Roughness-generation mechanism of Ru etching using Cl2/O2-based plasma for advanced interconnect

Abstract

We investigated the Ru-etching mechanism using a Cl2/O2-based plasma generated by a microwave electron cyclotron resonance etching system. When 20% of Cl2 gas was added to an O2 plasma, the Ru etch rate was the highest and a smooth surface was obtained by forming volatile RuO4 and RuClxOy. However, when Ru was etched using an O2-rich plasma with less than 10% Cl2 gas added, nonvolatile RuO2 was also formed, which caused surface roughness. When using Cl2-rich plasma, the formation of nonvolatile RuClx caused surface roughness by forming micro masks. We also evaluated line-and-space Ru patterns with 32 nm pitch and 95 nm pitch using Cl2/O2-based plasma. Selectivity over the Si3N4 mask was the highest when 20% of Cl2 gas was added to an O2 plasma due to the high etch rate of Ru. The Ru-sidewall profile and roughness depended on the duty cycle of wafer-bias power. When wafer bias was applied continuously, the Ru sidewall was tapered because the Si3N4 mask widened due to the deposition of by-products. When the ion flux decreased by reducing the duty cycle of the wafer-bias power, the Ru pattern was vertically etched, but larger sidewall roughness was generated. The sidewall roughness was thought to be caused by nonvolatile RuOx and RuCly formed on the sidewalls. The sidewall Ru was etched by O and Cl radicals during the off period of the wafer-bias power. To reduce the sidewall roughness, we suggested a C-coating that uniformly protects the sidewall from being etched by adding passivation gas to the Cl2/O2 plasma.