The solid phase reaction of Fe thin films with (111) Si substrate was investigated at constant annealing temperature and time (700°C, 7 minutes) as a function of the initial iron film thickness (from 5 nm to 27.5 um in 2.5 nm steps). The formed phases were analysed by X-ray diffraction, Rutherford backscattering and transmission electron microscopy and optical microscopy.
After annealing FeSi phase was detected in the thinner samples. Samples with Fe layers thicker than 12.5 nm contained a β-FeSi2 phase. This special phase sequence was explained with the help of a nucleation controlled phase formation model, taking into consideration the critical radius of nuclei of the new phase. The advantages of using the film thickness as a variable during investigation of solid phase thin film reactions and the probable substrate effects are also discussed.