Some Insights into Atomic Layer Deposition of MoN_x Using Mo(CO)₆ and NH₃ and Its Diffusion Barrier Application

Deposition providing precise control of the film thickness, low deposition temperature, and noncorrosive byproducts is essential for the efficient fabrication of barrier layers in semiconductor devices. Here, molybdenum nitride (MoNx) is deposited for Cu diffusion barrier application at a relatively low temperature (180–300 °C) by atomic layer deposition (ALD) using molybdenum hexacarbonyl [Mo(CO)6] and ammonia gas (NH3). The density functional theory calculations indicate favorable thermodynamics during the chemisorption of Mo(CO)6 on −NH2-terminated Si clusters at 200 °C, confirming the suitability of Mo(CO)6 for ALD. However, the films deposited beyond the ALD temperature window are of similar importance though decomposition-induced growth of the film is evident; nevertheless, other advantages exist (e.g., higher growth rate and improved film characteristics). Furthermore, a few experiments using NH3 plasma as a reactant show further scope for this process. The as-grown MoNx film using thermal ALD is mostly amorphous with significant O impurity. Poorly structured nanocrystalline h-MoN and cubic Mo2N phase are formed in the film above 225 °C. These phases are converted into the crystalline cubic Mo2N phase upon annealing at higher temperature (500–700 °C) in a hydrogen atmosphere. The resistivity of the MoNx films decreases sharply with deposition temperature and is significantly reduced further upon post-annealing. The properties of the as-deposited and annealed films are characterized in detail by secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy. The barrier properties against Cu diffusion for the as-deposited thin films (less than 10 nm) grown at 225 and 275 °C are analyzed using ex situ X-ray diffraction (XRD) and electrical impedance spectroscopy (EIS). EIS helps to determine the failure of the MoNx barrier layer qualitatively by comparing different EIS spectra obtained after annealing at different temperatures. Both XRD and EIS analyses show that the ALD-MoNx film deposited at the higher ALD temperature makes a better Cu diffusion barrier layer. This could be attributed to the higher density of the ALD MoNx film grown at 275 °C compared with that deposited at 225 °C.