23 research outputs found
Laser-to-droplet alignment sensitivity relevant for laser-produced plasma sources of extreme ultraviolet light
We present and experimentally validate a model describing the sensitivity of
the tilt angle, expansion and propulsion velocity of a tin micro-droplet
irradiated by a 1 {\mu}m Nd:YAG laser pulse to its relative alignment. This
sensitivity is particularly relevant in industrial plasma sources of extreme
ultraviolet light for nanolithographic applications. Our model has but a single
parameter: the dimensionless ratio of the laser spot size to the effective size
of the droplet, which is related to the position of the plasma critical density
surface. Our model enables the development of straightforward scaling arguments
in turn enabling precise control the alignment sensitivity.Comment: 7 pages, 5 figure
Drop fragmentation by laser-pulse impact
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.</p
Drop fragmentation by laser-pulse impact
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.Comment: about to be submitted to JF
Controlling ion kinetic energy distributions in laser produced plasma sources by means of a picosecond pulse pair
The next generation of lithography machines uses extreme ultraviolet (EUV)
light originating from laser-produced plasma (LPP) sources, where a small tin
droplet is ionized by an intense laser pulse to emit the requested light at
13.5 nm. Numerous irradiation schemes have been explored to increase conversion
efficiency (CE), out of which a double-pulse approach comprising a weak
picosecond Nd:YAG pre-pulse followed by a powerful pulse is considered to be
very promising [1]. Nevertheless, even for such CE-optimized schemes, ion
debris ejected from the plasma with kinetic energies up to several keV remain a
factor that hampers the maximum performance of LPP sources. In this letter we
propose a novel pre-pulse scheme consisting of a picosecond pulse pair at 1064
nm, which decreases the amount of undesirable fast ions, avoids
back-reflections to the lasers and enables one to tailor the target shape.Comment: 12 pages, 3 figures, 45 reference
Expansion Dynamics After Laser-Induced Cavitation in Liquid Tin Microdroplets
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.Comment: 7 pages, 6 figure
Mass Loss from a Stretching Semitransparent Sheet of Liquid Tin
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
Mass loss from a stretching semitransparent sheet of liquid tin
\u3cp\u3eWe 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.\u3c/p\u3
The transition from short-to long-timescale pre-pulses: Laser-pulse impact on tin microdroplets
We experimentally study the interaction of intense laser pulses with metallic microdroplets and the resulting deformation. Two main droplet deformation regimes have previously been established: that of sheet-type expansion after impact of "long"(typically >10 ns) pulses governed by incompressible flow and that of spherical expansion by internal cavitation after impact of "short"(typically <100 ps) pulses governed by shock waves, i.e., strongly compressible flow. In this work, we study the transition between these regimes by scanning pulse durations from 0.5 to 7.5 ns, where the boundaries of this range correspond to the limiting cases for the employed droplet diameter of 45 μm. We qualitatively describe the observed deformation types and find scaling laws for the propulsion, expansion, and spall-debris velocities as a function of pulse duration and energy. We identify the ratio of the pulse duration to the acoustic timescale of the droplet as the critical parameter determining the type of deformation. Additionally, we study the influence of fast rise times by comparing square-and Gaussian-shaped laser pulses. These findings extend our understanding of laser-droplet interaction and enlarge the spectrum of controllable target shapes that can be made available for future tin-droplet-based extreme ultraviolet sources
Laser-to-droplet alignment sensitivity relevant for laser-produced plasma sources of extreme ultraviolet light
\u3cp\u3eWe present and experimentally validate a model describing the sensitivity of the tilt angle, expansion, and the propulsion velocity of a tin micro-droplet irradiated by a 1 μm Nd:YAG laser pulse to its relative alignment. This sensitivity is particularly relevant in industrial plasma sources of extreme ultraviolet light for nanolithographic applications. Our model has but a single parameter: the dimensionless ratio of the laser spot size to the effective size of the droplet, which is related to the position of the plasma critical density surface. Our model enables the development of straightforward scaling arguments, in turn enabling precise control of the alignment sensitivity.\u3c/p\u3
The transition from short-to long-timescale pre-pulses: Laser-pulse impact on tin microdroplets
We experimentally study the interaction of intense laser pulses with metallic microdroplets and the resulting deformation. Two main droplet deformation regimes have previously been established: that of sheet-type expansion after impact of "long"(typically >10 ns) pulses governed by incompressible flow and that of spherical expansion by internal cavitation after impact of "short"(typically <100 ps) pulses governed by shock waves, i.e., strongly compressible flow. In this work, we study the transition between these regimes by scanning pulse durations from 0.5 to 7.5 ns, where the boundaries of this range correspond to the limiting cases for the employed droplet diameter of 45 μm. We qualitatively describe the observed deformation types and find scaling laws for the propulsion, expansion, and spall-debris velocities as a function of pulse duration and energy. We identify the ratio of the pulse duration to the acoustic timescale of the droplet as the critical parameter determining the type of deformation. Additionally, we study the influence of fast rise times by comparing square-and Gaussian-shaped laser pulses. These findings extend our understanding of laser-droplet interaction and enlarge the spectrum of controllable target shapes that can be made available for future tin-droplet-based extreme ultraviolet sources