Laser produce plasma extreme ultraviolet (EUV) source, which has the advantages of small size, high stability and adjustable output wavelength, plays a significant role in the application of EUV lithography. The EUV source with high conversion efficiency is an important part of EUV lithography system. We carried out the experiment of dual-pulse irradiated Gd target to realize the stronger 6.7nm EUV emission output. A planar Gd target is irradiated by a 8 ns full width half maximum pre-pulse Nd:YAG laser operating at its second (532 nm) harmonic to generate a pre-plasma. The pre-plasma is subsequently reheated by a 8 ns full width half maximum main Nd:YAG laser operating with wavelength of 1.06 μm. Firstly, we computed the contribution of transition arrays of the form 4p-4d and 4d-4f from their open 4d subshell in charge states Gd18%2B-Gd27%2B, and transition arrays of the form 4d-4f from their open 4d subshell in charge states Gd14%2B-Gd17%2B on the near 6.7nm EUV source. Subsequently, the experiment results of the dual pulse laser irradiated Gd target show that the intensity of 6.7nm peak EUV emission decreases first, then increases and drops again due to the plasma density decreases gradually when the delay time between the pre-pulse and main-pulse increases. The strongest intensity of 6.7nm peak EUV emission generated when the delay time of 100ns. At the same time, the spectrum efficiency is higher when the delay time of 100ns, which is 33% higher than that of single pulse laser. In addition, the experiment results show that the half width of EUV spectrum produced by dual pulse in the delay between 10-500ns is narrower than the case of signal laser pulse due to the method of dual pulse can suppress the self-absorption effect. The half width is narrowest when the delay of 30ns, which is about 1/3 time of EUV spectrum width generated by a single pulse. At the same time, the narrowing of Gd EUV spectrum improves the spectral utilization efficiency near the 6.7nm wavelength radiation within 0.6% bandwidth.
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