Abstract

The effect of focal spot size on in-band $13.5\mathrm{nm}$ extreme ultraviolet (EUV) emission from laser-produced Sn plasmas was investigated for an EUV lithography light source. Almost constant in-band conversion efficiency from laser to $13.5\mathrm{nm}$ EUV light was noted with focal spot sizes from $60\text{to}500\mu \mathrm{m}$. This effect may be explained by the opacity of Sn plasmas. Optical interferometry showed that the EUV emission must pass through a longer plasma with higher density when the focal spot is large, and strong reabsorption of EUV light was confirmed by a dip located at $13.5\mathrm{nm}$ in the spectrum.

© 2006 Optical Society of America