Xe laser-produced plasma with a gas-jet target is considered a promising λ = 11.2-nm radiation source for a possible extension of the EUV (Extreme UltraViolet) lithography. EUV spectra of the plasma radiation obtained with the aid of both a grating spectrograph and Bragg mirrors are presented. The absorption of the EUV radiation in the cold peripheral gas has been eliminated in another experiment by means of irradiating the target with a wide defocused laser beam that resulted in an increase of the EUV output by an order of magnitude. In that experiment, the conversion efficiency at λ = 11.2 nm amounted to 3.9%.
A more than tenfold increase in the intensity of EUV radiation from a laser-produced plasma source is observed experimentally under the illumination of a Xe gas-jet target with a wide laser beam covering its densest part compared with the radiation intensity at the traditional geometry, when the beam is sharply focused on the jet axis. As an explanation, it is assumed that in this case almost the whole dense part of the jet is heated and ionized under the action of laser irradiation and becomes transparent to the quanta of the studied radiation. The thickness of the absorbing peripheral shell of a laser spark decreases accordingly, that leads to the increase in the radiation intensity reaching the observer.
Absorption of the laser pulse energy in the plasma has been shown to change from 8.5% at irradiation of a gas-puff target with a beam narrow-focused onto a near-central area of the jet up to 65% at the wide defocused irradiation. An analysis of this phenomenon is based on a hypothesis that plasma density decays significantly during the 10ns laser pulse due to hydrodynamic expansion of the hot plasma. A similarity of the EUV (Extreme UltraViolet) intensity and the laser light absorption as functions of the laser beam diameter suggests a revision of the conventional idea of a strong EUV radiation self-absorption in a cold peripheral shell of the laser plasma whereas the high absorptivity of the laser radiation by the plasma looks like a major feature to gain high efficiency of an EUV source.
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