The use ofQ‐switched ruby laser and multiscanning electron‐beam annealing to produce the reaction of thin Ti and Ni films deposited onto silicon single crystals has been studied. Rutherford Backscattering (RBS),16O(d, p)17O* nuclear reaction, scanning electron microscopy (SEM) observation, and x‐ray diffraction were used to characterize the reacted layers. It was found that laser annealing produces a reaction only at the metal‐semiconductor interface: the reacted layers are not uniform in composition and more similar to a mixture than to a well‐defined phase. On the contrary, the silicide layers produced by multiscanningebeam result from the solid‐state reaction of the whole metal film and have a layered structure with well‐defined phase composition and sharp interfaces both between the silicide phases and the underlying semiconductor in Ti/Si system. It was observed that the TiSi2growth mechanism duringeirradiation cannot be explained with the parabolic ’’diffusion controlled’’ mechanism operating in the standard furnace annealing. All our observation seems to indicate that the growth mechanism is a ’’nucleation controlled’’ process, in which the growth speed of the disilicide is limited by the speed of ejection of oxygen from a TiSi2layer. In a Ni/Si system, only the NiSi phase could be obtained as a very uniform layer after thee‐beam irradiation; the impossibility of obtaining the Ni2Si phase indicates that, in these conditions, the ’’first‐phase nucleation law’’ is no more valid.