The practical importance of the reactions between a semiconductor and a metal system cannot be overstated. Fundamental physics itself offers also a wide variety of interesting phenomena. Unlike the "clean" metal-metal reactions nucleation or interface control is observed for some metal-silicon reactions. In this work we show our results for bi- and trilayered Al/a-Si, Cu/a-Si and Ni/a-Si model system. Our main investigation method was atom probe tomography, which allowed the local nanoscale investigation of the interfaces. Both the Cu- and Ni-Si system is characterized by a strong asymmetry in respect to the stacking order. The metal on Si transition was much broader and this allowed an almost instantaneous nucleation of the product phase. The Si on metal interface remained sharp and a considerable annealing time was required for the appearance of the phase. The subsequent growth of the Cu3Si layer followed a linear kinetics.