Gd plasma source modeling at 6.7 nm for future lithography

Plasmas containing gadolinium have been proposed as sources for next generation lithography at 6.x nm. To determine the optimum plasma conditions, atomic structure calculations have been performed for Gd11+ to Gd27+ ions which showed that n=4 - n=4 resonance transitions overlap in the 6.5 – 7.0 nm region. Plasma modeling calculations, assuming collisional-radiative equilibrium, predict that the optimum temperature for an optically thin plasma is close to 110 eV and that maximum intensity occurs at 6.76 nm under these conditions. The close agreement observed between simulated and experimental spectra from laser and discharge produced plasmas indicates the validity of our approach.