Formation of WO₃ Thin Films from RF Sputtered Tungsten Films by Air Annealing: A Cost-Effective Approach

Abstract

WO3 thin films were prepared by RF sputtering metallic tungsten onto glass substrates, followed by thermal oxidation through annealing in air. This technique is straightforward, cost-efficient, and time-effective, achieving high deposition rates of 16 nm/min on average at 200 W magnetron power for the highly homogeneous W-metallic films. SEM/EDX analysis showed that after annealing at 450ᵒC in air, the RF sputtered 269 nm thick metallic W films with a round grain morphology (~30 nm) turned into 420 nm thick nearly stoichiometric transparent WO3 (tungsten (VI) oxide) film, with a dramatically changed morphology of aggregated crystal rods approximately 1 µm long. XRD and Raman spectroscopy confirmed a biphasic crystal structure, with a dominant monoclinic phase and a minor tetragonal phase. XPS analysis revealed the characteristic W4f7/2 and W4f5/2 electron peaks associated with the W6+ oxidation state, with no evidence of W5+ species, indicating a stoichiometric nature of the WO3 films.