An examination of the influence of target composition and viewing angle on the extreme ultraviolet spectra of laser produced plasmas formed from tin and tin doped planar targets is reported. Spectra have been recorded in the 9–17nm region from plasmas created by a 700mJ, 15ns full width at half maximum intensity, 1064nm Nd:YAG laser pulse using an absolutely calibrated 0.25m grazing incidence vacuum spectrograph. The influence of absorption by tin ions (SnI–SnX) in the plasma is clearly seen in the shape of the peak feature at 13.5nm, while the density of tin ions in the target is also seen to influence the level of radiation in the 9–17nm region.

One key aspect in the drive to optimize the radiative output of a laser-produced plasma for extreme ultraviolet lithography is the radiation transport through the plasma. In tin-based plasmas, the radiation in the 2% bandwidth at 13.5 nm is predominantly due to 4d-4f and 4p-4d transitions from a range of tin ions (Sn7+ to Sn12+). The complexity of the configurations involved in these transitions is such that a line-by-line analysis is, computationally, extremely intensive. This work seeks to model the emission profiles of each ion by treating the transition arrays statistically, thus greatly simplifying radiation transport modeling. The results of the model are compared with experimental spectra from tin-based laser-produced plasmas.

Spectra from laser-produced plasmas of elements within the range 62 Z 74 (Z is the atomic number) are known to contain extensive regions of line-free continua throughout the VUV and XUV regions. These continua arise primarily from recombination while strong line emission is inhibited by the complexity of the electronic configurations involved which, as a result of 4f collapse, for the most part contain an open 4f subshell. The energies and compositions of the lowest configurations of rare-earth ions along the I-, Xe- and Cs-isoelectronic sequences are investigated and it is concluded that they consist of fixed parity sets of highly mixed or `compound' states built from configurations containing variable numbers of 5s, 5p and 4f electrons. These are described within the statistical unresolved transition array model. The strongest transitions which occur in these sequences for ionized samarium are explored using configuration-averaged Hartree-Fock calculations, while detailed line and level statistics are extracted for the lowest 4f 5d and 5p 5d transitions for Sm IX. The calculations predict a redistribution of single-configuration oscillator strengths from the stronger lines to the weaker ones in a full configuration-interaction (CI) basis which essentially smooths the spectra. It is concluded that both the level density and CI effects are such as to produce level distributions equivalent to nuclear compound states and combine to produce emission so complex that it is essentially `band-band' in nature (supercomplex spectra) leading to a spectrum which is a line-free continuum.

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