Mechanism of forming defects in roll-to-roll hot embossing of micro-pyramid arrays: II. Numerical study

Roll-to-roll (R2R) hot embossing is a continuous imprinting technique for the replication of large-area microstructures. Polymer deformation and recovery behavior is more complicated in this process than in the traditional plate-to-plate mode, especially for the embossing of 3-dimensional (3D) structures, such as the micro-pyramid structure used for reflective film. As a result, forming defects of poor shape and dimensional accuracy are more likely to emerge and are of greater diversity, which may negatively influence product quality and must be eliminated. In our previous experimental study, we have defined several types of forming defects that are commonly seen in embossed micro-pyramid arrays and have discussed the influence of processing parameters on the forming defects systematically. What is presented in this article is devoted to the simulation analysis of the forming mechanism of three typical types of forming defect: the platform-like defect, the bubble-like defect and the collapse-like defect. A 3D finite element model is established for the simulation analysis, in which the polymer resist is treated as a viscoelastic material based on the general Maxwell model. The simulation results clearly reveal the correlation between the forming defects and the processing conditions, indicating that the platform-like defect and the bubble-like defect are caused by material transport problems while the collapse-like defect results from polymer recovery. The underlying mechanisms for the forming defects disclosed in this research can provide effective guidelines for better process control as well as for improved design of the R2R hot embossing system.