The purpose of the present invention is to avoid a decrease in the mechanical strength of a Si substrate because of the repetition of ion-implantation and annealing processes. As ions are implanted while the Si substrate surface temperature is kept at as low as −60° C. or less. Then the Si substrate is heated to recover the implantation defects caused by the ion-implantation. Such a combination of the low temperature ion-implantation and the annealing processes is repeated as required.