# Uniform Pressing Mechanism in Large-Area Roll-to-Roll Nanoimprint Lithography Process
## 2 R2R NIL Pressing System
### Conventional Pressing System
It can be confirmed from Figure 3b that when pressing both ends of the imprinting roller with a total force of 4000 N, the ends of the contact area showed a high pressure and the center of the roller showed a low pressure. Generally, the material properties in the FE analysis are assumed to be elastic and the conditions are ideal; however, the physical properties of the rubber skin were not accurate and the conditions are not ideal. Therefore, verification was conducted through an actual experiment. An imprint pressing experiment was conducted to confirm the non-uniform pressure distribution on the contact area, the nip between the master roller and the imprinting roller, due to bending deformation during the actual pressing process. The experiment was conducted using a large-area R2R NIL system; the pressure and pressure distribution at the contact area were measured using a pressure measurement film (PMF, Model PRESCALE LLLLW, Fujifilm, Tokyo, Japan). The experimental equipment and configuration are shown in Figure 4a. The pressure distribution was measured upon the master roller after pressing with 200 kgf by using each of the pneumatic cylinders installed at both ends of the imprinting roller. The measurement results are shown in Figure 4b. After the pressing process, the PMF experienced color change. The change was analyzed and then averaged over an area, 1 mm\({}^{2}\), to transform to the measured pressure value. Since the actual measured values contain noise, the measured data were fitted using MATLAB; through this, it was found  
Figure 4: Imprinting module of conventional R2R NIL system used in experiments: (**a**) Setup of pressure measurement film on conventional R2R NIL system for experiments; (**b**) Pressure distribution along roller. Figure 3: Pressure distribution along roller during simulation (FEM): (**a**) Imprinting module of conventional R2R NIL used in simulation (FEM); (**b**) Distributed pressure on the contact area. that the measured pressure at both ends of the contact area tends to be higher than the value at the center. However, the pressure at the central part is low and at both ends the pressure tends to be high compared to the value in FE analysis. Additionally, the maximum values of the pressure at the two ends are not the same in the real situation. However, the tendency of the pressure distribution from the central part toward both ends appears to be similar to the FE simulation. In Figure 3b, the contact pressure does not have a symmetrical distribution about the center of the roller. This means that other causes are involved. Other than the bending, there are three more reasons why non-uniform pressure distribution exists when pressing the roller. During the pressing process, there are roller skin deflection, waviness error in the contact area due to roundness error and the straightness error of the roller and roller misalignment [16]. Other than roller deflection, all factors are caused during system manufacture. The best way to improve these problems is to improve the manufacturing process. However, to improve these problems, it is necessary to use precision machining and a precise assembly process, and thus the cost may be very high. The waviness error of the roller causes a problem in which contact is not made uniformly during the pressing process; this can be improved to a certain degree by using the rubber skin coating on the surface of the imprinting roller. However, the combination of roller misalignment and roller skin deflection causes non-uniform pressure distribution on the contact area, and because the roller rotates during the imprinting process, the pressure distribution changes with the passage of time. Therefore, there is a limitation to using the conventional roller pressing system to eliminate contact pressure non-uniformity.