Effects of residual metal impurities after RCA (Radio Corporation of America) standard cleaning (alkaline and acid rinses) on the generation of ac surface photovoltages (SPVs) are investigated usingn‐type silicon wafers. Aluminum (Al) and iron (Fe) in the native oxide induce a negative charge, causing high ac SPVs inn‐type wafers. The ac SPV dependency on the concentration of Al and Fe is determined. Nickel and zinc, however, have little effect on the generation of ac SPVs. In commonly employed cleaning processes, Al is the major impurity in the native oxide, and thus the ac SPV technique is applicable to nondestructive diagnostics for quality control in cleaning processes.