Compound Phase Formation in Thin Film Structures

Thin film compound formation during solid phase reaction, reactive deposition, ion-beam synthesis, and ion beam mixing is discussed in terms of the Effective Heat of Formation (EHF) model. This model defines an effective heat of formation ΔH′, which is concentration dependent. By choosing the effective concentration of the interacting species at the growth interface during solid phase reaction, to be that of the liquidus minimum, the model correctly predicts first phase formation during formation of silicides, germanides, aluminides, and other metal-metal binary systems. The ability to predict phase formation sequence and phase decomposition is also illustrated. The EHF model is also used to describe amorphous and metastable phase formation as well as the effect of impurities and diffusion barriers on phase formation. In the case of reactive deposition, the effective concentration is controlled by the rate at which thin film deposition is carried out and the temperature of the substrate. In this way epitaxial phases such as CoSi2 and NiSi2 can be formed directly at temperatures much lower than normally needed during solid phase reaction. During ion-beam synthesis silicon-rich compounds are expected to form during metal implantation into silicon- and metal-rich compounds for silicon implantation into a metal substrate. For ion-beam mixing, the effective concentration is not controlled by the mixing process at low temperatures, but by the liquidus minimum of the system at higher temperatures. For both ion-beam mixing and ion-beam synthesis, however, much work still needs to be done to correlate effective concentration with the various experimental parameters. The general trends of compound formation in these systems do, however, also correlate well with the predictions of the EHF model.