Analysis for roughness-generation mechanism in Ru etching using Cl2/O2-based plasma for advanced interconnect

We investigated roughness generation mechanisms of Ru etching using Cl2/O2-based plasma generated by a microwave electron–cyclotron-resonance etching system. The influence of the edge roughness of the Si3N4 mask pattern on that of the Ru line was analyzed using a high-voltage critical-dimension scanning electron microscope, which was used to nondestructively and simultaneously measure both linewidth roughness (LWR) of the Si3N4 mask pattern and that of the Ru line. The measurement results show that the LWR of the Ru line does not necessarily depend on that of the Si3N4 mask. The LWR of the Si3N4 mask became smaller when the ion flux increased, because the surface of the redeposited Si-containing by-product formed on the Si3N4 mask became smoother due to ion bombardment. On the contrary, the roughness of the Ru sidewall depends on the balance between flux of ion, which generates nonvolatile RuO2 and RuClx on the Ru sidewall, and that of radical, which etches Ru isotopically. To reduce the LWR of the Ru line, we examined adding passivation gas to the Cl2/O2 plasma. Vertical and low roughness Ru etching was achieved using a C-added plasma. Excessive redeposition of by-products on the sidewall of the Si3N4 mask was considered to be suppressed, while the Ru sidewall was protected from etching by a thin and uniform C-adsorption on Ru sidewall. We also analyzed the influence of the grain boundaries of Ru on the LWR. We found roughness generation due to the grain boundary becomes more serious as the LWR decreases.