Etching behavior of Si-containing polymers as resist materials for bilayer lithography: The case of poly-dimethyl siloxane

This work is focused on the plasma development of siloxanes investigated as model Si-containing photoresist components that show a promise for bilayer lithography at 157 nm and other Next Generation Lithography technologies. In such lithography, the image is developed in the top photosensitive polymer and transferred to the (usually thick) organic underlayer by means of O2-based plasma etching. In this work particularly, the issue of line edge roughness (LER) induced by transfer etching and its reduction by means of plasma processing optimization is addressed. The experimental results reveal that low values of line-edge roughness are obtained in a high-density plasma reactor, if an F- but not O-containing etching first step is used in appropriate plasma conditions. The effect of different etching chemistries and processing conditions on imaging layer roughness formation is demonstrated with the aid of scanning electron microscopy images and image analysis for quantifying LER, and atomic force microscopy (AFM) for measuring surface roughness. X-ray photoelectron spectroscopy analysis of etched PDMS is used to show the evolution of the chemical modification of the PDMS layer, to measure the top oxide thickness, and to correlate both to processing conditions. In situ interferometry and ellipsometry are used to determine the etch resistance of the imaging PDMS layer and the selectivity of the transfer etching process. It is demonstrated that optimum LER correlates well with plasma processing conditions that ensure a nonselective first etching step prior to a highly selective main etching.