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Laser-produced Sn plasmas used to generate extreme ultraviolet (EUV) light for lithography cause the release of Sn ions and neutrals in the EUV source chamber. These Sn atoms condense and deposit on the multilayer collector optic, which reduces its ability to reflect EUV light. This lowers the source throughput and eventually necessitates downtime for collector cleaning. In this paper, an in situ plasma-based collector cleaning technique is presented and experimentally demonstrated. First, the technique is shown to completely clean a 300 mm diameter stainless steel dummy collector. Second, simulations and secondary ion mass spectroscopy depth profiles show that the technique does not erode the real multilayer mirrors. Finally, EUV reflectivity measurements demonstrate the ability of the technique to restore EUV reflectivity to Sn-coated multilayer mirrors. This technique has the potential to be used in conjunction with source operation, eliminating cleaning-related source downtime.

Experimental measurements on angular distribution of ions and ion-energy spectra from planar slab targets of Al, Ti and binary targets of Al, Ti with different stoichiometries are reported. The plasma was produced by a 125 mJ, 5 ns, 1.06 µm Nd:YAG laser, incident on a planar target at a fixed angle of −45°. The laser was focused on an ablation area of about 0.3 mm2 at a laser intensity of about 1010 W cm−2. The characteristics of the ions were obtained using a retarding-potential analyser and a quartz crystal. Results on time-of-flight spectra, ion fractions, average ionization, angular distributions of particles and their kinetic energy are presented. Full width at half maximum of the angular distributions was obtained and the results are discussed. Measurements on total integrated energy per particle are also reported. They show evidence of energy transfer from light to heavy particles in agreement with the theoretical simulation results of earlier workers. Ion acceleration due to an in-built electrostatic potential is discussed in detail and its effect was found to be marginal.

Charge resolved average integrated kinetic energy of ions from plasmas produced from, monoatomic targets of copper and tungsten as well as binary targets of copper and tungsten, with two different stoichiometric compositions, have been obtained using 130 mJ-5 ns Nd:YAG laser pulse, at a focal intensity of about 8 x 10 9 W/cm 2. It is concluded that ionacceleration due to built-in electrostatic potential is not significant and the kinetic energy spectra are determined by the recombination during the plasma expansion. On the basis of the numerical results obtained from Monte-Carlo simulation, a significant energy-transfer from the lighter to the heavier ions seems reasonable in the case of binary targets.

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