High-resolution ion backscattering has been used to study the initial stages of Ni silicide formation on Si(111). NiSi mixing is observed, following Ni deposition on clean Si(111) at room temperature, up to coverages of ∼ 8 × 1015 Ni atoms/cm2. The ultra-thin mixed layers are disordered and have a composition close to Ni 2Si. Ion backscattering and high-resolution cross-section TEM observations show the films to grow out from small 3D silicide islands which are formed at the initial deposition stage. A new cluster growth model for the formation of silicide films on clean Si is derived from the results. © 1985.