Dramatically Enhanced Oxygen Uptake and Ionization Yield of Positive Secondary Ions with C60+ Sputtering

To explore C-60(+) sputtering beyond low-damage depth profiling of organic materials, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) were used to examine metallic surfaces during and after C-60(+) sputtering. During C-60(+) sputtering, XPS spectra indicated that the degrees of carbon deposition were different for different metallic surfaces. Moreover, for some metals (e.g., Al, W, Ta, Ti, and Mo), the intensity of the O Is photoelectron increased significantly during C-60(+) sputtering, even though the instrument was under ultrahigh vacuum (<5 x 10(-7) Pa). This result indicated that the rate of oxygen uptake was greater than the rate of C-60(+) sputtering. This behavior was not observed with the commonly used Ark sputtering. To measure the oxygen uptake kinetics, pure oxygen was leaked into the chamber to maintain a 5 x 10(-6) Pa oxygen environment. The C-60(+)-sputtered surface had a clearly increased rate of oxygen uptake than the Ark-sputtered surface, even for moderately reactive metals such as Fe and Ni. For relatively nonreactive metals such as Cu and Au, a small amount of carbon was implanted and no oxygen uptake was observed. High-resolution XPS spectra revealed the formation of metal carbides on these reactive metals, and the carbon deposition and enhanced uptake of oxygen correlated to the carbide formation. Because oxygen enhances the secondary ion yield through surface passivation, the enhanced oxygen uptake due to C-60(+) sputtering could be beneficial for SIMS analysis. To examine this hypothesis, C-60(+) and Ark were used as primary ions, and it was found that the intensity enhancement (because of the oxygen flooding at 5 x 10(-6) Pa) was much higher with C-60(+) than with Ark. Therefore, oxygen flooding during C-60(+) sputtering has a great potential for enhancing the detection limit due to the enhanced oxygen uptake.