PURPOSE: To form the shallow junction having excellent electrical characteristics by a method wherein the development of residual defect in a MOS transistor manufactured by performing the step for making a silicon substrate surface preamorphous is obviated by avoiding the existence of the residual defect in source/drain regions.

CONSTITUTION: After making the surface of a silicon substrate 22 in a transistor forming region amorphous by Si implanting step (1), a gate electrode 26 is formed through the intermediary of a gate oxide film 28. Later, BF2 is ion- implanted both for forming source/drain and lowering the resistance of the gate electrode 26. At this time, BF2 can be implanted even in the interface between the gate electrode 26 and the gate oxide film 28 also by the channelling effect of the gate electrode 26. Later, the implanted BF2 is activated by lamp annealing step to recrystallize the amorphous region. Through these procedures, a MOS semiconductor device having fine pattern and obviating the residual defect (c) increasing the leakage current can be manufactured.