Determination of Stopping Cross Sections by Rutherford Backscattering

Abstract

The stopping cross section, ε, of light ions in solids determines the accuracy of depth distribution measurements by RBS. Errors quoted for individual measurements of stopping cross section are often within 2 -3% but agreement between different authors is mostly no better than ± 10 %. Stopping cross sections have been measured by many authors by transmission of ions through thin films or by backscattering from thick targets. Two basic methods exist to determine stopping cross sections from Rutherford backscattering spectra: 1) ε can be determined from the energy width ∆E of a thin surface film (10,20). 2) ε can be calculated from the height H (counts/channel) of the spectrum (8,11,21,22). Both methods imply approximations. The errors introduced by these approximations are investigated by applying the different methods to artificial spectra generated by a computer program, assuming Rutherford backscattering and a known stopping cross section. The results show that all approximations cause errors which are within a few % if M_1/M_2 ≪ 1. For larger M_1/M_2 ratios, the ∆E-method is generally more accurate. The computer-generated spectra are further compared to measured backscattering spectra. The agreement is generally good in the upper energy range of the spectrum (E_1 ≃1/2 KE_o). At low primary energies (E_(He) < 500 keV) increasing deviations of the measured spectra from the calculations are observed due to a background produced at least in part by plural scattering. Backscattering measurements were performed with ^4He in the energy range 200–2000 keV on films of Au, Pt, Si0_2, and Ta_20_5 using both methods. The data obtained for Au and Pt agree well with those of other authors. They are 5 – 10% lower than those given in the tables of Ziegler & Chu(6). ε for Si0_2 is found 20% lower than obtained from (6) applying Bragg’s rule. The validity of ε measurements depends critically on the calibration procedure. In our case calibrated thin film targets of Au and Pt were used. The mass per area has been measured by microbalance at two different institutions with good agreement.