A one-dimensional (1D) collisional relativistic particle-in-cell (PIC) code with ionization processes has been developed to investigate the key semiconductor manufacturing device,<i> i.e.</i>, the extreme ultraviolet (EUV) light source from laserproduced plasmas (LPP). Unlike hydrodynamic approach, the kinetic model describes laser heating, energy transport and ultrafast electron dynamics with least approximations. The two major numerical effects of PIC simulations, <i>i.e.</i>, numerical self-heating and numerical thermalization, are also studied and mitigated in the collisional PIC model. The integrated numerical model is achieved by simulating the dense plasma using collisional PIC model and estimating EUV emission and mean opacities according to the respective weighted oscillator strengths of tin ions with charged states varying from 5+ to 13+.
High-efficiency fiber-based extreme ultraviolet driver in the alignment-free configuration has been experimentally achieved with a maximum intensity of 6.4×10<sup>10</sup> W/cm<sup>2</sup> on target at a repetition rate of 20 kHz. The output EUV signal within 10~20 nm in wavelength was confirmed with a Si/Zr-coated x-ray photodiode by varying numbers of Be and Al foil filters. The measured spectral range is consistent with that obtained by the weighted oscillator strengths of Sn<sup>8</sup>+ to Sn<sup>+13 </sup>ions using an one-dimensional hydrodynamic code coupled with the ionization model of collisional-radiative equilibrium. The driver is based on a 1064-nm nanosecond coiled ytterbium all-fiber laser system in diode-seeded master oscillator power amplification. With an overall optical efficiency up to 56%, it can deliver a 1.16-mJ, 117-kW, 6.1-ns laser pulses with a FWHM linewidth of 10 nm and beam propagation factor of M<sup>2</sup>~1.55. The full advantages of using fiber laser for a movable LPP EUV metrology source are revealed.
High power fiber laser amplifier cascade can be simplified using double-pass scheme due to improvement of overall efficiency, especially for amplifiers with small input seed or high stored energy. The yield of stimulated Raman scattering (SRS) in the double-pass scheme is comparable to the level in amplifiers using counter-directional-pumped single-pass scheme if the pumping configuration is appropriate, even though the interaction length becomes twice for double pass scheme. In the study, insertions of Raman strippers along the active fiber with double-pass scheme is proved to be another choice to effectively suppress SRS besides the utilization of photonic band-gap fibers.
Intense nanosecond emission with spectral broadening from 980 to 1600 nm was generated with peak power up to 117 kW, close to the damage threshold of fiber fuse. Both laser amplification and nonlinear conversion were simultaneously employed in a fiber power amplifier giving power scaling free from significant depletion. In a diode-seeded all-PM-fiber master oscillation power amplifier system under all normal dispersion, a core-pumped preamplifier using double-pass scheme can significantly improve the energy extraction. This produced the pulse energy of 1.2 mJ and duration of 6 ns with a conversion efficiency of 66% at the moderate repetition of 20 kHz, which is consistent with the coupled laser rate equations including the stimulated Raman scattering. For the comparable nonlinear strength in each stage from single to few modes, the onset and interplay of four kinds of fiber nonlinearities can be addressed.
By limiting the core diameter of 15 μm at maximum with NA=0.07±0.01 for the near-diffraction-limited output (<i>V</i> <3.6), we successfully generate the pulse with the peak power of 36 kW and the duration of 4.6 ns in FWHM at the repetition rate of 20 kHz. To the best of our knowledge, the signal pulse energy corresponding to 264 μJ is the highest to date in the diode-seeded 15-μm all-fiber MOPA system with the efficiency of 35%. The success in the energy/power scaling is attributed to the further raise of input energy for more extracted energy, the tradeoff between the Raman-limited signal energy and the amplifier slope efficiency for more signal energy ratio, and the proper adjustment of both pump wavelength and power for avoiding coat damage without forced cooling.