Abstract: A method is provided for forming WSix gate electrode films with tunable Si/W atomic ratios, low oxygen and carbon film impurities, and work functions suitable for advanced semiconductor devices. The method includes providing a substrate containing a high-k film in a process chamber, maintaining the substrate at a temperature between 450° C. and 550° C., and performing a plurality of deposition cycles to form a WSix gate electrode film on the high-k film. According to embodiments of the invention, each deposition cycle includes exposing the substrate to a first process gas containing W(CO)6 vapor to thermally deposit a W metal film with a thickness between 0.1 nm and less than 2 nm, and exposing the W metal film to a second process gas containing SiH4 to form a WSix film having a Si/W atomic ratio controlled by self-limited Si incorporation into the W metal film. The method further includes patterning the WSix gate electrode film and high-k film to form a gate stack.