The propulsion of a liquid indium-tin micro-droplet by nanosecond-pulse laser impact is experimentally investigated. We capture the physics of the droplet propulsion in a scaling law that accurately describes the plasma-imparted momentum transfer, enabling the optimization of the laser-droplet coupling. The subsequent deformation of the droplet is described by an analytical model that accounts for the droplet's propulsion velocity and the liquid properties. Comparing our findings to those from vaporization-accelerated mm-sized water droplets, we demonstrate that the hydrodynamic response of laser-impacted droplets is scalable and independent of the propulsion mechanism.

Extreme ultraviolet (EUV) lithography is currently entering high-volume manufacturing to enable the continued miniaturization of semiconductor devices. The required EUV light, at 13.5 nm wavelength, is produced in a hot and dense laser-driven tin plasma. The atomic origins of this light are demonstrably poorly understood. Here we calculate detailed tin opacity spectra using the Los Alamos atomic physics suite ATOMIC and validate these calculations with experimental comparisons. Our key finding is that EUV light largely originates from transitions between multiply-excited states, and not from the singly-excited states decaying to the ground state as is the current paradigm. Moreover, we find that transitions between these multiply-excited states also contribute in the same narrow window around 13.5 nm as those originating from singly-excited states, and this striking property holds over a wide range of charge states. We thus reveal the doubly magic behavior of tin and the origins of the EUV light.

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.

We experimentally re-evaluate the fine structure of Sn 11+...14+ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole M 1 transitions, in a wavelength range covering 260 nm to 780 nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of ab initio calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited. arXiv:1605.04236v1 [physics.atom-ph]

We analyze the complex level structure of ions with many-valence-electron open [Kr] 4d m subshells (m=7-4) with ab initio calculations based on configuration-interaction many-body perturbation theory (CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra of Sn 7+ -Sn 10+ ions were obtained using an electron beam ion trap. Semi-empirical spectral fits carried out with the orthogonal parameters technique and cowan code calculations lead to 90 identifications of magnetic-dipole transitions and the determination of 79 energy ground-configuration levels, questioning some earlier EUV-line assignments. Our results, the most complete data set available to date for these ground configurations, confirm the ab initio predictive power of CI+MBPT calculations for the these complex electronic systems.

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.

The cavitation-driven expansion dynamics of liquid tin microdroplets is investigated, set in motion by the ablative impact of a 15-ps laser pulse. We combine high-resolution stroboscopic shadowgraphy with an intuitive fluid dynamic model that includes the onset of fragmentation, and find good agreement between model and experimental data for two different droplet sizes over a wide range of laser pulse energies. The dependence of the initial expansion velocity on these experimental parameters is heuristically captured in a single power law. Further, the obtained late-time mass distributions are shown to be governed by a single parameter. These studies are performed under conditions relevant for plasma light sources for extreme-ultraviolet nanolithography.