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We experimentally study the mass partitioning of a fragmenting liquid sheet formed after the impact of a ns-laser pulse on a tin microdroplet, and its dependence on laser pulse energy and droplet size. We present the temporal evolution of individual liquid fractions: the sheet and its bounding rim, ligaments protruding from the rim, and droplets shed by the ligaments, applying machine learning to analyze subresolution fragments. Our results show that the temporal evolution of the mass partitioning between the sheet, rim, ligaments, and fragments is independent of the deformation Weber number - following Wang and Bourouiba [J. Fluid Mech. 935, A29 (2022)] for the analogous droplet-pillar impact case, extending the work to larger Weber numbers and to a system where the timescale of deformation is fully decoupled from impact. The full mass partitioning is accounted for by quantifying the further contributions unique to the laser-droplet impact case: that of a centrally located mass remnant, and the mass ablated by the laser pulse. These findings can be employed to optimize the mass utilization of the liquid tin that is used as target material in the production of extreme ultraviolet light for nanolithography.

© 2022 American Physical Society. We thank Alexander Klein for providing the double-frame camera utilized in this study. This work was carried out at the Advanced Research Center for Nanolithography (ARCNL), a publicprivate partnership of the University of Amsterdam (UvA), the Vrije Universiteit Amsterdam (VU), the Dutch Research Council (NWO), and the semiconductor equipment manufacturer ASML. This project received funding from the European Research Council (ERC) through Starting Grant No. 802648 and is part of the Vidi research program with Project No. 15697, which is financed by NWO., We experimentally investigate the speed of fragments produced by ligament breakup in the laser-induced deformation of tin microdroplets into axisymmetric sheets. The experiments were carried out covering a wide range of droplet diameters and laser-pulse energies. In addition to fragments produced by end-pinching, we also observe fragments shed via Rayleigh-Plateau breakup of long ligaments at late times. A double-frame backlit camera was used to obtain the speeds of the fragments u(f) and the time of their detachment t(d). We show that by normalizing u(f) to the initial expansion speed of the sheet R-0, all data collapse onto a single, universal curve that is a function of the dimensionless time t(d)/tau(c) only, where tau(c) is the capillary time. This universal curve is explicitly independent of the droplet's Weber number. The collapse of u(f) is supported by energy conservation arguments. Our findings enable the prediction of the instantaneous speed and position of the fragments shed from liquid tin targets used in state-of-the-art extreme ultraviolet nanolithography, facilitating the design of effective mitigation strategies against microparticulate debris., Unión Europea. Horizonte 2020, Dutch Research Council (NWO), Depto. de Óptica, Fac. de Ciencias Físicas, TRUE, pub

We experimentally and numerically investigate the early-time hydrodynamic response of tin microdroplets driven by a ns-laser-induced plasma. Experimentally, we use stroboscopic microscopy to record the laser-induced dynamics of liquid tin droplets and determine the propulsion speed (U) and initial radial expansion rate (Ṙ0). The ratio of these two quantities is a key parameter to be optimized for applications in nanolithography, where laser-impacted tin droplets serve as targets for generating extreme ultraviolet light. We explore a large parameter space to investigate the influence of the tin droplet diameter, laser beam diameter, and laser energy on the Ṙ0/U ratio. We find good agreement when comparing the experimentally obtained U and Ṙ0 values to those obtained by detailed radiation-hydrodynamic simulations using RALEF-2D. From the validated simulations, we extract the spatial distribution of the plasma-driven pressure impulse at the droplet-plasma interface to quantify its influence on the partitioning of kinetic energy channeled into propulsion or deformation. Our findings demonstrate that the width of the pressure impulse is the sole pertinent parameter for extracting the kinetic energy partitioning, which ultimately determines the late-time target morphology. We find good agreement between our full radiation-hydrodynamic modeling and a generalized analytical fluid-dynamics model [H. Gelderblom, J. Fluid Mech. 794, 676 (2016)0022-112010.1017/jfm.2016.182]. These findings can be used to optimize the kinetic energy partition and tailor the features of tin targets for nanolithography.

We study the fragmentation of a liquid drop that is hit by a laser pulse. The drop expands into a thin sheet that breaks by the radial expulsion of ligaments from its rim and the nucleation and growth of holes on the sheet. By combining experimental data from two liquid systems with vastly different time and length scales, we show how the early-time laser-matter interaction affects the late-time fragmentation. We identify two Rayleigh-Taylor instabilities of different origins as the prime cause of the fragmentation and derive scaling laws for the characteristic breakup time and wavenumber. The final web of ligaments results from a subtle interplay between these instabilities and deterministic modulations of the local sheet thickness, which originate from the drop deformation dynamics and spatial variations in the laser-beam profile.

We experimentally study the morphology of a radially expanding sheet of liquid tin, formed by nanosecond-pulse Nd:YAG laser impact on a spherical microdroplet. Specifically, the sheet thickness profile and its time evolution are captured in detail over a range of laser-pulse energies and for two droplet sizes. Two complementary methods to determine this thickness are employed and shown to be in excellent agreement. All obtained thickness profiles collapse onto a single self-similar curve. Spatial integration of the thickness profiles allows us to determine the volume of the sheet. Remarkably, less than half of the initial amount of tin remains in the sheet under conditions relevant for industrial sources of extreme ultraviolet light, where these thin tin sheets serve as target material. Further analysis shows that the dominant fraction of the mass lost from the sheet during its expansion ends up as fine fragments. We propose that such mass loss can be minimized by producing the sheet targets on the shortest possible time scale. These findings may be particularly valuable for ongoing developments in state-of-the-art nanolithography.