Titanium and cobalt silicides are used widely in microelectronics fabrication. There are limitations for both silicides. With TiSi2, a linewidth dependence of sheet resistance for lines narrower than 0.35 mu m has become dramatic. The transformation from the high-resistivity C49 phase to the low-resistivity C54 TiSi2 phase is nucleation limited. With CoSi2 there is much less linewidth dependence of the sheet resistance, but more Si is consumed to form the silicide. With the use of NiSi, these problems can be avoided as reported so far in literature. In this article we shall report an investigation of rapid thermal silicidation of nickel on single crystalline silicon wafers in the annealing range of 150-1150 degrees C. It has been found that there are five zones in the dependence of sheet resistance on silicidation temperatures as follows : below 175, 175-350, 350-650, 650-900, and above 900 degrees C. In order to extensively study the phase sequence for Ni/Si reactions and the kinetics of nickel silicide formation corresponding to the sheet resistance zones, the spectroscopic ellipsometry (SE) was used to characterize the thin silicide films. The sensitivity and usefulness of SE characterization for studying the intermetallic growth processes and the application of an appropriate algorithm for extracting the information from experimental data is demonstrated. The results obtained by this nondestructive SE technique are compared with measurements done by the well-established but destructive techniques, Rutherford backscattering,; scanning electron microscopy, and energy dispersive x-ray spectrometry. A physical picture of nickel silicidation as a function of temperature is also presented.