Recent progress in source development for lithography at 6.x nm

Abstract

In 2009 industry announced that sources would be needed at 6.x nm for future lithography. The brightest sources in this wavelength region are plasmas containing gadolinium and terbium. The strongest lines result from 4d–4f lines in the spectra of Ag-like Gd XVIII and Tb XIX through Rh-like Gd XX and Tb XXI. Assuming collisional radiative equilibrium, the optimum plasma temperature for producing these species is expected to be in the range 100–130 eV. Time integrated experimental spectra have been recorded with a variety of lasers and a maximum measured conversion efficiency (CE) for 150 ps pulses was 0.4%. The extreme ultraviolet (EUV) emission was observed to be anisotropic while the ion energy decreases with decreasing pulse length. The optimum laser intensity for efficient 6.7 nm EUV emission was determined to be close to 7 × 1013 W cm−2, which gives an electron temperature of ∼ 130 eV. The use of prepulses increases the CE which is limited by plasma opacity. To improve radiation transport low initial density targets and/or low electron density plasmas such as CO2 LPPs are required.