Investigating the Composition of Tin Vapor

Tin vapor is used in the EUV source of lithography machines and the liquid-tin divertor of fusion devices. However, in both cases, this tin vapor’s composition is unknown, whether it be atoms, clusters of atoms, or (a size distribution of) nanodroplets. Measuring and understanding the composition of the tin vapor would allow for future improvements in both these fields. For EUV lithography, it could help in the quest for a more efficient EUV generation method. It could also help to create a more reliable heat exhaust in the field of fusion. Thus, in this thesis, the following question will be answered: "What is the composition of tin vapor targets used in the EUV light creation process, and which factors influence this composition?"

An existing experimental setup is improved with multi-wavelength absorption imaging in the UV range, UV shadowgraphy. A model is created to predict and explain experimentally measured spectra. This model incorporates atomic absorption and Mie scattering on nanodroplets. Code is written to automate the measurements and find the extinction from the captured images.

Detailed extinction spectra between 225 to 405 nm are presented. These spectra show a background extinction level caused by Mie scattering, with narrow peaks in extinction on top. These narrow peaks are atomic resonances. One such spectrum is matched to the model to find its density, atomic mass ratio, temperature, and nanodroplet size. The model matches the general trend in the experimental data. However, there are some aspects where the model differs from the data. The substantial decrease in the background extinction as the wavelength becomes longer, caused by Mie scattering, and the width and strength of the atomic resonances cannot be matched perfectly by the model. The mismatch in the resonances is expected to be caused by unmodeled density-related broadening effects. Further research into the refractive index is proposed to match the decay in the extinction due to Mie scattering. The influence of some parameters, such as spatial position and timing, on the observed spectra are also presented. All spectra show similar profiles.

To conclude, the novel UV shadowgraphy metrology has been implemented with success. The research question has been partly answered using this metrology by proving the presence of atoms and nanodroplets in tin vapor for the first time. Future work to find the cause of the model mismatch is proposed to allow for the final determination of the composition of tin vapor.