Effect of Chain Conformation on the Polymer Deformation for a Nano-scale Molding Process

Abstract

In recent years, many studies related to the manufacturing of sub-100nm structures have been carried out in an effort to develop devices and core components for the future of science and industry. However, material exhibits different behaviorial tendencies at the micro/nano-sclae level than they do at the macro level. Existing micro/nano-scale molding processes have been developed based on continuum mechanics, such as deformation in the viscoelastic region or flow behavior in the viscous region. However, the deformation of a polymer during the molding process can be changed from that in the macro scale by decreasing the dimension of the molded parts below the polymer chain size. That is, with downsized patterns and process dimensions, the size effects of polymer chains such as the polymer conformational dynamics and the entanglement effect should be considered in the analysis and process design. In this study, nano-scale deformation of entangled polymer chains was investigated by thermal nanoimprint lithography (T-NIL) experiments. The dependence of the processability on the ratio of the polymer chain size, represented by the radius of gyration (Rg), and the cavity diameter was investigated by varying the pressure and film thickness for the imprinting process. The T-NIL process was implemented under the condition where only segmental deformation in the molecular chain occurs rather than in the entire chain network of the polystyrene (PS)-film. The results of the experiment of the bulk thick film revealed that if the cavity diameter was smaller than the diameter of gyration (2Rg) of the polymer, the processability decreased as the cavity diameter decreased, whereas no significant difference was observed if the cavity diameter was greater than 2Rg. Moreover, under a higher imprinting pressure, hardly any improvement in the processability was observed in the cavity diameter less than 2Rg, whereas considerable improvement was observed in a large cavity diameter. These results were also verified with the local density variation of the imprinted patterns. In this study, the effect of the film thickness on the polymer deformation was also investigated. When the initial film thickness is larger than 15Rg, the filling height was converged regardless of the film thickness at the given process conditions (temperature, pressure, molecular weight). On the other hand, when the initial film thickness is less than 15Rg, the filling height decreased for each cavity region. In addition, the dimensional analysis was performed based on the result of the experiment, and proposed relationship between dimensionless parameters. Finally, the apparent viscosity was reversely estimated using the filling ratio results, and a correlation with the film thickness was obtained. Through the estimation of the apparent viscosity, the physical meaning about the processability depending on the initial film thickness was analyzed in the continuum perspective. Furthermore, it is confirmed that the analytic solution applied the apparent viscosity model well predicted the actual filling height. Thus, the one master curve representing processability for different film thickness could be obtained from analytic approach. In addition, the numerical simulation applied the apparent viscosity model was investigated for different initial film thickness. The estimated apparent viscosity model well predicted the flow behavior compared to the existing Carreau-Yasuda model.