A spatio-temporal study of variable composition laser-produced Sn plasmas

Abstract

Laser-produced Sn plasmas are at present a major contender in the challenge to find a suitable replacement for the currently used excimer-laser technology, which has wavelengths of 248 and 193 nm, and that is utilized in projection lithography. These wavelengths are to be superseded by soft x-ray sources in the 13.5 nm wavelength regime for utilization in extreme ultraviolet lithographic (EUVL) technologies. To date, considerable international efforts have been channelled into the experimental realization and optimization of various tin based EUV sources. Therefore, in order to compliment these experimental accomplishments we have undertaken a spatio-temporal study of the free electron number density, atomic number density, average charge state and expansion kinetic energy of Sn and SnO2 plasmas. This has been achieved by coupling the collisional radiative equations to the one-dimensional Lagrangian fluid dynamic model MED103 (MEDUSA), thus obtaining the spatial and temporal histories of the aforementioned variables within a laser-produced plasma of spherical geometry, generated using a Gaussian laser pulse at 1064 nm. The evolution of ion stages Sn4+ to Sn13+ within a fluid cell is also presented. In addition, the dependence of the Sn fractional ion populations upon the atomic number density within variable composition plasmas of binary mixtures formed from Sn and oxygen and Sn combined with samarium is investigated. The overwhelming influence of both the atomic and free electron number densities within these plasmas is highlighted.