Enhancing The Undergraduate Experience: Measuring Film Thicknesses Using A Helium Ion Beam

Students performed Rutherford back scattering (RBS) experiments using 1.5 MeV helium ion (+1) beam from a Van de Graaff accelerator. The ion beam was incident normally on target samples located in a scattering chamber. The samples were single and multilayered thin films on substrate, prepared using a vacuum evaporator. The scattered ions were measured using a particle detector located at 150 from incident beam direction. The elastically scattered particle energies were predicted from kinematical scattering factor (derived using conservation of energy and momentum) and experimentally verified. Using samples of known elemental thicknesses, the data was normalized and thicknesses of layers in unknown samples determined. For multilayer samples, the scattered particle spectra were analyzed for particle yield per ion, energy shift in the centroid and energy at the leading edge for the elemental peak. The Rutherford scattering formula and the kinematical scattering factor together with the energy loss of the helium ion through the various layers were used to identify elemental layers, their positions and thicknesses. The multilayer films were identified as gold-tungsten-tin (Au-W-Sn). The normalization procedure allowed the thicknesses to be determined with uncertainties of a few percent.