Spectral Characterization of Plasma Soft X-ray Sources for Lithographic Applications

Abstract

The optimization of soft x-ray production from a laser-produced plasma source is an important issue for several current lithography schemes including proximity printing and point projection. Recent experiments at Lawrence Livermore National Laboratory by R. Spitzer and T. Orzechowski indicate that the conversion efficiency required by realistic system designs can be achieved using moderate laser intensities for a few select materials. Additionally, spectral data were collected for the two most promising materials, Sn and Au, as a function of laser wavelength and intensity in the region spanning 62 Å to 248Å (50 eV to 200 eV). Computer simulations of these experiments delineate the overall plasma characteristics of temperature and density. These quantities in turn predict the detailed spectral output in the region of interest, placing stringent demands on the accuracy of the simulation. The synthetic spectra produced are very sensitive to the quality of the atomic rate processes and to the number of ionization stages included. Results will be presented for Sn and compared to the experimental results. Previously unreported line indentifications for Sn will be assigned on the basis of accurate multi-configurational Dirac-Fock calculations yielding a more complete characterization of plasmas in this intermediate electron temperature regime than has been attempted before. This characterization provides more insight into the sensitivity of the desired spectral output to the choice of material. More general conclusions can be drawn about the extent to which computational simulations can limit parameter space studies in the search for optimal materials in laser-produced plasma x-ray sources.