2 μm-wavelength laser pulses on tin microdroplets for nanolithography: properties of the plasma and emitted ions

The properties of an extreme ultraviolet (EUV) source driven by hundreds-of-nanoseconds-long laser pulses of λ_laser = 2 µm wavelength are investigated through radiation-hydrodynamic simulations. A time-integrated EUV source is imaged by taking an Abel transform of the local net in-band (13.5 nm +/- 1%) emissivity, revealing a bright focus point with a lower-intensity 'tail' due to the ablation-induced motion of the droplet. Ion emissions are studied with two types of simulations: first a 'short pulse' (27 ns-long) case with laser intensity I_laser ∼ 10^11 W cm^-2 that can be readily benchmarked with present experimental systems. Then, a 'long pulse' case is studied, in which the droplet is irradiated until it is fully vaporized, which takes ∼150 ns. The kinetic energy-resolved ion spectra in both cases feature a high-energy peak on the order of keVs. At lower kinetic energies the spectra are substantially different: the short pulse spectrum exhibits much higher ion numbers due to plasma cooling at the end of the pulse, which is not present in the long pulse case.