Enhanced planarization technology by reactive ion beam etching and using a nanoimprint resist

In the pursuit of high-end optics, the fabrication of ultra-smooth surfaces is essential to minimize quality degradation and light scattering and to improve optical performance. One example of this are mirrors that are produced using single-point diamond turning (SPDT), a process that introduces surface roughness necessitating subsequent post-processing. This study investigates the use of ion beam planarization (IBP) with a low-viscosity nanoimprint photoresist applied via spin coating as a novel method for turning mark removal. The study evaluates the effectiveness of this nanoimprint resist in reducing surface roughness across various spatial frequencies on specially prepared silicon substrates with line patterns up to a spatial wavelength of 30 \(\upmu \)m. The degree of planarization (DoP) achieved with the nanoimprint resist was compared to that of a conventional photoresist. Additionally, the etch transfer of the smooth resist surface to the silicon substrate was demonstrated using reactive ion beam etching (RIBE) under normal ion incidence. Through a gradient etching process, the selectivity - the removal rate ratio of resist to substrate - was varied around a value of 1, assessing the process stability. The results demonstrated that the nanoimprint resist exhibits superior leveling properties compared to established photoresists, consistently achieving a DoP of 6% irrespective of spatial wavelengths. In contrast, the standard photoresist showed a significant loss of DoP at spatial wavelengths exceeding 9 \(\upmu \)m. Furthermore, successful pattern transfer of the smooth resist morphology into the substrate was accomplished, with a RMS-roughness reduction achieved from 13 nm to 0.5 nm. Variations in selectivity around the optimal value of 1 generally exhibited the predicted behavior, though a slight shift of the optimum from theoretical predictions towards lower values was observed and discussed.