Efficient Generation of Extreme Ultraviolet Light From Nd :YAG-Driven Microdroplet-Tin Plasma

Abstract

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.

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

“…Comparison to experiment. In order to benchmark our present calculations, we have made comparisons with experimental laserproduced tin-plasma spectra recorded for a variety of laser intensities, which determine the plasma properties such as temperature and degree of ionization 38 . Guided by our radiationhydrodynamics simulations (see below), we performed ATOMIC calculations at predicted ranges of plasma temperatures and densities, and compared these to the measured spectra to ascertain the specific sets of conditions that lead to best agreement between modeling and experiment.…”

“…Guided by our radiationhydrodynamics simulations (see below), we performed ATOMIC calculations at predicted ranges of plasma temperatures and densities, and compared these to the measured spectra to ascertain the specific sets of conditions that lead to best agreement between modeling and experiment. The experimental spectra were obtained by irradiating a molten Sn microdroplet, 30 μm in diameter, with a 15-ns-long Nd:YAG laser pulse having a flat-top spatial profile of 96 μm diameter 38,39 . The emission in the EUV regime is recorded using a wavelength-calibrated spectrometer 40 .…”

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

“…In the experiments, temporally box-shaped pulses of 15 ns duration with a flat-top spatial beam profile (115 μm in diameter) and energies of 270 mJ/pulse are used at a 10-Hz repetition rate. The resulting laser intensity of 1.7 × 10 11 W/cm 2 is relevant for the efficient production of EUV light [62]. Hydrogen is introduced in the vacuum vessel as a buffer gas, at a pressure of about 1 mbar, to prevent contamination of viewports.…”

“…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].…”

“…[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].…”