High shape-accuracy of surface roughnesses upon nano-moulding with optical elastomers

Nano-imprint and nano-moulding with polymers have quite some history already, but usually, those polymers are not specifically selected to allow for optical applications, where absorption and damping due to volume scattering have to be as small as possible. Defined optical (surface) scattering plates represent one such application, i.e., transparent plates with specific surface roughness morphologies. Typically optical scattering plates are made of glass, and the roughness is produced with a mask-less, self-organizational reactive ion etching (RIE) process. As this technology is relatively expensive, it would be beneficial to replicate such surfaces by imprint or moulding into a polymer. Since the scattering characteristics are not only related to micron-sized but also to nm-sized features on the surface, the right choice of the monomer solution is crucial. Here we use the PDMS-based transparent elastomer Elastosil® RT 601 A/B by Wacker Chemie, Munich, Germany, with very good results. We conduct two nano-moulding steps, the first one making the transition from the master to the negative form, the second one from the negative to the positive form. In the case of such scattering plates, the shape-accuracy can be characterized by the very application for which the plates are intended, i.e., by optical scattering. For other applications, scattering might still be an option for characterization. In our case, the transmissive scattering characteristics of the master, the negative form, and the positive form are nearly identical, which reveals the high accuracy of the nano-moulding process. Moreover, the fact that there are nearly no differences between the scattering characteristics of the negative form on one side and the master or the positive form on the other side is a manifestation of ‘Babinet's principle’. Together with the very good achieved accuracy, this finding allows to confine the process to only one moulding step further on and, thus, to the inverse morphology in the polymer for scattering applications.