TiSi2 strap formation by Ti–amorphous-Si reaction

Abstract

This report describes the formation of a TiSi2 strap in combination with the self-aligned titanium silicide (salicide) technology. The TiSi2 strap is formed by reaction of amorphous silicon (a-Si) with the underlying Ti layer. It was determined that sputter deposition of the Ti and a-Si had to be done in one deposition cycle, otherwise interface contamination would prevent the Ti–a-Si reaction and give rise to extensive silicon diffusion from the active areas. Since TiSi2 straps are formed over diffusion areas as well as over oxide surfaces, the influence of the substrate on the Ti/a-Si reaction was determined. It was found that for straps deposited on oxide substrate the properties of the silicide layer formed were determined by the Ti/a-Si atomic ratio. A titanium-rich strap resulted in a high-resistivity silicide layer due to TiSi compound formation. Stoichiometric straps formed low-resistivity TiSi2 layers with a thin-TiN top layer and silicon-rich straps also resulted in a low-resistivity TiSi2 layer but with a silicon enrichment at the surface. On a mono-Si substrate no influence of the sputtered Ti/a-Si atomic ratio could be found. Only low-resistivity TiSi2 layers were formed. In case of silicon-rich Ti/a-Si ratio the excess silicon disappears from the layer and regrows onto the Si substrate. The roughness of TiSi2 straps, mainly observed for stoichiometric or silicon-rich straps on oxide substrates, was found to be related to the presence of argon (and probably hydrogen) incorporated in the layer during sputter deposition. Special attention was paid to strap formation at oxide/diffusion area edges. Possible void formation by local silicon consumption could not be detected. Strap/TiSi2 salicide transitions are also very smooth and showed no substrate defects.