Atomic layer deposited nanocrystalline tungsten carbides thin films as a metal gate and diffusion barrier for Cu metallization

Tungsten carbides (WC_x) thin films were deposited on thermally grown SiO₂ substrates by atomic layer deposition (ALD) using a fluorine- and nitrogen-free W metallorganic precursor, tungsten tris(3-hexyne) carbonyl [W(CO)(CH₃CH₂C ≡ CCH₂CH₃)₃], and N₂ + H₂ plasma as the reactant at deposition temperatures between 150 and 350 °C. The present ALD-WC_x system showed an ALD temperature window between 200 and 250 °C, where the growth rate was independent of the deposition temperature. Typical ALD characteristics, such as self-limited film growth and a linear dependency of the film grown on the number of ALD cycles, were observed, with a growth rate of 0.052 nm/cycle at a deposition temperature of 250 °C. The ALD-WC_x films formed a nanocrystalline structure with grains, ∼2 nm in size, which consisted of hexagonal W₂C, WC, and nonstoichiometric cubic β-WC_1−x phase. Under typical deposition conditions at 250 °C, an ALD-WC_x film with a resistivity of ∼510 μΩ cm was deposited and the resistivity of the ALD-WC_x film could be reduced even further to ∼285 μΩ cm by further optimizing the reactant pulsing conditions, such as the plasma power. The step coverage of ALD-WC_x film was ∼80% on very small sized and dual trenched structures (bottom width of 15 nm and aspect ratio of ∼6.3). From ultraviolet photoelectron spectroscopy, the work function of the ALD-WC_x film was determined to be 4.63 eV. Finally, the ultrathin (∼5 nm) ALD-WC_x film blocked the diffusion of Cu, even up to 600 °C, which makes it a promising a diffusion barrier material for Cu interconnects.