Phase composition of sputter deposited tungsten thin films

Abstract

Sputter deposited tungsten thin films are studied by X-ray diffraction. Two phases can be identified: $\alpha$-W and $\beta$-W based on the observed (110), and (200) and (210) Bragg reflections, respectively. With increasing film thickness (50 to 200 nm), the phase composition shifts from $\beta$-W towards $\alpha$-W. No influence of the base pressure (3 × 10⁻³–3 × 10⁻⁵ Pa) on the phase composition is observed. Also the influence of the argon pressure (0.3 to 0.7 Pa) is rather weak. The strongest shift towards $\alpha$-W composed thin films is obtained by increasing the discharge power (50 to 250 W). This trend is further studied by energy flux measurements using a calorimetric probe. These measurements rule out a strong change of the substrate temperature, and an impact of the energy flux scaled by the deposition rate (total energy per deposited atom). Test particle Monte Carlo simulations reveal the importance of the momentum of the reflected argon neutrals on the phase composition. The maximum energy of these species is mainly defined by the discharge voltage, and is higher than the directional dependent displacement energy of W. Despite the significant correlation between phase composition and the number of displacement per deposited atom, there is a strong scatter of the phase composition. As the deposition conditions were varied in random way, changes of the target erosion profile, and the changing discharge voltage over each series are probably partially responsible for the observed scatter. This scatter is also enhanced by the long term changes in the phase composition towards the more thermodynamic stable $\alpha$-W phase.