Spectral Efficiency Extreme Ultraviolet Emission from CO₂ Laser-Produced Tin Plasma Using a Grazing Incidence Flat-Field Spectrograph

A grazing incidence flat-field spectrograph using a concave grating was constructed to measure extreme ultraviolet (EUV) emission from a CO₂ laser-produced tin plasma throughout the wavelength region of 5 nm to 20 nm for lithography. Spectral e±ciency of the EUV emission around 13.5 nm from plate, cavity, and thin foil tin targets was studied. By translating the focusing lens along the laser axis, the dependence of EUV spectra on the amount of defocus was investigated. The results showed that the spectral effiency was higher for the cavity target in comparison to the plate or foil target, while it decreased with an increase in the defocus distance. The source's spectra and the EUV emission intensity normalized to the incident pulse energy at 45^o from target normal were characterized for the in-band (2% of bandwidth) region as a function of laser energy spanning from 46 mJ to 600 mJ for the pure tin plate target. The energy normalized EUV emission was found to increase with the increasing incident pulse energy. It reached the optimum value for the laser energy of around 343 mJ, after which it dropped rapidly.