Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma

Schupp, Ruben; Torretti, Francesco; Meijer, Randy; Bayraktar, Muharrem; Sheil, John; Scheers, Joris; Kurilovich, Dmitry; Bayerle, Alex; Schafgans, A. A.; Purvis, Michael; Eikema, K.S.E.; Witte, Stefan; Ubachs, Wim; Hoekstra, Ronnie; Versolato, Oscar

doi:10.1063/1.5117504

Cited by 30 publications

(53 citation statements)

References 40 publications

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“…Instead, a single-temperature, single-density approach is here employed. Indeed, recent experiments by Schupp et al 22 have indicated that a dominant fraction of the EUV emission may be produced in such a quasi-stationary 24 single-density, singletemperature region. For such a medium, the spectral flux I λ can be determined using the simple solution I λ ¼ B λ 1 À expðÀτÞ ½ , with the optical depth τ defined as the product between α λ and the transport path-length L. The temperature of the opacities are chosen such that the calculated charge state contributions matched the observed one.…”

Section: Resultsmentioning

confidence: 99%

“…The tenfold decrease in laser wavelength λ increases the critical plasma electron density n c by two orders of magnitude, n c ∝ λ −2 . This higher critical density causes EUV radiation to be created in plasma regions of higher density, and with overall larger optical depth 22 . Significant self-absorption of the emitted radiation in such dense, partially opaque plasma could lead to a broadening of the spectral emission out of the 2% bandwidth of interest, reducing efficiency.…”

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confidence: 99%

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Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

et al. 2020

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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.

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Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

et al. 2020

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“…The Nd:YAG-laser-produced tin plasma relevant for the production of extreme ultraviolet light is hot (∼30 eV) and dense (∼10 21 e − cm −3 ) (see, e.g., Refs. [62,72,73]). At the laser intensity of 1.7 × 10 11 W/cm 2 , used in our experiments, given the density set by the laser wavelength [1], the temperature of the plasma produces a charge-state distribution containing mostly Sn XII-Sn XV ions [73] that is optimal for emitting EUV light at 13.5 nm [62,72].…”

Section: Diagnostics Of the Afterglow Of Laser-produced Plasmamentioning

confidence: 99%

“…[62,72,73]). At the laser intensity of 1.7 × 10 11 W/cm 2 , used in our experiments, given the density set by the laser wavelength [1], the temperature of the plasma produces a charge-state distribution containing mostly Sn XII-Sn XV ions [73] that is optimal for emitting EUV light at 13.5 nm [62,72]. After the laser pulse ends, the plasma quickly cools down while continuing its free, quasispherical expansion [74].…”

Section: Diagnostics Of the Afterglow Of Laser-produced Plasmamentioning

confidence: 99%

Time- and space-resolved optical Stark spectroscopy in the afterglow of laser-produced tin-droplet plasma

et al. 2020

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The afterglow emission from Nd:YAG-laser-produced microdroplet-tin plasma is investigated, with a focus on analyzing Stark effect phenomena and the dynamical evolution of the plasma. Time-and space-resolved optical imaging spectroscopy is performed on 11 lines from Sn I-IV ions, in the 315-425-nm wavelength range. Stark shift-to-width ratios serve as the basis for unambiguous experimental tests of atomic physics theory predictions. Experiment and theory, where available, are found to be in poor agreement, and are in disagreement regarding the sign of the ratio in several cases. Spectroscopic measurements of the Stark widths in tandem with Saha-Boltzmann fits to Sn I and Sn II lines, establish the evolution of the local temperature and density of the plasma afterglow, 20-40 ns after the end of the 15-ns-long temporally box-shaped laser pulse. A clear cool-down from ∼2 to 1 eV is observed of the plasma in this time window, having started at ∼30 eV when emitting extremeultraviolet (EUV) light. An exponential reduction of the density of the plasma from ∼10 18-10 17 e − cm −3 is observed in this same time window. Our work is relevant for understanding the dynamics of the decaying, expanding plasma in state-of-the-art EUV nanolithography machines.

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“…The scaling of the here relevant inverse bremsstrahlung absorption coefficient k L ∝ λ 2 n 2 e , with wavelength λ and electron density n e indicates that shorter wavelength light is absorbed less efficiently at equal plasma density but because of the higher critical electron density (n c ∝ λ −2 ), the shorter wavelength laser light can penetrate into denser plasma regions leading to an overall increased absorption of the laser light, with the absorption taking place in regions of higher emitter and absorber density. This may benefit the obtainable source brightness but an associated increase in optical depth [32,33] leads to increased broadening of spectral features outside the in-band region relevant for EUV lithography. This broadening may limit the obtainable CE.…”

Section: Introductionmentioning

confidence: 99%

Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

Schupp

Behnke

Bouza

et al. 2021

J. Phys. D: Appl. Phys.

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The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µm) pulse creates the EUV-emitting plasma. Irradiating 30-to 300 µm-diameter targets with 2 µm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 µm drive laser intensity (0.6-1.8 × 10 11 W cm −2 ) and pulse duration (3.7-7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 µm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 µm-wavelength drive laser. Emission spectra, recorded in a 5.5-25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 µm case, while in the 2 µm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 µm-driven plasmas produced from disk targets when compared to 1 µm-driven plasmas, providing strong motivation for the use of 2 µm lasers as drive lasers in future high-power sources of EUV light.

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Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma

Cited by 30 publications

References 40 publications

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

Time- and space-resolved optical Stark spectroscopy in the afterglow of laser-produced tin-droplet plasma

Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

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