Atomic layer deposited Mo2N thin films using Mo(CO)6 and NH3 plasma as a Cu diffusion barrier

Abstract

Thin films of molybdenum nitride (Mo2N) are prepared using sequential exposure of molybdenum hexacarbonyl [Mo(CO)6] and NH3 plasma in a plasma-enhanced atomic layer deposition (PEALD) reactor. Several process parameters such as the deposition temperature, plasma power, and post-annealing conditions are systematically investigated to achieve the best quality films. The superior growth kinetics is evident with a significantly higher growth per cycle (GPC) value with lower incubation period for this PEALD process (~1.1 Å, ~36 cycles) when compared to thermal ALD (~0.3 Å, ~63 cycles), both carried out at 200 °C. The growth rate of the Mo2N film reveals a significant jump above 215 °C, indicating a severe decomposition of Mo(CO)6, however, polycrystalline γ-Mo2N films with face-centered-cubic structure are evident within the deposition temperature range of 200–230 °C. The sharp decrease in the resistivity of the as-grown Mo2N films is observed with increasing deposition temperature, film thickness, and plasma power. The resistivity could be further lowered by a post-annealing process and the lowest resistivity of ~395 μΩ cm is achieved for the thin film deposited with 300 watt plasma power and annealed at 700 °C. Finally, the Cu-diffusion barrier capability of an extremely thin film (~7 nm) of as-deposited Mo2N is evaluated in Cu/PEALD-Mo2N/Si structure. The X-ray diffractometry analysis confirms that such a thin layer can successfully prevent the diffusion of Cu up to 500 °C and significant Cu3Si formation was observed only at 600 °C and above. The gradual failure of the diffusion barrier upon annealing is further investigated using electrical impedance (EI) analyses in two different modes. In-plane EI measurements directly indicate the change in the Cu layer at the top, and the formation of Cu3Si can be inferred from the through-plane mode. A comparison with PEALD and thermal-ALD-grown MoNx from EI analyses further reflects the superiority of plasma-enhanced process towards fabricating diffusion barrier layer.