Method for assaying copper in silicon wafers

A method for assaying copper in silicon wafers, the method comprising:

• forming polysilicon layers on front surfaces and rear surfaces of two or more p-type silicon wafers having same characteristics as silicon wafers being assayed;
• polishing at least one of the p-type silicon wafers and thereby removing the polysilicon layer on one of the front surface and the rear surface;
• subjecting the polished p-type silicon wafer to a heat treatment at a temperature within a range of 200 to 450° C. for 5 to 60 minutes;
• dissolving the polysilicon layer on the heat-treated p-type silicon wafer with a mixed acid including at least hydrofluoric acid and nitric acid;
• quantitatively determining an amount of copper components within the mixed acid after dissolution of the polysilicon layer on the heat-treated p-type silicon wafer, and thereby obtaining an amount of the copper components after the polishing;
• dissolving the polysilicon layer on at least one of the other p-type silicon wafers which are not subjected to the polishing, with a mixed acid including at least hydrofluoric acid and nitric acid;
• and
• quantitatively determining an amount of copper components within the mixed acid after dissolution of the polysilicon layer, and thereby obtaining an amount of the copper components before the polishing.