Stability of masking materials for pattern transfer of lithographic masks into fused silica by atmospheric pressure plasma jet etching

Abstract

Masking of thin films and bulk materials is traditionally applied for the transfer of micron patterns into the functional material according to the requirements of the application. For optical purposes, lithographically produced micron patterns are transferred by plasma/ion etching, which is a traditional technology in microelectronics and other micron technologies. However, pattern transfer by atmospheric pressure plasma etching can help to save time and cost for a future sustainable production. Therefore, the pattern transfer of lithographic resist masks into fused silica using atmospheric pressure reactive plasma jets (APPJ) was studied as a new approach of micropatterning.

First the etch rates of the potential masking materials, e.g. photoresists, as well as of fused silica as substrate are studied in dependence on the APPJ etching parameters, in particular on the gas composition (O₂/CF₄) and the dwell time of the APPJ tool's footprint. Typical etch rates of the masking materials are in the range of 140 to 370 nm·s⁻¹ whereas the fused silica has a rate of 25 to 80 nm·s⁻¹. The surface morphology of masking materials changes during etching and features additional nanoscale roughness and waviness. The surface roughness of the etched masking materials and the fused silica are 2 to 5 nm rms and 1.5 nm rms for etch depths of ∼3000 nm and ∼ 600 nm, respectively. Finally, the pattern transfer by APPJ of a diffraction grating with a period of 15 μm, depth of 230 nm and a roughness below 2 nm rms into fused silica was demonstrated.