Opacity Effect on Extreme Ultra-Violet Radiation from Laser Produced Tin Plasmas

Opacity effects on the extreme ultra-violet (EUV) emission from laser-produced tin (Sn) plasmas have been experimentally investigated. EUV light source for a microlithography is getting a lot of attention as a challenging application of the laser-produced high-Z plasmas. Laser produced Sn plasma is an attractive 13.5 nm light source due to its compactness and its high emissivity. Opacity as well as emissivity of the laser-produce Sn plasma is quite high for the 13.5 nm light, therefore the 13.5 nm light from deep inside region of the Sn plasma is absorbed strongly during transporting through a surrounding plasma, and is unavailable. The opacity is a critical parameter to investigate optimum conditions for the EUV generation, however no reliable database had been available. Absolute opacity structure of a uniform Sn plasma, whose electron temperature is 30 - 40 eV, has been measured in the EUV range (10 - 20 nm of wavelength) for the first time. Experimental results indicate that control of optical thickness of the laser-produced Sn plasmas is essential to obtain high efficient and narrow band EUV radiation from the laser-produced tin plasmas.