Optimization of the size ratio of Sn sphere and laser focal spot for an extreme ultraviolet light source

The effect of the ratio of Sn sphere diameter to laser focal spot size (SD/FSS) on conversion efficiency (CE) from laser to in-band (2%) 13.5nm extreme ultraviolet (EUV) light was investigated by fixing the laser spot size and irradiating variable diameter spheres. It was found that a minimum SD/FSS, i.e., 2.5, is necessary to produce high in-band CE, which is 15% higher than planar targets. Two-dimensional plasma density profile maps showed that the density of the dominant in-band EUV emission region and the size of the surrounding absorbing plasma can be manipulated by geometric effects of the SD/FSS ratio.