Characterizing and optimizing the density profile of double-pulse laser-produced Sn-based plasmas

The plasma density profile plays a key role in the generation of 13.5 nm light for an extreme ultraviolet lithography (EUVL) source from laser-produced (LP) Sn-based plasmas due to the importance of opacity. We propose to characterize and optimize the plasma density profile to enhance conversion efficiency and mitigate debris using a double laser pulses technique. The basic idea is to separate generation and heating processes of the EUV plasma. Cold plasma is first generated by a low energy prepulse and then a main pulse heats the plasma to temperatures favorable for emitting efficient 13.5 nm EUV light. In this way, the plasma density profile can be controlled by verifying pre-pulse energy and the interval between preand main pulses. Higher conversion efficiency compared with solid density Sn should be expected due to the lower opacity. The minimum number of atoms required to generate efficient EUV light can be obtained using a low energy pre-pulse. And most of the atoms will be ionized; ions are much easier to be stop by applying electric and magnetic fields. In that way, a clean target supply method can be expected. Comprehensive parameters of plasma (density profile), EUV light (conversion efficiency, spectrum, angular distribution), and debris (production, energy spectrum, angular distribution) will be measured in detail. The physics dominating the role of plasma density profile on the properties of EUV light and debris will be investigated by comparing experimental data with the results of radiation hydrodynamic simulations.