Effects of SiO2∕Si3N4 hard masks on etching properties of metal gates

Reduced etching rates of advanced metal gates (TaN, TiN, and HfN) using SiO2∕Si3N4 hard masks are observed in Cl2 plasma. Si and O released from hard masks react with metal surfaces newly exposed to the plasma during the etching, thereby forming metal oxides. The metal oxides formed on the etched surface retard the etch rates. The suppression of etch rates with hard mask is more obvious for TiN than for TaN and HfN, because Ti oxides are readily formed on the etched TiN surface due to their low Gibbs free energies of formation. The surface of TiN degrades with etching time with SiO2 mask, because etching rates of Si oxides and Ti oxides are different in the (TiO2)1−x(SiO2)x residues remaining on the etched surface. In contrast, a conventional poly-Si electrode does not show the mask effects on etch rates and surface roughness.