LOW-LEVEL COPPER CONCENTRATION MEASUREMENTS IN SILICON-WAFERS USING TRACE-ELEMENT ACCELERATOR MASS-SPECTROMETRY

Accelerator mass spectrometry (AMS) is now widely used in over 30 labo ratories throughout the world to measure ratios of the abundances of l ong-lived radioisotopes such as Be-10, C-14, Cl-36, and I-127 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 st able isotope impurities,Copper concentrations as low as 1 part per bil lion have been measured in silicon wafers. In this letter, we demonstr ate the use of TEAMS to measure previously unknown copper concentratio n depth profiles in As-implanted Si wafers at a few Darts per billion. To verify the TEAMS technique, the samples from the same wafer were m easured with secondary ion mass spectrometry, which showed the same pr ofiles, albeit plateauing out at a concentration level six times highe r than the TEAMS measurement. The ability to measure at these levels i s especially significant in light of the recent moves towards the use of copper interconnects in place of aluminum in integrated circuits. ( C) 1998 American Instutitue of Physics.