Highly conductive Tungsten Carbide Thin Films Grown by Atomic Layer Deposition Using a Novel Fluorine-Free Tungsten Precursor for Copper and Ruthenium Interconnects

Abstract

This dissertation investigates fluorine-free tungsten carbide (WCₓ) thin films grown by plasma- enhanced atomic layer deposition (PEALD) as ultrathin diffusion barriers for advanced Cu and Ru interconnects. A newly developed fluorine-free metal–organic precursor, W(N-MePyr)(CO)₅, was employed with H₂-based reactants over a substrate temperature range of 150–300 °C. Reactant screening using molecular H₂, molecular NH₃, H₂ plasma, and NH₃ plasma identified H₂ plasma as the optimal reactant, providing the lowest resistivity and a W-rich carbide composition. Under the optimized condition of 250 °C and 200 W, the PEALD process exhibited typical ALD behavior with self-limiting growth and a linear increase in thickness per cycle, yielding a growth per cycle (GPC) of 0.94 Å . Structural and compositional analyses confirmed the formation of a nanocrystalline W-rich β- WC₁₋ₓ-type phase with a W/C ratio of ~2.5 and low levels of oxygen and nitrogen impurities. The corresponding film resistivity was ~200 μΩ·cm and showed negligible dependence on film thickness, indicating that a pronounced size effect does not appear even in the ultrathin regime. The influence of key process parameters, such as substrate temperature and H₂ plasma power, on film composition, microstructure, and electrical properties was systematically examined to establish an optimal process window for high-quality fluorine-free WCₓ films. Diffusion-barrier performance was evaluated using Cu(40 nm)/PEALD-WCₓ(≈2.6 nm)/Si and Ru(40 nm)/PEALD-WCₓ(≈2.6 nm)/Si stacks under rapid thermal annealing. Cu diffusion into Si was effectively suppressed up to 600 °C, and Ru diffusion was suppressed up to 900 °C. These results demonstrate that fluorine-free PEALD-WCₓ films combine low resistivity, stable electrical behavior at small thickness, and strong diffusion-blocking capability, making them promising candidates for next-generation Cu and Ru metallization.