Thin platinum films of 164 and 303 Å thickness are deposited on (100) silicon and annealed at temperatures ranging from 180 to 300 °C in a nitrogen furnace for various times ranging from 1 min to 200 h. Sheet resistance (Rs) measurement shows a four-stage silicide formation sequence: (1) the Rs increases to a maximum value; (2) the Rs then decreases to a minimum value; (3) the Rs increases once again and reaches a second maximum value; and (4) the Rs decreases to a final stable value. X-ray diffraction analysis indicates that the initial phase grown is Pt12Si5 which corresponds to the sheet-resistance increase in the first stage. This phase is also identified by the transmission electron diffraction analysis. This is the first time that the Pt 12Si5 phase formed by the Pt/Si direct reaction is observed. The second stage corresponds to the growth of Pt2Si as confirmed by the x-ray diffraction and Auger spectroscopy analyses. The third stage corresponds to the growth of PtSi and the final stage corresponds to the increase of PtSi grain size. Samples with initial Pt films of 500, 800, and 2000 Å thickness were also prepared and analyzed by x-ray diffraction. The increase of film thickness suppresses the growth of Pt12Si 5, and for Pt films thicker than 800 Å the initial phase grown becomes Pt2Si. The film thickness dependence is explained by the partial lattice-match-induced preferred phase growth effect. The effective absorption coefficient (Kse) of the Pt/Si structure after annealing is determined by ellipsometry measurement, and the activation energy of the Pt12Si5 phase formation derived from the Rs -time plot or the Kse -time plot is determined to be 1.5-1.6 eV.