Characteristics of ion debris from laser-produced tin plasma and mitigation of energetic ions by ambient gas

Extreme ultraviolet lithography is most promising for the next generation lithography. However, debris from laser-produced plasma, particularly energetic ions, severely decreases the lifetime of extreme ultraviolet optics. We measured the characteristics of ions from tin plasma by the time of flight method with a frequency-doubled Nd: YAG laser at the intensity of 3.5×1010 W/cm2 (532 nm, 8 ns). Our measurement shows that the maximum and peak of tin ions energies from plasma under the above experimental parameters are about 4.2 and 1.8 keV, respectively. Moreover, it is found that kinetic energy angular distribution of tin ions can be fitted by cos0.8(θ), where θ is the angle with respect to the target normal. We also investigated the mitigation effect of argon, helium gases to the tin ions, and found that tin ions from the plasma can be mitigated effectively at the pressure ∼38 mTorr for argon or ∼375 mTorr for helium, respectively.