Electronic energy loss of high-energy ions in matter.

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 (RBS) [1] or elastic recoil detection analysis. 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 electronic energy loss. Knowing the stopping cross section accurately is critically important to improve the ion beam analysis methods. Besides of this, it is also essential in applications as for example ion implantation, fusion research, or medical applications like hadron therapy.

An energetic ion that penetrates matter loses kinetic energy due to interactions with the electrons and the nuclei of the target [1]. The process is described by the stopping cross section which relates the energy loss to the unit path length normalized by the atomic density. 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 selected ion/target combinations [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.

We will introduce novel experimental as well as data analysis and interpretation approaches to study the stopping cross section. One of the novel experimental approaches is to derive the value from RBS spectra that are collected on pure thin films at different kinetic energies ranging from ~400 keV up to 4 MeV. Another novel experimental approach is to use ultra-thin films that are prepared as a self-supporting thin film by using a combination of reactive ion etching and wet chemical etching in the imec cleanroom and measuring the energy loss in transmission geometry. You will develop novel data-analytical methods to determine the most probable stopping cross section by combining values in the literature [2] and your own data. Starting from existing semi-empirical models [4], you will propose and publish a new parametrization for the stopping cross section, including their uncertainties.

The study will expose you to the research in a high-energy accelerator laboratory and give you the opportunity to gain a very deep insight into the physics of solid-state and other materials. You will experience preparing samples in the clean room of imec, as for example self-supporting thin films. Besides, you will learn to use advanced computational methods for data-analysis and deepen your knowledge in uncertainty estimation. The newly obtained parametrization of the stopping cross section will be published as open data and it is expected that it will have a high impact on a large community.

[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/