Highly Conductive Tungsten Carbide Thin Films with a Suppressed Size Effect Prepared by Plasma-Enhanced Atomic Layer Deposition

High Resolution Image Download MS PowerPoint Slide Conformal and precise control over film thickness, noncorrosive byproducts, and high electrical conductivity are crucial for diffusion barrier layer of semiconductor devices interconnects. The present study, for those requirements, investigates tungsten carbide (WC x ) thin films prepared by atomic layer deposition (ALD) using a fluorine-free metal–organic W precursor, ( N -methylpyrrolidine)tungsten(0) pentacarbonyl and H 2 plasma as a reactant. The process exhibited characteristic ALD behavior such as self-limiting surface reactions for both the precursor and H 2 plasma and a linear increase of films’ thicknesses with increasing number of ALD cycles (growth per cycle: ∼0.94 Å at 250 °C). The as-grown WC x films are W-rich non stoichiometric (W/C ≈ 2.5), with nanocrystalline features dominated by the β-WC 1– x phase, and show ∼80% step coverage in 3D trenches with an aspect ratio of ∼1.5. The films exhibited a low resistivity of ∼220 μΩ·cm that remained nearly unchanged down to ∼3.6 nm, indicating a suppressed size effect that is an advantageous feature for ultrathin barriers. Density functional theory (DFT)-based transport interpretation indicates that the suppressed size effect shown at the present ALD-WC x film is attributed to its extremely short electron mean free path (λ = 0.22 nm) together with a nanocrystalline microstructure, leading to negligible grain boundary scattering. DFT calculations were also performed to elucidate the surface adsorption mechanisms of W precursor and the ligand dissociations by hydrogen reactants. Diffusion barrier performance was evaluated by rapid thermal annealing of Cu (50 nm)/WC x (2.6 nm)/Si and Ru (50 nm)/WC x (2.6 nm)/Si stacks, where Cu–silicide and Ru–silicide were not observed up to 550 and 900 °C, respectively. Overall, the fluorine-free ALD-WC x process investigated in this study provides conformal, conductive ultrathin films with effective thermal diffusion-barrier performance, suggesting their potential as diffusion-barrier layers for Cu and Ru metallization.