Study of the electronic stopping cross section of light ions in noble metals

A multitude of new materials are being developed and investigated to enhance the performance of the future generations of logic, interconnect, and memory devices. To characterize the nanometer scale components, imec uses amongst others ion beam analysis methods, like for example Rutherford backscattering spectrometry [1]. This project concerns the study of the underlying physical phenomena that are used to interpret the experimental data from ion beam analysis. The goal is to develop a methodology to improve the accuracy of the stopping cross section.

An energetic ion that penetrates matter loses kinetic energy due to collisions with the electrons and the nuclei of the target [1]. The process is described by the stopping cross section, epsilon = 1/n dE / dx, which relates the energy loss (dE) to the unit path length (dx) normalized by the atomic density (n). Knowing the stopping cross section accurately is critically important. Besides of the importance for ion beam analysis, it is also essential in applications as for example ion implantation, fusion research, and medical applications. 

The stopping cross section depends on the kinetic energy (E) and the atomic number of the moving ion (Z1), and on the elemental composition of the material in which it penetrates. Thanks to numerous studies [2], the reported stopping cross section of various ions has an accuracy of around 10% for most elements and around 3% to 5% for ion/target combinations that are encountered in ion beam analysis [3]. Still, the accuracy of the stopping cross section is one of the limiting factors to reach to a better accuracy in ion beam analysis. 

In the Master thesis subject, we investigate a new approach to derive the stopping cross section value from Rutherford backscattering (RBS) experiments. First, dedicated very pure thin films will be deposited on a substrate (for example a layer Au on silicon). Then, the RBS spectra will be collected for different kinetic energies ranging from ~400 keV up to 4 MeV (by using the accelerator in the IMBL or at imec). The stopping cross section values will be obtained from the analysis of the various spectra, and we will estimate the uncertainties on the obtained values.

The newly obtained values of the stopping cross section will be compared with existing experimental values in the literature [2] and with semi-empirical models [4]. For the studied target/ion combination you will combine your own experimental data with existing data to arrive at a new parametrization of the stopping cross section. Besides of the stopping power, you will also analyse and estimate the error on the obtained parametrization.

[1] https://en.wikipedia.org/wiki/Stopping_power_(particle_radiation)  

[2] data base available at https://www-nds.iaea.org/stopping/  

[3] https://en.wikipedia.org/wiki/Ion_beam_analysis  

[4] SRIM http://www.srim.org/