Ion-backscattering analysis of tungsten films on heavily doped SiGe

The technique of ion backscattering has been used to obtain information on the reactions taking place at the interface between a sputtered tungsten film and a heavily doped silicon-germanium alloy substrate. Significant differences were observed for n-type and p-type SiGe substrates. In both cases silicon was observed to migrate into the tungsten at temperatures above 650 °C and form a layer of atomic composition, WSi2. There was little or no diffusion of silicon beyond the WSi2 layer for the samples on n-type substrates, but on p-type SiGe, silicon diffused through the WSi2 layer to the surface of the tungsten film. Tungsten films on single-crystal silicon showed small amounts of tungsten-silicon diffusion and little or no WSi2 formation for anneals at and below 700 °C. Samples with n-type SiGe substrates exhibited little or no tungsten-germanium diffusion; whereas, samples on p-type substrates annealed to 675 °C or higher showed significant migration of tungsten and/or germanium. The oxidation of the sputtered tungsten films was also investigated by ion backscattering. A 1200-Å-thick W film was completely converted to tungsten oxide (atomic composition WO3) by annealing at 650 °C, which inhibited WSi2 formation. In addition, the growth of WO3 on a thick W film was observed as a function of time.