Description
State-of-the-art nanolithography uses extreme ultraviolet (EUV) radiation at 13.5 nm emitted from tin laser-produced plasma (LPP), driven by a 10.6 µm-wavelength CO2-gas laser, to create the smallest features on semiconductor devices. Advances in solid-state laser technologies have led to the alternative concept of a 2 µm-wavelength-driven LPP as a promising candidate for a more efficient and more powerful EUV source. At this drive laser wavelength a high conversion efficiency of laser energy into EUV radiation of 4% has been demonstrated [Appl. Phys. Lett. 123, 234101 (2023)], but a full picture of the plasma has not been shown to date.
In this study we characterize the partitioning of all emissions of a 2 µm laser driven plasma to have a full picture of the energy sinks. We use our custom-built high energy 2 μm laser system, developed at ARCNL, to heat a tin sheet into an EUV emitting plasma. Plasma emissions are measured by an extensive diagnostic suite: In-band EUV light (13.5nm±1%) is measured, correcting for the emission anisotropy, by a set of EUV photodiodes. The EUV emission is combined with the emission spectrum measured in 5-260nm in order to reconstruct the full radiative emission. Ion emissions are measured with a set of retarding field analyzers, reconstructing the charge-state-resolved ion kinetic energy spectrum. Together with laser reflection and kinetics of the remaining neutral tin this accounts for the energy sinks in the plasma.
We can for the first time reconstruct 100% of the incoming laser energy. With ~70%, most of the energy goes into photons, the largest part of which is emitted in the 5-80 nm wavelength range. Plasma ions contribute ~30% of the energy, and, contrary to available theory and numerical simulations, we find that this fraction decreases as a function of the laser intensity. This is a key insight in finding pathways to optimize plasma for industrial application. Our results offer insight into the energy partitioning of EUV-emitting plasmas for research and industrial nanolithography alike, and provide a benchmark for plasma light source development.