The Formation and Structure of CVD W Films Produced by the Si Reduction of WF6

In this paper, we present data on the formation and structure of CVD W films deposited by the Si reduction of Formula . Although the great majority of CVD W films deposited for IC applications are ostensibly deposited by Formula reduction, the Si reduction of Formula always occurs first, even in the presence of copious amounts of Formula . Therefore, it is the Si reduction reaction that determines the nature of W/Si interface, and, in turn, such important properties as contact resistance and leakage current. We have found that Si reduced W films deposited between 210° and 700°C are porous and discontinuous, and are probably not effective barriers to the further diffusion of Formula . The discontinuous structure of the films provides a simple mechanism for the growth of thick W films by Si reduction; such thick films have been observed with increasing frequency as of late and have been enigmatic, because the reaction between Formula and Si was thought to be self-limiting. An unusual temperature dependence of W film growth is reported. Films deposited below 310°C self-limit at about Formula coverage (~60Å of full dense W). In the temperature range 320°-450°C, film coverages up to Formula (~950Å, at 340°C), are observed. In the higher temperature range, 500°-700°C, film coverages of about Formula (200Å) are observed. The temperature dependence and the self-limiting behavior of film growth (regardless of the ultimate film thickness at which the film self-limits), are believed to be controlled by some surface-catalyzed reaction. Thus, the temperature dependence of the sticking coefficient of a reactive intermediate probably controls the reaction. This kind of model has the potential to explain why minor changes in the Si surface due to cleaning variations, for example, can result in radically different W film growth behavior.