Ultimate Efficiency of Extreme Ultraviolet Radiation from a Laser-Produced Plasma

An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived. The maximum efficiency is achieved when the plasma expansion distance during laser heating is equal to the laser absorption length. The dependence of the radiation efficiency on the plasma density is confirmed by experiments using a particle-cluster target. By creating a relatively uniform density plasma with a 300 microm diameter by dispersing SnO(2) particles coated on a Si wafer, the conversion efficiency at 14 nm, as high as 4 times as that for a Sn plate target, is achieved.