The bimetallic layers of Co/Ti were deposited by ion beam sputtering on silicon implanted with arsenic. The sample of Co/Ti/Si was treated by a multi-step annealing under nitrogen ambient. The behaviuor of arsenic during the reaction of ternary Co/Ti/Si system was studied by Rutherford backscattering sepctrometry (RBS). The results show that a multi-layer structure of TiN(O)/Co-Ti-Si/CoSi2/Si was formed by the reaction and a number of arsenic atoms were segregated from silicon substrate to the interlayer of Co-Ti-Si compound.