Pack-extraction method combined with inductively coupled plasma mass spectroscopy to monitor metal contaminants on surfaces of silicon wafers
A method to extract micro-contaminants detrimental to silicon (Si) devices is developed and its impact on clean technology is discussed, focusing on upgrading the oxide film performance of densely packed devices. Detrimental metal impurities (Fe, Cu, Al, Ni and Zn) on surfaces of Si wafers from three vendors were successfully extracted with a prototype pack-extraction method (PEM), in which sample wafers were individually enclosed in a cleaned Teflon bag containing an aqueous acid solution. Each solution, which contained extracted contaminants, was then analysed with inductively coupled plasma mass spectroscopy (ICP-MS). The results could help reduce undesirable impurities that degrade device characteristics. To analyse impurities on surfaces and in the Si bulk separately, we developed an improved PEM, in which impurities were extracted by successively heating the wafer with three different acid solutions ((i) HCl/H2O, (ii) HF/H2O and (iii) HF/HNO3/H2O). By using this improved PEM combined with ICP-MS, we analysed the impurities on the oxide surfaces, in the oxide and in Si bulk separately (including the SiO2-Si interface) for oxidized wafers contaminated intentionally with impurities of Fe, Al, and Cu prior to oxidation. The analysis showed that Al and Fe mainly segregated at the uppermost layer of the oxide, whereas Cu was distributed not in the oxide, but mainly in the Si bulk and SiO2-Si interface.