The effect of focal spot size on in-band 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 EUV light was noted with focal spot sizes from . 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 in the spectrum.
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