A semiconductor device fabrication method by which the thermal stability of nickel silicide can be improved. Nickel (or a nickel alloy) is formed over a semiconductor substrate on which a gate region, a source region, and a drain region are formed. Dinickel silicide is formed by performing a first annealing step, followed by a selective etching step. By performing a plasma treatment step, plasma which contains hydrogen ions is generated and the hydrogen ions are implanted in the dinickel silicide or the gate region, the source region, and the drain region under the dinickel silicide. The dinickel silicide is phase-transformed into nickel silicide by performing a second annealing step.