The effect of target material concentration on EUV near 6.7nm and out-of-band radiation of laser-produced Gd plasma

We have prepared various low-density Gd targets to investigate the effect of concentration on the extreme ultraviolet (EUV) radiation near 6.7nm and out-of-band emission from laser-produced plasma source. Results show that an increased EUV radiation intensity and narrow-band spectra from the low-density target was obtained. Comparing with the original solid Gd target, the intensity and spectral purity of the 6.7 nm EUV radiation from the 40% mass concentration Gd target are enhanced 45% and 74.5%, respectively. And the FWHM of the spectra are decreased to one-half of that from the solid target. The measurements of the plasma electron density for various concentration targets shown that the low-density target exhibited lower plasma electron density, which reduced the self-absorption effect and the yield of the low-charge ions in the plasma, resulting in the enhancement of EUV radiation intensity as well as narrower band spectra output. Finally, the out-of-band radiation in the range of 325-385 nm from the 40% concentration Gd target is decreased to approximately 60% of the solid target case, while the angular distribution profile was significantly uniform. Our findings are beneficial to the Gd target as a new promising source candidate for the EUV nanolithography in the future.