Conversion efficiency of a laser-produced Sn plasma at 13.5 nm, simulated with a one-dimensional hydrodynamic model and treated as a multi-component blackbody

Efficiency optimization of a stable and debris free plasma source at 13.5 nm, is at the forefront of current extreme ultraviolet lithographic (EUVL) research efforts. To date, 1-2.5% soft x-ray conversion efficiencies (CEs) within a 2% bandwidth (BW) around 13.5 nm and into 2 pi steradians have been attained experimentally for laser-produced plasmas containing Sri at power densities of 0.5-5 x 10(11) W cm(-2). In order to complement these experimental endeavours, we have undertaken to study the CE, for the given wavelength regime, in the optically thick limit. We have achieved this by coupling time-dependent and steady-state collisional-radiative (CR) equations to the output of the one-dimensional hydrodynamic code MED103 (MEDUSA), where a solid sphere of radius 50 mu m was uniformly irradiated by a high intensity laser pulse with a Gaussian temporal profile. The ion populations obtained from these CR results were then used in an integrated spatio-temporal figure of merit (FOM) together with in-band weighted dipole oscillator strengths and transition energies. The maximum FOM, when divided by the laser energy, was found to occur in the range of peak power densities of 2-3 x 10(11) W cm(-2) for the steady-state and time-dependent models, respectively. The hydrodynamic variables of these peak power densities were then used in a radiative transfer calculation in which the many-celled spherical plasma was treated as a multi-component blackbody. It is found that CEs of 3.5-6% within the 2% BW per 2 pi steradians may be achieved. These results are of particular relevance to EUVL technologies where a minimum CE of 3% is required by industry.