Finite Element Simulations of Filling and Demolding in Roll-to-Roll UV Nanoimprinting of Micro- and Nanopatterns

Roll-to-roll UV nanoimprinting is a powerful method for the mass fabrication of nano- and microstructured surfaces, which are highly interesting for many technological applications (e.g., in the fields of optics, electronics, biomimetic, and microfluidics). When setting up a production process based on this technique, one of the main challenges is the prevention of defects (mainly entrapped air during filling and fractures during demolding). This can be cost- and time-intensive as it is mainly done by trial and error. An improved theoretical understanding of defect generation and its prediction for certain material and process parameters is therefore desirable. To accomplish this, we developed COMSOL-based two-dimensional (2D) and three-dimensional (3D) computer simulations for the two key stages of UV nanoimprinting (filling and demolding) and validated them by corresponding roll-to-roll as well as step-and-repeat experiments. Regarding filling, the investigated parameters are template and substrate contact angles; resin viscosity, velocity, and thickness during filling; as well as feature geometry. In summary, it is beneficial for filling to have low template contact angles; high substrate contact angles; low resin viscosity and velocity; as well as inclined sidewalls, low-aspect-ratio features, and a sufficient resin thickness (whereby lack of one of these factors can be compensated by others). Interestingly, nanoscale features are much easier to fill than microscale features in practice (which is not due to reduced bubble trapping but due to enhanced gas dissolution). Regarding demolding, we studied the sidewall angle, fillet radius, size, and elastic modulus of the features. In addition, we compared demolding by roll-to-roll and by step-and-repeat considering the radius of rotation and we decoupled bending, adhesion, and friction to investigate their relative contributions. We could demonstrate quantitatively that for demolding, it is advantageous to have small features, inclined sidewalls, rounded corners, and a large radius of rotation. The dominant effect for nanostructures is adhesion, whereas for microstructures, it is friction. Moreover, demolding by tilting (step-and-repeat) exerts less stress on the imprint than demolding using a roll-to-roll approach. Finally, we present a 3D demolding simulation that identifies the most vulnerable positions of a geometry. From the lessons learned from our filling and demolding simulations, we could demonstrate the defect-free roll-to-roll UV nanoimprinting of a challenging pattern (cuboids with vertical sidewalls).