We experimentally investigate the emission of EUV light from a mass-limited laser-produced plasma over a wide parameter range by varying the diameter of the targeted tin microdroplets and the pulse duration and energy of the 1-μm-wavelength Nd:YAG drive laser. Combining spectroscopic data with absolute measurements of the emission into the 2% bandwidth around 13.5 nm relevant for nanolithographic applications, the plasma's efficiency in radiating EUV light is quantified. All observed dependencies of this radiative efficiency on the experimental parameters are successfully captured in a geometrical model featuring the plasma absorption length as the primary parameter. It is found that laser intensity is the pertinent parameter setting the plasma temperature and the tin-ion charge-state distribution when varying laser pulse energy and duration over almost 2 orders of magnitude. These insights enabled us to obtain a record-high 3.2% conversion efficiency of laser light into 13.5-nm radiation and to identify paths towards obtaining even higher efficiencies with 1-μm solid-state lasers that may rival those of current state-of-the-art CO 2-laser-driven sources.
The measurement of the propulsion of metallic microdroplets exposed to nanosecond laser pulses provides an elegant method for probing the ablation pressure in dense laser-produced plasma. We present the measurements of the propulsion velocity over three decades in the driving Nd:YAG laser pulse energy, and observe a near-perfect power law dependence. Simulations performed with the RALEF-2D radiation-hydrodynamic code are shown to be in good agreement with the power law above a specific threshold energy. The simulations highlight the importance of radiative losses which significantly modify the power of the pressure scaling. Having found a good agreement between the experiment and the simulations, we investigate the analytic origins of the obtained power law and conclude that none of the available analytic theories is directly applicable for explaining our power exponent.
Experimental scaling relations of the optical depth are presented for the emission spectra of a tin-droplet-based, 1-lm-laser-produced plasma source of extreme-ultraviolet (EUV) light. The observed changes in the complex spectral emission of the plasma over a wide range of droplet diameters (16-65 lm) and laser pulse durations (5-25 ns) are accurately captured in a scaling relation featuring the optical depth of the plasma as a single, pertinent parameter. The scans were performed at a constant laser intensity of 1.4 Â 10 11 W/cm 2 , which maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for industrial EUV lithography. Using a one-dimensional radiation transport model, the relative optical depth of the plasma is found to linearly increase with the droplet size with a slope that increases with the laser pulse duration. For small droplets and short laser pulses, the fraction of light emitted in the 2% bandwidth around 13.5 nm relative to the total spectral energy is shown to reach high values of more than 14%, which may enable conversion efficiencies of Nd:YAG laser light into-industrially-useful EUV radiation rivaling those of current state-of-the-art CO 2 -laser-driven sources.
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