Characterization of out-of-band radiation and plasma parameters in laser-produced Sn plasmas for extreme ultraviolet lithography light sources

Abstract

Out-of-band (OOB) radiation, in contrast to the in-band radiation at 13.5 nm in a 2% bandwidth, emitted from dense tin plasmas generated by a laser was investigated for application as an extreme ultraviolet lithography light source. It was found that the continuum spectrum, which overwhelms the atomic and ionic line emissions, is responsible for the intense OOB radiation. The spectral distribution of the continuum emission matches that of blackbody radiation with a temperature of about 10 eV. The OOB radiation can be considerably suppressed by employing a minimum-mass target and short-pulse laser irradiation. Spectroscopic observations were made to examine the spatiotemporal behavior of the plasma immediately after laser irradiation. Prominent line broadening due to the Stark effect in the high-density plasma was observed, from which the variation of the electron density was deduced. The electron density and temperature on the target surface were 1017–1018 cm−3 and a few eV’s, respectively, in the first 200 ns after laser illumination. In addition, the expansion velocities of neutral and singly ionized tin atoms were deduced from time-of-flight transients in the spectral emission.