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Open Access Publications from the University of California

Using Temporally Synthesized Laser Pulses to Enhance the Conversion Efficiency of Sn Plasmas for EUV Lithography


We have studied the laser pulse shape dependence of the conversion efficiency of λ = 1.03 μm laser pulse energy into 13.5 nm extreme ultraviolet (EUV) emission from a Sn laser-produced plasma. Laser pulses of arbitrary temporal shape ranging from hundreds of picoseconds to several nanoseconds were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. Measurements show that the conversion efficiency favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A 35% increase in conversion efficiency was obtained when Q-switched pulses were substituted by square pulses of a similar duration. Experiments conducted irradiating a Sn target with a sequence of two time-delayed 250 ps pulses showed a 30 percent increase in the EUV yield respect to a single pulse of the same total energy when the pulse separation was optimum at 2.1 ns. This suggests that re-heating of the plasma with delayed laser pulses could be used to improve the EUV yield. The spectroscopic characterization of EUV emission and in-band EUV images that characterize the source size are also presented.

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