Low-level copper concentration measurements in silicon wafers using trace-element accelerator mass spectrometry

Abstract

Accelerator mass spectrometry (AMS) is now widely used in over 30 laboratories throughout the world to measure ratios of the abundances of long-lived radioisotopes such as Be111, C14, CI36 and I127 to their stable isotopes at levels as low as 10−16. Trace-element AMS (TEAMS) is an application of AMS to the measurement of very low levels of stable isotope impurities. Copper concentrations as low as 1 part per billion have been measured in silicon wafers. In this letter, we demonstrate the use of TEAMS to measure previously unknown copper concentration depth profiles in As-implanted Si wafers at a few parts per billion. To verify the TEAMS technique, the samples from the same wafer were measured with secondary ion mass spectrometry, which showed the same profiles, albeit plateauing out at a concentration level six times higher than the TEAMS measurement. The ability to measure at these levels is especially significant in light of the recent moves towards the use of copper interconnects in place of aluminum in integrated circuits.