HEAVY HITTERS

This thesis investigates the atomic-scale interactions of tin ions (Sn) in EUV light
sources, which are essential for modern chip lithography. EUV light, with a wavelength
of 13.5 nm, is generated by irradiating a tin droplet with a laser, creating a hot plasma of
tin ions. In addition to the desired light, energetic tin particles are also released, which
can damage the collector mirror; a hydrogen buffer gas is used to slow these particles
down. Optimizing these sources requires a detailed understanding of how tin ions
interact with gases and surfaces.
The research is built around three pillars. First, ion–gas interactions were studied: the
stopping power of hydrogen for Sn1+ ions was measured and found to be lower than
previously assumed. A transition in the energy dependence around 600 eV was
observed, in good agreement with the results of a new semi-classical collision model.
Second, ion–surface interactions were investigated: argon, krypton, and xenon ions
were directed at a polycrystalline ruthenium surface. The results show that surface
roughness only partly explains the absence of an expected scattering peak, pointing to
the role of many-body interactions for heavy projectiles. Measurements of ion-induced
electron emission from Sn1-5+ show that kinetic electron emission occurs at velocities
much lower than expected, while potential electron 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
electrostatic lensing caused by their grids. A new gridless design exhibits a constant,
energy-independent transmission.
In summary, this work shows that simplified models fall short of fully describing these
interactions, but that a more complete atomic-scale description is now within reach.