PURPOSE: To realize an accurate measurement of Fe-B concentration by comparing the value of a specific relation of minority carrier diffusion length before and after activation of a p-type silicon wafer with a predetermined value obtained from a specific correlation graph thereby making a selection between presence and absence of oxygen deposition.

CONSTITUTION: The minority carrier diffusion length is measured for a silicon wafer where the Fe-B is bonded at Step 1 and the value Lb is stored. The minority carrier diffusion length is then measured under dissociated state at Step 2 and the value La is stored. The operation result (Lb-La) /Lb is compared with a constant C at Step 5 and if (Lb-La) /Lb<C, the operation is shifted to Step 6 where the operation of Fe-B concentration for the wafer is interrupted before the operation returns back to Step 1. A decision is then made than the wafer has an oxygen deposition defect and the wafer is separated. The constant C is determined from the correlation graph of Lb and Lb-La according to a formula; C= (Lb-La) /Lb. This method realizes an accurate measurement of Fe-B concentration.