Multi-Diagnostic Characterization of Laser-Produced Tin Plasmas for EUV Lithography

TL;DR

The paper presents SparkLight, a multi-diagnostic platform that integrates EUV emission spectroscopy, polarization-separated Thomson scattering, and laser interferometry to characterize laser-produced tin plasmas for EUV lithography. By measuring $n_e$ and $T_e$ with spatial and temporal resolution and cross-validating across diagnostics, the study maps the plasma conditions associated with EUV emission and assesses spectral purity around the 13.5 nm band. Key findings show $T_e$ in the 9–15 eV range and $n_e$ on the order of $10^{17}$–$10^{18}$ cm$^{-3}$ in the 120–270 μm region, with the bulk of EUV emission originating within ~150 μm of the target and peaking near 13.6–13.8 nm, indicating suboptimal in-band emission under the explored conditions. The integrated diagnostic workflow provides a robust framework for validating radiation-hydrodynamic models and guiding future optimization of tin-based EUV sources, with planned upgrades to access the near-target region and higher-energy operation.

Abstract

We present a comprehensive characterization of laser-produced tin (Sn) plasmas relevant to extreme ultraviolet (EUV) lithography using a multi-diagnostic suite integrated into the new experimental platform, "SparkLight". Tin plasmas are generated by irradiating a continuously moving tin-coated wire with laser pulses (1064 nm, 10 ns, up to $5.7\times10^{10}$ W/cm$^2$) and probed via coherent Thomson scattering, laser interferometry, and EUV emission spectroscopy. Thomson scattering measurements reveal electron temperatures and densities that decay with distance from the target. Densities derived from Thomson scattering are cross-validated against laser interferometry, showing excellent agreement. Correlating the results of these laser diagnostics with spatially resolved EUV spectroscopy suggests that the bulk of useful EUV emission originates within 150 $μ$m of the target and is generated under suboptimal plasma conditions. This work demonstrates a practical integrated approach for plasma characterization in EUV source development.

Figures (12)

• Figure 1: Schematic of the "SparkLight" facility showing a vacuum chamber, target (tinned wire), Thomson scattering, laser interferometry, and EUV emission spectroscopy diagnostics. Optical paths are color-coded: red— drive beam, green— 532 nm probe beam used for laser interferometry and Thomson scattering (part of the optical path related to Thomson scattering shown in blue), and yellow— laser beam profiling. Acronyms are defined as follows: F: optical focal length in cm, M: mirror, BS: beamsplitter, BP: bandpass.
• Figure 2: Schematic of the compact polarization-separated Thomson scattering diagnostic. A Wollaston prism splits the collected signal into s- and p-polarized components.
• Figure 3: (a) Emission spectra from a laser-produced tin plasma obtained at different laser intensities. (b) and (c) Spatially and spectrally resolved emission spectrum obtained at the laser intensity value used during Thomson scattering experiments. All spectra were accumulated over 200 laser shots (1064 nm, 10 ns) onto a tin-coated wire.
• Figure 4: Signals separated by polarization using a Wollaston prism and imaged with an emICCD camera. The s-polarized signal consists of Thomson scattering and the s-polarized part of the plasma self-emission, the p-polarized signal contains the remaining part of the unpolarized self-emission. The signals were collected using a 1200 g/mm grating, $270\pm20\ \mu m$ from the target, integrated over 40 laser shots per sequence, and averaged over 10 sequences.
• Figure 5: (Top) 2D image of the Thomson scattering signal. (Bottom) Vertically integrated spectral intensity showing distinct sidebands (electron plasma features, EPW) and a strong central feature attributed to an under-resolved ion acoustic wave (IAW). The signal was collected using a 1200 g/mm grating, $270\pm20\ \mu m$, integrated over 40 laser shots per sequence, and averaged over 10 sequences.