ULTRA-HIGH-VACUUM CVD OF W AND WSI2, FILMS BY SI REDUCTION OF WF6

Thin films of W and WSi2 have been deposited on Si(100) by silicon reduction of WF6 in a UHVCVD system. The deposition experiments were performed in a cold-wall reactor with a WF6 pressure ranging from 1 X 10(-5) to 1 X 10(-4) Torr. It was found that the UHVCVD process gave quite different results compared to conventional W CVD carried out at higher pressures. Pure tetragonal WSi2 films could be deposited at 550 degrees C without any post-annealing. A more complicated growth behaviour was observed at lower temperatures, where beta-W generally was deposited prior to the formation of WSi2. At 475 degrees C, hexagonal WSi2 was formed after a deposition time of a few minutes probably by a solid-state transformation of the beta-W film. XRD analysis suggested that an amorphous phase with unknown composition was involved in the transformation. For the experimental conditions used in this paper, hexagonal WSi2 was formed at temperatures as low as 400 degrees C. Finally, it was concluded that the formation of WSi2 observed in this study is due to the low WF6 pressures used in the UHVCVD process.