The initial stages of growth in chemical vapour deposition (CVD) of W
from WF6 have been studied in an ultra-high vacuum system using CVD Ti
Si2 films as substrates. The influence of different surface conditions
on the substrate-reduction process was investigated employing three d
ifferent substrates: (i) oxide-free TiSi2; (ii) TiSi2 covered with a 1
0 Angstrom SiO2 film and (iii) TiSi2 covered with a mixture of SiO2 ac
id TiO2. The very low total pressures of WF6 (3 x 10(-4) Pa) used in t
he experiments reduced the rate of the substrate-reduction step and ma
de it possible to follow the initial growth stages in detail by X-ray
photoelectron spectroscopy (XPS). It was found that the oxide-free TiS
i2 substrate was highly reactive towards WF6. Significant amounts of T
iF3 were formed on the surface during the nucleation stage and incorpo
rated at the W/TiSi2 interface. TiSi2 with a SiO2 film was, as expecte
d, less reactive than an oxide-free silicide substrate. No indication
of tungsten oxide formation could be observed in the XPS analysis. W g
rowth occurred by penetration of the oxide him and reduction of WF6 by
the underlaying silicide. A further reduction of reactivity was obser
ved using a surface consisting of a mixed oxide of TiO2 and SiO2 prepa
red by storing the TiSi2 films in humid air. An interaction between W
and O was observed on the mixed oxide surface. This was attributed to
the presence of hydroxyl groups on the surface.