Determination of trace metallic impurities on 200-mm silicon wafers by time-of-flight secondary-ion-mass spectroscopy

Surface contamination on silicon wafers is an important issue in integrated circuit fabrication. The concentration of many surface metallic impurities is below the detection limit of total reflection x-ray fluorescence (TXRF). Vapor phase decomposition—atomic absorption spectroscopy (VPD-AAS) cannot reveal the lateral distribution of these impurities even though the detection limits can be as low as 107 atoms/cm2. The time-of-flight secondary-ion-mass spectroscopy (TOF-SIMS) technique can deliver sensitivity close to that by VPD-AAS for many metallic impurities. For example, the detection limits of Fe, Al, Na, and K are 6×108, 3×108, 7×107, and 3×107 atoms/cm2, respectively. In addition, this technique can disclose the lateral distribution of the impurities as well as information on organic contaminants. This article addresses some of the latest development of TOF-SIMS in the determination of surface impurities of 200-mm silicon wafers. When choosing conditions such that the analysis is integrated throughout the native oxide layer, good quantitative correlation with TXRF and VPD-AAS data can be achieved.