Helium plasma modification of Si and Si3N4 thin films for advanced etch processes

To achieve the etching of silicon nitride spacers with a perfect anisotropy and an almost infinite selectivity, an alternative method consisting of two sequential steps-surface modification in hydrogen or helium plasma followed by the selective removal of modified layers in wet solutions or by exposure to gaseous reactants-was recently proposed. In this paper, molecular dynamics (MD) simulations of low-energy (5-250 eV) He+ bombardment on c-Si and amorphous Si3N4 surfaces are performed to understand all mechanisms driving the light ion implantation in this process. The modification of the Si and Si3N4 substrates with the ion dose is investigated, as well as their structure and composition at steady state. Simulations show a self-limited ion implantation with a surface evolution composed of two stages: a rapid volume transformation (with almost no sputtering) followed by a slow saturation and the formation of a stable He-implanted layer at steady state. The mechanisms of ion-induced damage (Si-Si or Si-N bond breaking, He trapping) induce the creation of a less dense modified layer, full of He-filled cavities and containing weaker bonds, which can facilitate its subsequent removal in a second step. The impact of the ion energy on the modified thickness and sputtering is studied. Finally, a comparison between MD and experiments performed on Si3N4 substrates is reported, showing a both qualitative and quantitative agreement. Published by the AVS.