DEPTH PROFILING OF ULTRATRACE CHROMIUM, I RON, NICKEL, AND COPPER IN SILICON-WAFERS BY ELECTROTHERMAL VAPORIZATION ICP-MS/

A method of etching micro thin layers for silicon wafers by a wet chem ical technique has been developed. Ultratrace concentrations of Cr, Fe , Ni, and Cu in the etching solution were determined by electrothermal vaporization ICP-MS (ETV/ICP-MS). Silicon wafers 0.01 to 10 mum thick could be dissolved by controlling the acidities of HF and HNO3 in the etching solution. The thickness was calculated from the Si concentrat ion of the etching solution by ordinary spectrophotometry using the Mo lybdenum Blue method. Silicon wafers were etched by 10 ml of acid solu tions containing appropriate concentrations of HF and HNO3. After the etching process, an aliquot of the solution was used for Si measuremen t. The rest was dried, dissolved in water and then subjected to elemen tal analysis by ETV/ICP-MS. The micro depth profiling of Cr, Fe, Ni, a nd Cu of a reference sample was done using the proposed method and sho wed good agreement with that obtained by SIMS. When a few practical si licon wafer samples were also analyzed by this method, it was found th at different types of silicon wafer had clearly different depth profil es of elements. The detection limits were 0.005 ng/g for Cu and Ni, an d 0.01 ng/g for Cr and Fe.