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Extreme-ultraviolet radiation transport in small scale length laser-produced tin plasmas

  • Author(s): Sequoia, Kevin Lamar Williams
  • et al.
Abstract

The majority of the studies on laser-produced plasmas as an efficient extreme ultraviolet (EUV) light source have focused on relatively large plasmas produced at large laser facilities. However, to develop a commercially viable light source for EUV lithography, much smaller lasers and hence much smaller plasmas must be employed. Smaller plasmas behave quite differently than large plasmas in that the temperature and density are less uniform, and lateral expansion is more important. These differences affect the energy transport and, in particular, the radiation transport. This work studies the EUV radiation transport in small scale length tin plasmas, focusing on the effects of target geometry and laser pulse duration. Both planar and spherical tin targets were irradiated with an Nd:YAG laser operating at 1.064 [Mu]m. Conversion efficiency of laser light to 13.5 nm radiation (in-band), EUV emission spectrum, two-dimensional in-band emission profile, and the plasma electron density were measured experimentally. These measurements provide insight into where the laser is absorbed, where the in- band emission is produced, and how the radiation is transmitted. The plasma evolution in these experiments were simulated with a two-dimensional radiation hydrodynamic code, while the radiation transport and atomic kinetics where modeled with a collisional radiative code. Additional experiments were conducted using planar targets where the pulse duration was varied from 0.5 ns to 16 ns to understand the effects of laser pulse duration. It was found that the optimum plasma temperature for efficient generation and transmission of in-band emission is 20 eV. This is lower than the previously reported optimum temperature of 30 eV. The use of a 1.064 [Mu]m heating laser results in overheating of the plasma in a region that is much too dense to transmit the in-band emission. This overheating is necessary for the plasma to reach the optimum temperature in the region where the density is low enough to transmit the in-band emission

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