A Rutherford backscattering study using 2 MeV He+ ions has shown that the principal contaminants heavier than Si remaining on silica‐sol polished silicon surfaces are , and a heavy metal in the range . Preoxidation cleaning using and was found to be erratic in the ability to yield surfaces free of Ca and Cu and always left heavy metal contaminants. Cleaning with and solutions always removed all elements heavier than chlorine to below the level of detectability. Both cleaning treatments left . Breakdown voltage measurements indicated that junction quality was not predominantly controlled by surface impurities present after preoxidation cleaning, although the diode quality on cleaned wafers was much higher than on as‐polished wafers.