Study Of Plasma Characteristics Of Gd EUV Light Source

Laser-produced Gd plasma is the best candidate of the next generation lithography because of its small size, high brightness and providing a strong in-band extreme ultraviolet(EUV) radiation around 6.7nm. However, there are still many problems to study laser-produced Gd plasma EUV Source. Firstly, the conversion efficiency of EUV in-band radiation should be improved. Secondly, laser-produced Gd plasma source can emit strong out-of-band(OOB) radiation, which causes seriously damage to the EUV lithography collection system and reduce the photoetching resolution. Thirdly, the debris of plasma source also need to mitigate effectively. All these properties are closely related to the dynamics characteristic of laser-produced plasma. We studied characteristics of laser-produced Gd plasma based on the above factors.In this thesis, an Nd:YAG laser with 10 ns pulse width and output wavelength of 1064 nm is used to irradiate Gd metal target and Gd-doped glass target for studying the EUV in-band and out-of-band radiation of the two targets. The results show that the EUV in-band radiation is mainly caused by the transition of 4d-4f and 4p-4d from Gd17+-Gd25+ions, and the continuum spectrum is responsible for the intense OOB radiation which overwhelms the atomic and ionic line emissions. In order to further study on the characteristics of laser-produced Gd plasma, we compared the spectral evolution of plasma from the two targets. The temporal and spatial behaviors of electron temperature(Te) and density(Ne) of the Gd-doped glass target plasma are also analyzed, and experimental results show that temporal evolution of electron temperature and density of the plasma are found to be decayed exponentially with the increasing of delay time. At 125 ns after laser irradiation,electron temperature and density are 4eV and 1.2×1018cm-3 respectively, and then decreased to 1.5eV and 8×1017cm-3 with delay time of 250 ns. On the other hand, both the spatial evolution of electron temperature and density first increase with the distance away from the target and then decrease in the distance of 1-10 mm away from the target surface. When the probe location away from the target surface 6mm, electron temperature and electron density achieves maximum, 2.6eV and 8.5×1017cm-3 respectively.