Abstract

The optimization of soft x-ray production from a laser-produced plasma source is an important issue for several current lithography schemes. Recent experiments at Lawrence Livermore National Laboratory by R. Kauffman, D. Phillion, and R. Spitzer, indicate that the required conversion efficiency can be achieved using moderate laser intensities for a few select materials. Computer simulations of these experiments delineate the critical phenomena underlying these high conversion efficiencies, especially the role of the coupled hydrodynamic expansion and radiation flow in the plasma. By driving the plasma emission out of local thermodynamic equilibrium, photon output in the desired spectral region can be much greater than the black-body emissivity. These conditions can only be achieved under certain conditions which are determined by the incident laser intensity, essentially the electron temperature, and the excited electronic state populations within each ionization stage. Reliable simulations of the experiments require fairly detailed atomic physics databases since the prediction of photon output occurs in such a narrow spectral range. The computer program LASNEX is capable of coupling the hydrodynamic motion and a radiation field described by detailed atomic physics rates for laser produced plasmas. Two-dimensional simulations using this code are compared to the experimental results and are shown to be reliable in their interpretation of these results.

© 1992 Optical Society of America