Characterization of 1- and 2-$\mu$m-wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

TL;DR

This study compares EUV emission from tin microdroplet plasmas driven by 1- and 2-micron lasers, revealing a wavelength-dependent scaling that enhances EUV source efficiency for lithography.

Contribution

It demonstrates an inverse proportional scaling of plasma electron density and laser intensity with wavelength, supported by experiments and simulations, advancing EUV source development.

Findings

Similar spectra at fixed intensity ratios for 1- and 2-micron lasers

Scaling of plasma properties approximately as wavelength^{-1}

Enhanced spectral performance with 2-micron laser driving

Abstract

Experimental spectroscopic studies are presented, in a 5.5--25.5nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2-$\mu$m-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-$\mu$m-wavelength Nd:YAG laser light over a range of laser intensities spanning approximately $0.3-5 \times 10^{11}$Wcm$^{-2}$, under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1-$\mu$m to 2-$\mu$m laser intensities fixed at a value of 2.1(6), which is in good agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling $\sim \lambda^{-1}$ of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength $\lambda$. This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16-51$\mu$m at a constant laser intensity that maximizes the emission in a 2\% bandwidth around 13.5nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-$\mu$m- vs 1-$\mu$m driven plasma provides strong motivation for the development of high-power, high-energy near-infrared lasers to enable the development of more efficient and powerful sources of EUV light.