Characterization of 1- and 2−μm -wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

Abstract

Experimental spectroscopic studies are presented, in a 5.5-25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2µm-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-µmwavelength Nd:YAG laser light over a range of laser intensities spanning approximately 0.3 − 5 × 10 11 W cm −2 , under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1-µm to 2-µm laser intensities fixed at a value of 2.1(6), which is in good agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling ∼ λ −1 of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength λ . This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16-51 µm at a constant laser intensity that maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-µm-vs 1-µm driven plasma provides strong motivation for the development of high-power, high-energy nearinfrared lasers to enable the development of more efficient and powerful sources of EUV light.

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

“…After a set time delay the targets are irradiated with highintensity 2 µm-wavelength laser pulses. The pulses are produced in a master oscillator power amplifier (MOPA) [33,37] that was built following the work of Arisholm et al [36]. Signal and idler pulses having energies up to 180 mJ each are produced at wavelengths of 1.9 and 2.1 µm, respectively.…”

“…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.…”

“…A 2 µm wavelength, between the widely investigated 1 and 10 µm, is an interesting candidate providing intermediate plasma densities and hence optical depth. In addition to 2 µm systems based on difference frequency generation [33,36,37], high-power Big Aperture Thulium (BAT) laser systems, operating at 1.9 µm wavelength, are currently under development [38,39].…”

“…A broad body of literature has investigated experimentally the emission from undeformed liquid tin droplet targets [9,34,43,44], coated spheres [45] or solidplanar targets [46][47][48][49] using 1 µm solid-state lasers. First results from experiments using 2 µm laser light driving plasma from solid-planar tin targets [37] as well as from undeformed liquid tin droplet targets [33] have recently been presented. Both works demonstrate the potential of the 2 µm drive wavelength, showing a doubling of the obtainable CE over the 1 µm driver, to a 3% level for the solid-planar tin target.…”

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

“…Given a higher critical electron density (n c ∝ λ −2 ), * o.versolato@arcnl.nl the shorter wavelength laser light is absorbed in regions with higher emitter and absorber density. This may benefit the obtainable source brightness but an associated increase in optical depth [30,31] leads to increased broadening of spectral features outside of the for EUV lithography relevant inband region, defined as a 2% bandwidth centered at 13.5 nm. This broadening may limit the obtainable conversion efficiency (CE) of laser energy into in-band radiation into the backward hemisphere towards the laser origin.…”