Two-photon laser-induced fluorescence of hydrogen atom in EUV photoionized plasma

摘要

Hydrogen flow can be applied to buffer the ions produced by the extreme ultraviolet (EUV) source andreduce the deposition rate on collector in the EUV lithography. However, the hydrogen gas can also bephotoionized by the EUV radiation. Detailed information including absolute densities and temperature ofthe production under various conditions is needed in order to understand and predict their long-term impacton highly delicate optics in EUV lithography tools. Hydrogen radical is one of the most importantproduction due to EUV dissociative photoionization. Laser-induced-fluorescence (LIF) is used to observespatial distribution and temporal evolution of the hydrogen radical. In the experiment, hydrogen radical isexcited from 1s~2S to the 3s~2S and 3d~2D levels with two photons near 205.14 nm, and then, the fluorescenceat Balmer-α near 656.28 nm can be observed. The density is determined from the intensity of thefluorescence excitation spectrum, whereas the temperature is inferred from the line shape.