Debris And Radiation Of Pre-Pulse Laser-produced Gd Plasma Source At 6.7nm

For effective use of laser-produced-plasma(LPP) light source, LPP is desired to emit a narrow spectral peak because reflection spectrum of multilayer mirrors for guiding emission from the source is very narrow. While a Gd plasma has been studied extensively as an extreme ultraviolet(EUV) light source at around 6.8 nm, where La/B4 C multilayer is reported to have a high reflectivity with bandwidth of about 0.6%, all previous work using a Nd:YAG laser reported very broad spectra. This paper reports the first narrowing of the6.8nm peak in a Gd plasma produced by an Nd:YAG laser by using a pre-pulse to reduce self-absorption. The best peak narrowing is observed when the pre-formed plasma was heated by a 1064 nm main laser pulse with duration of 10 ns at the irradiation density of 4×1011 W/cm2 at delay time of 50 ns after a pre-pulse. The observed spectral width of about 0.3nm is about one fifth of the value for no pre-formed plasma. At the same time, the paper discussed the time variation of the two spectral line intensity of 7.2nm and 6.8nm under different pre-pulse conditions. On the other hand, the experiment found the peak wavelength of the 6.8 nm band shifted to longer wavelength and the peak broadened both for lower and higher laser irradiation density. In further the observed spectra are compared with those previously reported in various conditions., Finally, this paper discussed laser produced Gd plasma to reduce debris with pre-pulse, and the debris can be reduced mostly under 50 ns delay time condition.