Reactions of ruthenium cyclopentadienyl precursor in the metal precursor pulse of Ru atomic layer deposition

Abstract

Ruthenium is a promising material in the semiconductor industry and is investigated as the interconnect metal or as a seed layer for Cu interconnects. Non-oxidative reactants are required in a plasma-enhanced atomic layer deposition (PE-ALD) process for metals to avoid oxygen contamination. The PE-ALD of Ru has been explored experimentally, but the growth mechanism is not clear. In this paper, the reaction mechanism of the cyclopentadienyl (Cp, C₅H₅) precursor RuCp₂ with NH_x-terminated Ru surfaces that result from the plasma cycle is studied in detail by first-principle calculations. In this mechanism, Cp ligands are eliminated via hydrogen transfer and desorb from metal surface as CpH. The results show that on the NH_x-terminated Ru surface at typical ALD operating temperatures from 550 K to 650 K, Cp ligand elimination for single RuCp₂ is overall endothermic. Investigating the precursor coverage, a neighbouring RuCp₂ promotes the Cp ligand elimination reaction and the reaction energies for the first hydrogen transfer on (001) and (100) surfaces are negative. The first hydrogen transfer is the rate-limiting step and has high barriers, which are 1.40 eV for Ru(001) and 2.02 eV for Ru(100). The two Cp ligands may be completely eliminated on Ru(100) surface at sufficiently high temperature during the metal precursor pulse, resulting in one Ru atom on the surface. However, at most only one Cp ligand is eliminated on Ru(001) surface, resulting in an RuCp termination on (001) surface. The final surface coverages of final terminations after the metal precursor pulse are 0.85 RuCp per nm² on the NH_x-terminated Ru(001) surface and 1.01 (Ru + RuCp₂) per nm² on the NH_x-terminated Ru(100) surface. By contrast, the Cp ligand elimination of CoCp₂via hydrogen transfer is overall exothermic and the computed barriers are all moderate in the range of 0.52 eV to 0.85 eV. The orientation of surface NH_x species, which is driven by the metal lattice constant and metal–metal spacing, and the intrinsic characteristics of the precursor play an important role in the Cp ligand elimination via hydrogen transfer. These structures are vital to model the following N-plasma step.