PURPOSE: To obtain a dynamic RAM which has excellent shoftware error resistance and good electric characteristics by emitting a radiant beam to a semiconductor substrate which includes an impurity of 1017atoms/cm3 or more, generating a nondefect region in a depth of 1∼5μm from the surface of the substrate at least in a memory region forming portion, and forming a high density ultrafine defect region thereunder.

CONSTITUTION: A YAG laser beam which has 1.06μm of wavelength is emitted to the surface of a P type single crystal Si substrate 21 which includes oxygen of 1017atoms/cm3 or more as an impurity while scanning, oversaturated oxygen is precipitated near the surface of the substrate 21, and a nucleus 22 of ultrafine defect which has approx. 106atoms/cm3 are formed thereat. At this time, the output of the beam is controlled to limit the depth of producing the nucleus 22 to the depth of 1∼5μm from the surface, the surface layer is formed in nondefect state, and ultrafine defect region 35 of high density is formed in the presence of the nucleus 22.