Philipp Mika Wolf: Advancing in-situ studies using low-energy ions: Growth and modification of transition metal compounds

Abstract

Transition metal compounds are highly relevant materials with applications ranging from mechanical tools over catalysis to thin film electronics. Continued miniaturization in the field of integrated circuits and effects of high temperatures and reactive gasses manifesting in the near-surface region of mechanical tools call for techniques able to quantitatively resolve composition and structure of transition metal compounds at a sub-nm scale. In this thesis, a range of transition metal-based systems is studied with a particular focus on depth resolved analysis of structure and composition using Time-of-Flight Low-Energy Ion Scattering (ToF-LEIS). This work improves the accuracy in ToF-LEIS quantification by studying model systems of transition metals and critically assessing its capabilities in in-situ studies of near-surface processes caused by exposure to oxygen or high temperatures in technologically relevant transition metal compounds.

The analytical power of in-situ ToF-LEIS in the quantitative study of composition and structure in transition metal compounds is demonstrated on the example of Ti-based thin films and transition metal silicides. Oxide layers with nm thicknesses on Ti layers exposed to oxygen and the formation of an Al-rich surface layer on (Ti,Al)N above 750 °C are detected with a sub-nm depth resolution. Furthermore, the silicidation processes for Ti and Ni are investigated using ToF-LEIS to in-situ study structure and composition in a depth-resolved manner. For ultrathin Ni silicide, commonly used as contact layers in integrated circuits, an unprecedented phase transition sequence with a direct phase transition from orthorhombic δ-Ni2Si to epitaxial NiSi2‑x, skipping the NiSi phase present in thicker films is observed at 290°C.

To improve quantitative analysis using ToF-LEIS as well as our understanding of the underlying physics, two aspects of the interaction between slow ions and solids are studied. The electronic stopping cross section of Ti for light ions is investigated. The obtained comprehensive dataset offers valuable experimental data for a highly relevant transition metal, allowing substantially improved range and depth quantification in ToF-LEIS experiments. Furthermore, the interatomic potential is studied for W and Fe in combination with He and D, relevant atom pairings for future fusion reactors. This study offers critical experimental data for the prediction of plasma-wall interactions like sputter yields that will affect the operation, durability, and safety of fusion devices. Additionally, these results yield a better understanding of the quality of common interatomic potential models applied in ToF-LEIS, enhancing quantification of depth and composition.