Chemical and structural aspects of reaction at the Ti/Si interface

Chemical bonding and reactions at the Ti/Si(111) interface have been studied as a function of Ti overlayer thickness and annealing temperature and time. The chemical properties (composition, electronic structure, and reactivity) were observed under ultrahigh-vacuum conditions using photo-emission (ultraviolet and x ray) and Auger electron spectroscopies; the structural properties (phase and microstructure) were investigated by subsequent transmission electron microscopy; sputterdepth profiling was employed to complete the characterization of the material formed by interface reaction. At room temperature the clean Ti/Si interface remains unreacted, i.e., no intermixing of atoms across the interface occurs; this behavior is similar to that of V/Si but in contrast to the more common initial limited reaction (∼10 Å depth) which takes place for other transition-metal/Si interfaces, especially the near-noble metals. At sufficiently high temperatures (≥ 600°C), growth of the silicides TiSi and ${\mathrm{TiSi}}_2$ takes place over considerable distances (hundreds of angstroms or more) as expected from previous thin-film investigations. However, initial reaction at the interface can occur at temperatures considerably lower (∼300°C), and this reaction can extend over a considerable distance (∼100 Å or more) from the Si interface. This low-temperature intermixing process leads to structural properties quite different from those induced by silicide compound formation at higher temperatures: The reacted material forms Ti-Si mixtures with very small grains (∼10-30 Å in size), if not nearly amorphous material. Finally, if the low-temperature reaction has already progressed considerably, the kinetics of transformation to well-defined larger grain silicide phases is altered, and notably higher reaction temperatures are required to reach the same structural state. These observations suggest the possibility that the phase kinetics and microstructure associated with the Ti/Si interfacial reaction can be altered in the intermediate reaction stage observed at low reaction temperatures.