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 $300\mu \mathrm{m}$ diameter by dispersing ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ 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.

Figure 1

(a) Expanding ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ particles at $150\mu \mathrm{s}$ observed by laser scattering. (b) ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ particles were dispersed by giving a laser-induced shock.

Figure 2

Observed EUV efficiency at 14 nm for a Sn plate and for dispersed ${\mathrm{S}\mathrm{n}\mathrm{O}}_2$ particles. The delay time of generating the plasma after the laser-induced shock was $15\mu \mathrm{s}$.

Figure 3

The observed dependence of EUV intensity from a dispersed particle target on delay time of plasma generation after the shock for dispersion. EUV intensity at $50\mu \mathrm{s}$ delay was nearly 4 times higher of that for a Sn plate. The inserted curves show Eqs. (4, 7).