Nuclear Instruments and Methods in Physics Research B9 (1985) 1977200 North-Holland. Amsterdam 197 RUTHERFORD BACKSCATTERING (RBS) WITH LITHIUM IONS E. NORBECK, L.W. LI *, H.H. LIN, and M.E. ANDERSON Department of Physics and Astronomy, The Unwerslty of Iowa, Iowa Cit_v, Iowa 52242, USA Received 14 November 1984 and in revised form 2 January 1985 ‘Li ions at 4.5 MeV and 'He ions at 1.5 MeV were scattered at 170” from the same target under the same conditions. The distribution of elements as a function of distance from the surface is more clearly revealed with ‘Li than 4He. We have rewritten our RBS analysis program to handle all ion beams and to take into account the normal isotopic abundance of the target elements. With 4.5 MeV ‘Li ions the minority isotopes can not be ignored, even with silicon. At present, the accuracy for determining elemental depth profiles with ‘Li is limited by the accuracy of Li ion stopping powers, 1. Introduction Rutherford Backscattering Spectrometry (RBS) is often the method of choice for obtaining the concentra- tion vs depth profiles of elements in the upper microme- ter of smooth solids. The data shown below demon- strate the advantages of 4.5 MeV lithium beams over the more conventional 1.5 MeV 4He beams. Detailed analyses of depth and mass resolution are available [1,2]. We will not repeat these discussions here, rather we will show some data and discuss differences between RBS with 4He and with ‘Li. It will be seen that a combination of features is responsible for the superior qualities of the higher energy ‘Li spectra. 2. Experiment Except for the lithium beam, our RBS setup was conventional. The 100 pm thick, 50 mm2 solid state detector was operated at room temperature. The detec- tor resolution was about 9 keV for 4He and 15 keV for ‘Li. The scattered beam was detected at 170” with an angular width of about 2”. The ion beams were supplied by the CN Van de Graaff at The University of Iowa. The target was a round robin sample distributed by M. Strathman and S. Baumann at Charles Evans and Associates. It was a 0.1 mg/cm2 film of molybdenum silicide on a silicon substrate. 3. Data A study of figs. 1 and 2 reveals some significant features. The two figures are for the same sample under * On leave from Nankai University, People’s Republic of China. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) identical conditions except that in fig. 1 the beam was 4.5 MeV ‘Li and in fig. 2 it was 1.5 MeV 4He. In fig. 1 the scattering peaks for the three elements in the surface film are completely separated. Note the break in the scale between the Ar and MO peaks. In fig. 2 the Ar peak is barely separated from the MO peak and is not separated from the Si. It is the higher energy of the 4.5 MeV ‘Li that provides the advantages over 1.5 MeV 4He. The energy of the high energy edge of the MO peak is proportional to the beam energy. However, the width of the peak, which is proportional to the thickness of the MO silicide layer divided by the stopping power, is nearly constant for ‘Li beam energies from 0.5 to 5.0 MeV. Raising the ‘Li beam energy spreads out the spec- trum without significantly changing the depth resolu- tion, defined as the width of the peak divided by the detector resolution. Actually, the real depth resolution can be much worse if the features in the spectrum overlap. In fig. 1 it is clear that the Ar was only in the surface layer. In fig. 2 there is no way of knowing if the Ar was only in the surface layer or if it permeated the entire sample. A larger beam energy provides additional benefits. With larger signals from the detector, there are less problems with noise and electrical interference, and it is easier to generate logic signals for opening gates and triggering pile up rejection circuits. At 1.5 MeV the cross section for scattering from heavier elements falls below the Rutherford formula because the nucleus is partly screened by the K electrons. For 4He + Au this reduction is about 2%; for ‘Li + Au at 4.0 MeV the reduction is negligible [3-51. Increasing the 4He beam energy to 4.5 MeV gives quite different results. Fig. 3 shows what happens. The MO peak has become so narrow that nothing can be learned about the distribution of MO in the surface