Quantification of metal trace contaminants on Si wafer surfaces by Laser-SNMS and TOF-SIMS using sputter deposited submonolayer standards

Abstract

We have shown that quantitative determination of metal trace impurities in the range between 109 cm−2 and 1012 cm−2 on top of the native oxide of a Si wafer by laser postionization of sputtered neutrals in combination with time-of-flight mass spectrometry (Laser-SNMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) is possible. Trace metal standards with concentrations in the range between 109 cm−2 and 1012 cm−2 were used for calibration. These standards were prepared by sputter deposition and were independently controlled by Total Reflection X-Ray Fluorescence Analysis. Relative sensitivity factors for 12 metals on Si for Laser-SNMS were determined. Additionally, we compared postionization with different wavelengths (193 nm and 248 nm). With Laser-SNMS it was then possible to determine the influence of UV/ozone treatment on the measured surface concentration of metal species. The UV/ozone treatment is necessary to achieve high and reproducible useful yields for the metal species in TOF-SIMS. With this knowledge, we were able to determine relative sensitivity factors for the metals on UV/ozone-treated Si wafer surfaces measured by TOF-SIMS. Detection limits down to 108 cm−2 and 1012 cm−2 for sample surface areas of 100 μm in diameter and 1 μm in diameter, respectively, were found for both Laser-SNMS and TOF-SIMS.