investigation of Ruthenium Etching Induced by Electron beam Irradiation and O_2/Cl_2 Remote Plasma-Based Neutral Fluxes: Mechanistic Insights and Etching Model

Ruthenium (Ru) films have recently received attention in the microelectronics industry due to their unique physical and chemical properties. In this work, we investigated etching of Ru using an approach that combines surface functionalization using the effluent of a remote plasma source (RPS) fed with Ar/O₂/Cl₂ gas mixtures and electron beam (EB) irradiation. Simultaneous exposure of the Ru substrate to reactive fluxes from the RPS and the energetic EB source exhibits a synergistic effect: For combined fluxes, the Ru etch rate (ER) is greater than for separate RPS exposure or EB irradiation. The RPS generates reactive neutral species that functionalize the Ru surface through oxidation and chlorination. The flux of energetic (1 keV) electrons incident on the Ru surface induces Ru etching. A parametric study in which the electron flux density [proportional to the electron emission current (EC)], relative Cl₂ and O₂ flow rates, and RP power were varied was performed to examine the impacts of the neutral and electron fluxes on the Ru ER. The Ru etching reactions change from being electron flux-limited for small EC to neutral flux-limited etching for large EC or for small reactive gas flows. We also show that selective removal of Ru over Ta, which is important for applications like extreme ultraviolet photomask repair, can be realized for these process conditions. For energetic EB bombardment and Ar/O₂/Cl₂ RP exposure, a Ru/Ta etching selectivity of ∼6 can be realized. Spatially resolved x-ray photoelectron spectroscopy (XPS) has been performed to characterize the surface chemistry for (a) locations exposed to both reactive neutral and energetic electron fluxes and (b) areas only exposed to the reactive neutral flux produced by the RPS. The XPS results support an EB and RP induced Ru etching mechanism where Ru etching is based on the formation of volatile Ru-oxides, and in which the role of Cl is to assist in Ru oxidation. A surface etching model based on the consideration of the incident oxygen and chlorine fluxes, Langmuir adsorption limited surface functionalization, and EB bombardment causing volatilization of RuO₄ in the etching process has been developed. The model can successfully account for the major parametric observations of the Ru ER seen for the energetic EB irradiation and RPS-generated reactive neutral-induced etching process.