Characterizations of pulsed chemical vapor deposited-tungsten thin films for ultrahigh aspect ratio W-plug process

Tungsten (W) thin films were deposited using a modified chemical vapor deposition (CVD) process, called pulsed CVD, and the film properties were characterized as nucleation layers for a W-plug fill process. In this study, the deposition stage is composed of four steps, resulting in one deposition cycle: (i) reaction of WF₆ with SiH₄, (ii) inert gas purge, (iii) SiH₄ exposure, and (iv) inert gas purge. The W growth per cycle was extremely linear with a growth rate of ∼ 1.32nm/cycle at 400°C. The growth rate was further enhanced to 1.5-1.9 nm/cycle by increasing the SiH₄ flow rate in the first step and/or by adding H₂ in the first and the third steps. The W film deposited by pulsed CVD showed a much lower roughness (∼0.7 nm) and a better conformality at the contact holes with an aspect ratio of 14, compared to W films deposited by conventional CVD using WF₆ and SiH₄. The film resistivity was closely related with its phase (body-centered cubic α-W or primitive cubic β-W) and microstructure characterized by grain size as well as the film thickness (the "size effect"). Transmission electron microscopy analysis showed that H₂ addition into the first and third steps increased the grain size from ∼7 to ∼13 nm and prevented the film from forming a β-W phase with high resistivity, resulting in a lower resistivity of 100 μΩ-cm compared to that of the W film deposited without H₂ addition (210 μΩ-cm). H₂ addition was also effective in reducing the F and Si impurities in the films. Finally, the film resistivity was discussed on the basis of impurity, roughness, microstructure, and film phase.