Initial stages of growth during CVD of W on TiSi 2 substrates

Abstract

The initial stages of growth in chemical vapour deposition (CVD) of W from WF 6 have been studied in an ultra-high vacuum system using CVD TiSi 2 films as substrates. The influence of different surface conditions on the substrate-reduction process was investigated employing three different substrates: (i) oxide-free TiSi 2; (ii) TiSi 2 covered with a 10 Å SiO 2 film; and (iii) TiSi 2 covered with a mixture of SiO 2 and TiO 2. The very low total pressures of WF 6 (3 × 10 −4 Pa) used in the experiments reduced the rate of the substrate-reduction step and made it possible to follow the initial growth stages in detail by X-ray photoelectron spectroscopy (XPS). It was found that the oxide-free TiSi 2 substrate was highly reactive towards WF 6. Significant amounts of TiF 3 were formed on the surface during the nucleation stage and incorporated at the W/TiSi 2 interface. TiSi 2 with a SiO 2 film was, as expected, less reactive than an oxide-free suicide substrate. No indication of tungsten oxide formation could be observed in the XPS analysis. W growth occurred by penetration of the oxide film and reduction of WF 6 by the underlaying suicide. A further reduction of reactivity was observed using a surface consisting of a mixed oxide of TiO 2 and SiO 2 prepared by storing the TiSi 2 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.