Automatic Iterative Fitting of RBS Spectra from Multielement Samples
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is a method
for
performing
surface chemical analysis using a charged particle accelerator
and energy sensitive detectors. The technique relies on the kinematics of hard sphere scattering, which results in a different fractional energy transfer upon scattering that depends on the mass of the target atom. This makes it possible to analyze the elemental composition of a smooth surface to determine both the major alloying elements and impurity concentrations as a function of depth from the surface. Because of this capability, RBS is ideally suited to the analysis of surfaces consisting of multiple layers of two or more elements. Such surface structures frequently arise in ion beam mixing experiments where an inert gas ion beam is used to mix several layers of composition. Typically, this sort of analysis is performed by a computer code [1,2] which models the backscattering process with the appropriate experimental parameters: geometry, beam energy, total currrent, and a guess of the probable elemental concentration profile. The modeling program then produces a spectrum which is compared to the experimental data, and the experimenter changes the input concentration profile to produce a better spectrum match. An automatic fitting routine (SQUEAKIE) has been written by Borgesen, et.al. [3] which greatly simplifies the analysis of many spectra. In the present work, a similar analysis routine has been devised. The present code, BASF, is a modification of a manual fitting routine, RSVP[l], which has been revised to perform automatic iterative fitting of RBS spectra using only the experimental spectrum and the parameters defining the experiment as input. This new code was tested first against computer generated spectra and then on actual experimental spectra obtained from multicomponent samples prepared by vacuum evaporation and analyzed using a 2 MV tandem ion accelerator. A description of the code, its capabilities and test results are presented in the following sections.
Mat. Res. Soc. Symp. Proc. Vol. 54. c 1986 Materials Research Society
694
CODE DESCRIPTION The code (BASF - Backscattering Spectrum Fitter) creates a spectrum by modeling the important processes which contribute to the detector signal. These are the slowing down and straggling of the beam particles as they travel into the sample, the backscattering event itself, the slowing down and straggling of the particles on the way out of the sample, and the actual detection event. The slowing down of the particles is accomplished by computing stopping powers using a tabulated six parameter quintic fit to experimental and extrapolated data [4]. The stopping power in multielement regions is assumed to be that obtained by weighting the individual stopping powers with the element's concentration (Bragg's Rule). The energy straggling of the beam is modeled with the Bohr formula. Straggling results in loss of resolution in the final spectrum. Another source of resolution loss is the detector and electronics, which typically have an energy resolution of about 15 kV
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