Extending RBS analysis to very fragile, nanometer-thick foils

Abstract

We have applied Rutherford backscattering (RBS) to the analysis of very thin and very fragile, free-standing foils. These foils consisted of a thin metal or other layer encapsulated inside thin layers of a polycarbonate-like polymer. Thicknesses of the middle layers were ∼10 nm of Fe, ∼2 nm of Au and ∼7 nm of Ni with ∼12 nm of NaF. The encapsulating, polymer layers each had a thickness of about 75 nm. Application of appropriate sample mounting and RBS techniques has led to spectra with essentially no background. We determined the amounts and depth distributions of all elements with atomic numbers Z⩾6. Very low current densities (∼2 nA/mm 2) of 2.3 MeV He + ions were used to minimize heating by the beam and yet obtain useful statistics in a reasonable time. To monitor any changes in the foils during analysis, spectra were obtained in a sequence of low-fluence bombardments. After each bombardment the beam was interrupted, and the data was examined and stored. This procedure permitted us to monitor the data for any significant changes in the foil and allowed cooling of the target during the beam-off interval.