The effect of prolonged annealing (10 h) at low temperature (200 degrees C) has been studied in 20-nm Ni/Si(100) samples using Rutherford backscattering spectroscopy, X-ray diffraction, and four-point probe techniques. We observed that at 200 degrees C a considerable amount of Ni3Si2 and Ni31Si12 was formed. After 10 h annealing at 200 degrees C only a fraction of total amount of Ni has been converted into Ni3Si2 and Ni31Si12 phases and around 60% of nickel remains unreacted. Formation of the Ni3Si2 and Ni31Si12 silicides at 200 degrees C is slow and seems to saturate after approximately 140x10(15) atoms/cm(2). It is observed that after 2 h annealing the amount of both silicides is almost equal to that after 10 h annealing. After 10 h annealing at 300 degrees C there is no indication of either Ni3Si2 or Ni31Si12, and all Ni was consumed and only the stable NiSi phase is detected. (c) 2005 The Electrochemical Society. All rights reserved.