THE EFFECT OF DOPING ATOMS ON THE KINETICS OF SELF-LIMITING TUNGSTEN FILM GROWTH ON SILICON BY REDUCTION OF TUNGSTEN HEXAFLUORIDE

The reaction of WF6 with a Si substrate plays an important role in the metallization of Si active areas by tungsten chemical vapor depositio n (W-CVD). At typical low pressure W-CVD conditions, this reaction lea ds to self-limiting growth of a W film. The reaction, which takes plac e at the film surface, is maintained by Si diffusion through the W fil m and stops when the (rapid) supply of silicon becomes rate limiting. Although the mechanism controlling the reduction of WF6 by intrinsic S i is reasonably understood, little is known about the influence of dop ing atoms in the Si substrate. In this paper we describe the effect of doping atoms on the growth kinetics of tungsten films deposited by th e reaction of WF6 with highly doped Si at temperatures ranging from 27 5 to 360 degrees C. Doping atoms can have a large effect on the self-l imiting time of the WF6/Si reaction, the self-limiting thickness of th e W film, and the rate of the WF6/Si reaction. Similar to undoped Si s ubstrates, depositions on As-doped n(+) Si are controlled by WF6 gas-p hase diffusion and the self-limiting effect occurs earlier than on und oped Si; the W growth rate on N+ Si is linear in WF6 pressure and almo st independent of temperature. On B doped p(+) Si the WF6/Si reaction rate is decreased to such an extent that this surface reaction becomes rate limiting, instead of the WF6 gas-phase diffusion; the W growth r ate on p(+) Si is thermally activated and independent of the WF6 press ure. The results obtained with the doped Si substrates are discussed i n the light of a model for self-limiting W film growth on undoped Si s ubstrates.