Characterization of 1- and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>2</mml:mn><mml:mtext>−</mml:mtext><mml:mi>μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>-wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light
Ruben Schupp, L. Behnke, John Sheil, Zoi Bouza, Muharrem Bayraktar, W. Ubachs, R. Hoekstra, O. O. Versolato
Abstract
Experimental spectroscopic studies are presented, in a 5.5-25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2-m-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-m-wavelength laser light over a range of laser intensities spanning approximately (0.3-5) 10 11 W/cm 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-to 2-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 -1 of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength . 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 m at a constant laser intensity that maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-m-versus 1-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.