Trace element sensitivity for heavy ion backscattering spectrometry

Heavy Ion Backscattering Spectrometry (HIBS) is an ion beam analysis tool for measuring very low levels of surface contamination. HIBS uses low-energy (100-200 keV), heavy ions (C+, N+) for analysis combined with a thin carbon foil to range out the increased backscattering yield from the substrate. A new HIBS system has been built at Sandia National Laboratories under a cooperative research agreement with Sematech, which was made available as a user facility for Si wafer contamination analysis beginning in July 1995. A number of factors have influenced the detection limits which are being achieved, including detector efficiency, choice of beam species and energy, sputtering by the incident beam, and spectral background due to multiple scattering and random coincidences. Because of the increased sensitivity of this new system, we have identified sources of background which had not been a problem in a research prototype. These include problems with beam scattering in the chamber, molecular contamination in O and N beams, and forward scattering of H by the incident beam. In its first tests the system has demonstrated detection limits of similar to 6 X 10(9) atoms/cm(2) for Fe to similar to 3 X 10(8) atoms/cm(2) for Au on Si.