Abstract
We present the results of a joint experimental and theoretical study of plasma expansion arising from Nd:YAG laser ablation (laser wavelength λ = 1.064 μm) of tin microdroplets in the context of extreme ultraviolet lithography. Measurements of the ion energy distribution reveal a near-plateau in the distribution for kinetic energies in the range 0.03–1 keV and a peak near 2 keV followed by a sharp fall-off in the distribution for energies above 2 keV. Charge-state resolved measurements attribute this peak to the existence of peaks centered near 2 keV in the Sn3+–Sn8+ ion energy distributions. To better understand the physical processes governing the shape of the ion energy distribution, we have modelled the laser-droplet interaction and subsequent plasma expansion using two-dimensional radiation hydrodynamic simulations. We find excellent agreement between the simulated ion energy distribution and the measurements both in terms of the shape of the distribution and the absolute number of detected ions. We attribute a peak in the distribution near 2 keV to a quasi-spherical expanding shell formed at early times in the expansion.
Highlights
Laser-produced plasmas (LPPs) formed on tin microdroplets are established as the light source of choice in newgeneration lithography machines for high-volume manufacturing of integrated circuits below the 10 nm node [1,2,3]
We present the results of a joint experimental and theoretical study of plasma expansion arising from Nd:YAG laser ablation of tin microdroplets in the context of extreme ultraviolet lithography
To better understand the physical processes governing the shape of the ion energy distribution, we have modelled the laser-droplet interaction and subsequent plasma expansion using two-dimensional radiation hydrodynamic simulations
Summary
Laser-produced plasmas (LPPs) formed on tin microdroplets are established as the light source of choice in newgeneration lithography machines for high-volume manufacturing of integrated circuits below the 10 nm node [1,2,3]. A low-intensity prepulse is used to deform the droplet into an elongated disk-like target This target is irradiated by a second, high-energy CO2 laser pulse (λ = 10.6 μm) which generates a hot, EUV-emitting plasma. DE dΩ Z ηdet ηCR ΔE (E) ΔΩ where ΔE (E) is the energy-dependent absolute energy resolution, NZm(E) is the number of tin ions of charge state Z having kinetic energy in the range E ± (ΔE(E)/2) detected by the channeltron, ηdet is the detection efficiency of the channeltron, ηCR ( 1) is a correction for undetected counts due to high count-rate effects and ΔΩ is the solid angle of the input aperture of the ESA device. The absence of Sn11+–Sn15+ charge states may in part be attributed to the process of recombination, whereby free electrons in the expanding plasma recombine with these ions through processes such as three-body or radiative recombination [34]
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