A laser-produced plasma source based on thin-film Gd targets for next-generation extreme ultraviolet lithography

We have studied laser-produced plasma based on mass-limited thin-film Gd targets for beyond the current extreme ultraviolet (EUV) light source of 13.5 nm wavelength based on tin. The influences of the laser intensity on the emission spectra centered around 6.7 nm from thin-film Gd targets were first investigated. It is found that the conversion efficiency of the produced plasma is saturated when the laser intensity goes beyond 2 x 10(11) W cm(-2). We have systematically compared the emission spectra of the laser-produced plasma with the changes in the thicknesses of the thin-film Gd targets. It is proved that a minimum-mass target with a thickness of 400 nm is sufficient to provide the maximum conversion efficiency, which also implies that this thickness is the ablation depth for the targets. These findings should be helpful in the exploration of next-generation EUV sources, as the thin-film Gd targets will reduce the debris during the plasma generation process compared with the bulk targets.