establish a faithful resin coating process where typical spin-coating or drop-casting might obviously not be the best solution for coating a uniform resin film on a continuously feeding large-area substrate.
To this end, we present a facile, rapid, and scalable resin coating protocol by utilizing a controlled airbrushing technique. A UV-curable polymeric resin is diluted in a volatile solvent for smooth flowing, which is airbrushed over the substrate to leave a uniform thin resin film behind. This quick process is well-suited to the continuous and high-speed R2R NIL; we first review that a uniform nanoscale line (nanograting) pattern can be faithfully fabricated by conducting R2R NIL with a flexible nanograting mold onto the airbrushed resin films based on our previous work . By modulating the resin concentration and airbrushing time, the initial thin film thickness as well as the residual layer thickness (RLT) of the nanopatterns created by R2R NIL can be readily controlled. We systematically investigate the parametric effects of the airbrushing process on the resulting nanopattern quality in our experimental space; reducing resin concentration and airbrushing time can lead to thinner RLTs usually favorable in most applications, yet is prone to cause the nanoscale pores and discontinued lines due to solvent evaporation and/or insufficient resin. We also discuss how to further improve the quality and scalability along with the potential applications that can benefit from the airbrushing and other extended large-area coating methodology such as the doctor blade coating.
2 Results and discussion
2.1 Process overview
A full experimental detail of airbrushing is described elsewhere . Briefly, an airbrusher (compressed propellant blown at the maximum pressure of 0.15 MPa) connected to a compressed air cylinder and resist material vessel is mounted vertically over the substrate with the 15 cm distance. For the substrate materials, we choose polyethylene terephthalate (PET) films and Si wafers as the representatives of flexible and rigid substrates, respectively. In this study, we use epoxy-silsesquioxane (SSQ) mixed with 3 wt% photoacid generator as a UV-curable resist material  whose concentration can be modulated (e.g., 2, 5, 10, and 20 wt%) by diluting in propylene glycol methyl ether acetate (PGMEA). Airbrushing time of the diluted SSQ resist onto the target substrate is varied (e.g., 1, 2, and 3 s). A R2R NIL processing is performed on the SSQ-coated substrate by using the custom-built 400 mm-wide R2R NIL instrument , at the imprinting pressure of ~5 psi, followed by UV curing. Here, flexible molds bearing the 700 nm-period and 500 nm-deep nanograting and the microscale dot array are prepared by the soft lithography method  using polydimethylsiloxane (PDMS). The mold area can be scaled up as needed to wrap around the roll, by performing the tiling of master mold pieces in a slightly overlapped fashion .
2.2 Effect of resin concentration on nanopore generation
Though the nanopore generation may have an adverse influence for the normal purpose of obtaining smooth nanopattern surface, it can be useful for specific applications by utilizing its large surface area and/or rough topology. For instance, the nanopored nanostructures can make more advantageous templates for sensors and electrodes with increased surface areas, thereby enhancing the sensitivities and efficiencies. Additionally, the dimples in the nanopored surface can provide favorable spots for selective docking of the small particles such as cells , working as a functional nanoparticle sorting and trapping device .
2.3 Effect of airbrushing time on nanograting continuity
2.4 Further applications and advances for conformal, scalable, and high-speed coating
Not only to the RLT-controllable resin coating for R2R NIL and simple planar thin film fabrication, can the airbrushing method also be applicable to more specific uses and broader materials. As the microscale droplets out of an airbrusher nozzle can smoothly reflow along the surface topography , airbrushing further enables the conformal coating over the surfaces especially with large surface roughness, which otherwise demands special treatments in ordinary spin coating or drop casting. By airbrushing, many functional materials can be conformally coated on highly topographic and flexible surfaces, including photoresists , anti-sticking agents , and active layers in flexible photovoltaic cells [23–25].
In summary, we introduce the airbrushing method for conformal and high-speed coating of functional polymers and demonstrate its application in continuous and scalable R2R NIL with controlled RLTs and further potential uses. The polymer film thickness as well as the surface morphology and profile can be modulated by regulating the concentration of UV-curable polymeric resin with mixing the volatile solvent and its airbrushing time. Using more diluted resin and shorter airbrushing time can reduce the RLTs favourably for many applications, while generating the surficial nanopores and/or dewetting-driven fragments upon UV curing. Airbrushing can be practically applied to many unique applications by enabling the conformal coating over the highly topographic and flexible surfaces at high speed, including but not limited to various functional coatings, electronics, and energy conversion devices.
All authors have contributed to the writing of the manuscript. All authors read and approved the final manuscript.
We thank Bo Kyung Kim for assistance with SEM imaging and the National NanoFab Center (NNFC) for assistance with microfabrication.
The authors declare that they have no competing interests.
This study was supported by the Research Program funded by the Seoul National University of Science and Technology.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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