HEAVY HITTERS
This thesis investigates the atomic-scale interactions of tin ions (Sn) in EUV light sources, essential for modern chip photolithography. EUV light (13.5 nm) is generated by hitting a tin droplet with a laser, creating a hot tin plasma. Besides the desired light, energetic tin ions are also released, which can damage the collector mirror; a hydrogen buffer gas slows these ions down. Optimizing these sources requires detailed knowledge of how tin ions interact with gases and surfaces.
The research is built on three pillars. First, ion-gas interactions were studied: the stopping power of hydrogen for Sn1+ ions was found to be lower than previously assumed, with a transition in behavior around 600 eV that agrees well with a new semi-classical collision model. Second, ion-surface interactions were investigated: argon, krypton, and xenon ions impinging on a ruthenium surface show that surface roughness only partly explains why an expected scattering peak is absent, pointing to the role of many-body interactions for heavy projectiles. Measurements of ion-induced electron emission by Sn1+–Sn5+ show that kinetic electron emission already occurs at velocities much lower than expected, and that potential emission increases with charge state as expected. Third, measurement instrumentation was improved: conventional retarding field analyzers were found to produce unreliable energy distributions due to lensing effects caused by their grids. A new gridless design shows a constant, energy-independent transmission.
In summary, this work shows that simplified models fall short in capturing these interactions, but that a more complete atomic-scale description is now within reach.