Sample-thickness dependence and chemical effects in SIMS depth profiling of multilayer langmuir-blodgett films

Abstract

We have further investigated the potential of secondary ion mass spectrometry (SIMS) for sputter depth profiling of organic model systems. The samples consisted of multilayer films of arachidic acid (2–10 monolayers) which were deposited on gold backings using the Langmuir-Blodgett technique. Magnesium or cadmium ions served as markers for the individual monolayers in the film. Depth profiling was performed using raster scanned O 2 + or Ar + beams at energies between 2 and 8 keV. The initial stage of bombardment was characterized by preferential emission of H + and a rapid disintegration of the organic molecules. The C + and Mg + intensities increased initially, then passed through a maximum and finally decreased roughly exponentially. The erosion rate of the organic films was found to decrease by up to one order of magnitude as the film thickness increased from 2 to 10 ML. This effect is attributed to a thickness dependent contribution of backscattered primary ions to the sputtering yield. Comparison of the Au + profiles with previously reported results suggests that small holes may be formed during exposure of thin films to a focussed ion beam. Yield enhancement effects were observed for thick films bombarded with O 2 + ions.