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 similar to 1.32 nm/ cycle at 400 degrees 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 H-2 in the first and the third steps. The W film deposited by pulsed CVD showed a much lower roughness (similar to 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 alpha-W or primitive cubic beta-W) and microstructure characterized by grain size as well as the film thickness (the size effect). Transmission electron microscopy analysis showed that H-2 addition into the first and third steps increased the grain size from similar to 7 to similar to 13 nm and prevented the film from forming a b-W phase with high resistivity, resulting in a lower resistivity of 100 mu Omega-cm compared to that of the W film deposited without H-2 addition (210 mu Omega-cm). H-2 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. (c) 2005 The Electrochemical Society. All rights reserved.