EUV Plasma Processes

Understand exploding tin microdroplets

What determines deformation and fragmentation?

Advanced semiconductor manufacturing demands intense extreme ultraviolet (EUV) light sources. This light is generated through the interaction of nanosecond lasers with micrometer-sized liquid tin droplets. A critical aspect of this process is understanding and controlling the deformation dynamics of the droplet and hydrodynamic stability of the resulting thin sheet after laser impact, as these factors directly influence EUV light generation.

Studying these systems also provides valuable insights into fundamental fluid dynamics. Key areas of investigation include the force field distribution on the droplet following laser impact, cavitation effects within the droplet driven by shock waves and the full characterization of capillary-mediated processes such as droplet breakup, sheet atomization and jetting. These studies not only advance our understanding of laser-droplet interactions but also offer critical knowledge for improving the design and performance of EUV light sources, contributing to enhancing the performance and cost-effectiveness of EUV lithography. Modeling activities are done in collaboration with UvA’s Jalaal group.

Key insights to enable source predictive modeling

What emits that EUV light?

EUV light is emitted by a hot and dense laser-driven tin plasma, which consists of a large range of atomic charge states. Each has its own complex electronic energy level structure that gives rise to millions of transition lines. It seems extraordinarily convenient that this complex structure results in a narrow-bandwidth peak at 13.5 nm, perfect for lithography. What is the origin of this light; which fundamental interactions determine the properties of this peak? Ultimately, can we make similar predictions for other elements, which could enable us to find candidates for a new source of light, at even shorter wavelengths?

We investigate how the spectra of highly-charged tin ions depend on different fundamental interactions. A systematic study of interelectronic interactions (electron correlation) as well as higher order effects will enable us to pinpoint the most important contributions to the energy spectra. Ab initio electronic structure theory is used to do these calculations without any fitting to experimental data. All efforts are coordinated with the ARCNL Plasma Theory and Modeling group.

Push the fundamental limits of the conversion efficiency of laser light into useful EUV photons

What sets the fundamental limit?

Control expansion dynamics of laser-produced plasma

What is the cause of the ion energy distribution, how can it be controlled, and what does it cause?

The transient laser produced plasma (LPP) that produces the sought-after EUV photons, has densities of approaching that of a solid, and can reach temperatures up to 500 thousand Kelvins (~40 eV). This puts the LPP in the plasma world somewhere between lightning and the solar core. In this state, the plasma will not only produce the sought-after EUV photons, but also tin debris that is a result of the ablation of the tin droplet. This debris includes droplet fragments, neutral atoms, microdroplets and plasma ions. The plasma ions can have massive energies, ranging up to multiple keV, and has the potential to damage the optics close to the EUV source. Therefore, the emission of these ions needs to be controlled or (preferably) mitigated. To this end, the ion emission is studied in detail using retarding field analyzers (RFAs), which provide a charge-resolved ion energy spectrum. This provides further insights into the plasma expansion, ultimately leading to a better control of the ion spectra and thus less damage to the source optics. Highly energetic tin ions may be mitigated using a buffer gas. It is important to understand the interactions between the tin ions and the gas molecules. All efforts are coordinated with the ARCNL-Groningen Ion Interactions group.