Selective Etching of Ruthenium Using Electron Beam-Irradiation and Ar/O_2/DF_4 Remote Plasma-Based Surface Functionalization: Comparisons to Tantalum

Patterning of ruthenium (Ru) in the microelectronics industry has become important because of novel Ru applications, including back-end-of-line metallization. Selective etching and deposition of Ru over tantalum (Ta) are crucial for the repair of extreme ultraviolet photomasks. A further challenge is to reduce near-surface damage and interdiffusion at the interfaces of material layers, which is often generated when patterning is performed by ion bombardment. In this work, we investigated the etching of Ru and Ta by exposure to electron beam (EB) irradiation and reactive neutral fluxes provided by a remote plasma source (RPS) fed with Ar/O₂ gas mixtures. A synergistic effect is observed for Ru etching for simultaneous EB and remote plasma (RP) exposure as compared to isolated EB using the nonexcited feed gas mixture or RP exposure. The RP exposure functionalizes the Ru surface by oxidizing the Ru to nonvolatile RuO₂, and the electron flux can further oxidize the functionalized surface to volatile RuO₄ resulting in Ru etching. The Ru etch rate (ER) shows strong dependence on O₂ flow and EB emission current, which determine the oxygen neutral and electron fluxes to the Ru surface, respectively. The effect of increasing O flux by adding a small amount of CF₄ to the Ar/O₂ as a feed gas for RPS does not directly result in Ru ER improvement. This is likely due to the formation of nonvolatile Ru oxyfluoride, which cannot be removed by the electron flux for Ar/O₂/CF₄ gas mixtures. Following Ar/O₂/CF₄ remote plasma exposure, Ru etching with Ar/O₂ is subsequently enhanced for some time once the CF₄ flow is stopped. This effect is likely caused by the passivation of reactor walls by RP-generated fluorocarbon species and reduced recombination of reactive oxygen species necessary for Ru etching on the reactor walls, thus leading to a higher ER. Exposure of Ta to EB and Ar/O₂ RPS generated fluxes induces oxidation of Ta to nonvolatile Ta oxide, which is accompanied by an increase in layer thickness. The Ta oxidation rate decreases as the Ta oxide layer grows. With the addition of CF₄, RP only exposure induces Ta etching by the formation of volatile Ta fluoride, whereas with EB irradiation, Ta oxide forms. Utilizing the passivation effect induced by CF₄ addition and the differing responses of Ru and Ta to EB irradiation, we developed a process that enables selective removal of Ru over Ta. Surface chemistry and thickness measurements by spatially resolved x-ray photoelectron spectroscopy and ellipsometry suggest that the EB-induced materials’ modification likely arises from the promotion of surface oxidation.