Ion-backscattering analysis of tungsten films on heavily doped SiGe

Abstract

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.