# Uniform Pressing Mechanism in Large-Area Roll-to-Roll Nanoimprint Lithography Process
## 2 R2R NIL Pressing System  
### Conventional Pressing System  
For increased productivity, when applying the R2R pressing process, the axial length of the imprinting roller must be long for large-area NIL. To make conformal contact with the pattern roller on a wide substrate, a large force is applied to both ends of the imprinting roller. As shown in Figure 1a, the imprinting module of the conventional R2R NIL system includes a master roller made of steel and an imprinting metal roller coated with rubber skin. Typically, a patterned flexible mold is wound on and adhered to the master roller and the substrate is passed between the two rollers during the process. After the resin is deposited on the substrate, the mold pattern on the master roller is duplicated on the resin-coated substrate by the application of a force to external force points \(F_{1}\) and \(F_{2}\) of the imprinting. The two forces generate the contact pressure between the substrate and the pattern roller. At this time, bending deformation of the imprinting roller occurs, which causes non-uniform pressure distribution on the contact area. This can be predicted through simple static modeling, as shown in Figure 1b. Roller bending deformation is usually the largest contributor to nip pressure variations across the width. As seen in Figure 2, all end-loaded rollers will deflect and bow away from the nip, which causes pinching at the ends. Indeed, non-uniformity is caused on the metal roller's nips. Even with only about the outer 10-20% of the ends in contact, there will be a gap in the center. Rubber coating on the imprinting roller can help even out the nip load variation; however, the tendency is still there [16]. As shown in Figure 2b, stress-strain characteristics for rubber are mainly divided into three different regions. First, there is a linear deformation region where the stress linearly increases with the increase in strain. Second, a plateau region occurs, indicating that the stress remains constant despite the change in strain. Third, there is a densification region where the stress dramatically increases as the strain increases [17]. As shown in Figure 2a, the area at both ends of the rubber layer (skin) is densified due to the bending deformation of the roller. It can be seen that in this densification region, the pressure distribution in the contact area increases dramatically due to the high compressive elastic modulus of rubber. Calculating the imprinting roller (rubber-steel composite) bending deformation and the contact pressure variation and the flexural properties is difficult [18; 19; 20; 21]. Therefore, numerical solutions such as finite element modeling (FEM) analysis were used to analyze the mechanical behavior of composites [16; 22]. Using the static structural analysis of ANSYS (analysis system) 2019 R3, FEM analysis was performed, as shown in Figure 3a, to check the non-uniform pressure distribution at the contact area between the master roller and the imprinting roller due to bending deformation during the pressing process. The specifications of the imprinting roller and the master roller used in the analysis are the same as the conventional system (PNT, R2R  
Figure 1: Conventional R2R NIL pressing process: (**a**) Imprinting module of conventional R2R NIL system; (**b**) Free body diagram of conventional R2R NIL pressing process. Figure 2: (**a**) Roller bending deformation of imprinting roller when pushing against master roller; the master roller’s deformation is ignored, and the imprinting roller has rubber skin. (**b**) The curve of the compressive measure for rubber. NIL system) specifications used in the industry. For efficient and accurate analysis, the global mesh was generated with coarse size, and the area of interest where the contact occurred was generated with the element size set to fine size. The area of interest was generated as a hexahedral element with the mesh size set to 2 mm.