An ultrawideband electromagnetic metamaterial absorber is proposed that consists of double-layer metapatterns optimally designed by the genetic algorithm and printed using carbon paste. By setting the sheet resistance of the intermediate carbon metapattern to a half of that of the top one, it is possible to find an optimal intermediate metapattern that reflects and absorbs the EM wave simultaneously. By adding an absorption resonance via a constructive interference at the top metapattern induced by the reflection from the intermediate one, an ultrawideband absorption can be achieved without increasing the number of layers. Moreover, it is found that the metapatterns support the surface plasmon polaritons which can supply an additional absorption resonance as well as boost the absorption in a broad bandwidth. Based on the simulation, the $$90\%$$ 90 % absorption bandwidth is confirmed from 6.3 to 30.1 GHz of which the fractional bandwidth is 130.77$$\%$$ % for the normal incidence. The accuracy is verified via measurements well matched with the simulations. The proposed metamaterial absorber could not only break though the conventional concept that the number of layers should be increased to extend the bandwidth but also provide a powerful solution to realize a low-profile, lightweight, and low cost electromagnetic absorber.

This study proposes the design of a roll-to-roll system for flexible electronics that enables accurate and precise tension control. It analyzes the factors for change in the tension of a roll-to-roll system and develops a tension model for each section to successfully predict the tension that is applied to such a system, the sagging of film according to tension, and deformation due to residual stress. This series of modeling processes allow engineers to design a roll-to-roll system for flexible electronics. Both a velocity control method for the tension between in-feeder and out-feeder—where there is no change of roller radius—and torque control method for the tension in modules like the rewinder, where the roll radius changes, are proposed. A roll-to-roll system according to the proposed design procedure and tension control methods was manufactured and experimented on under various test conditions. The accuracy and precision of velocity control between the in-feeder and the out-feeder were 100.01% and 1.15%, respectively, whereas those of torque control between the out-feeder and the rewinder were 99.9% and 1.35%, respectively, both at one sigma. The experiments confirmed that the two proposed types of tension control methods were accurate and precise. The experimental result with a monitoring sensor showed that the modeling was valid and that an accurate roll-to-roll system minimizing tension reduction was built. This study successfully demonstrated roll-to-roll system design and control techniques that are applicable to various pieces of roll-to-roll process equipment.

One of the key challenges for adapting the roll-to-roll (R2R) process to flexible electronics production is to fulfill the tight registration requirements between layers. This study proposed a precise registration control for an R2R screen printing system based on a combination of registration error analysis, passive compensation, and active compensation. The registration error factors, such as film tension error, sintering error, reference alignment mark accuracy, screen printing error, and screen mask error, were evaluated. Meanwhile, with a refined correlation factor through the experiments, evaluation-based passive compensation was applied to the design layout for making a screen mask. Active compensation is based on combined control, which consists of the tension control and stage alignment control, reduces the registration error's varying term. As a result of passive and active compensation for polyethylene terephthalate film, achieving a machine direction registration error of 0 ± 9 µm (σ) and cross direction of 10 ± 4 µm (σ) was possible. The proposed R2R system with registration control was capable of accurate and precise registration over several hundred prints and was suitable for manufacturing flexible electronic devices.

Demand for high throughput manufacturing has recently increased in various fields, such as electronics, photonics, optical devices, and energy. Moreover, flexible electronic devices are indispensable in applications such as touch screens, transparent conductive electrodes, transparent film heaters, organic photovoltaics, organic light-emitting diodes, and battery. For these applications, a large-area roll-to-roll (R2R) process is a promising method for producing with high throughput. However, bending deformation of rollers is unavoidable in a large-scale R2R system, which produces non-uniformity in force distribution during processing and reduces the sample quality. In this study, we propose a new R2R imprinting module to mitigate the deformation by using an additional backup roller to achieve uniform force distribution. From numerical simulations, we found that there exists an optimal imprinting force for each backup roller length to obtain the best uniformity. Experimental results using a large-area pressure sensor verified the effectiveness of the proposed method. Finally, the R2R nanoimprint lithography process showed that the proposed method produces patterns of 100 nm width with uniform residual layer thickness, which are distributed across the substrate of 1.2 m width.