Interactions Between Ultrashort Pulses and Laser-Produced Tin Plasmas

Abstract

Inside commercial EUV nanolithography sources, micrometer-sized droplets of liquid tin are irradiated by a high-power CO2 laser, creating a plasma which emits the desired 13.5 nm light. However, small dense liquid spheres are an inefficient target shape for EUV production. To solve this, each droplet is irradiated by two laser pulses. Firstly, a low-energy pre-pulse irradiates the initially-spherical droplet, deforming it via hydrodynamic expansion into a shape more favourable for EUV production. Afterwards, a time-delayed high-energy main pulse irradiates this deformed droplet. Typically, pre-pulses with nanosecond durations are used to deform the target from a sphere into a thin disk. An alternative approach involves using pre-pulses with picosecond or femtosecond durations. These shorter pulses create an intense shock wave on the droplet surface, which propagates through the droplet focusing at its center, where a bubble is formed through a process called cavitation. This bubble rapidly expands, rupturing the droplet, which then breaks up into fine particles. These “cloud” targets have in some cases shown higher conversion efficiencies than disk targets, with the drawback of additional debris created by the violent transformation process. To study the dynamics of cloud targets, we designed and built a laser system capable of producing laser pulses with durations ranging from 220 fs to 100 ps. The system includes a 1064 nm source based on Nd:YVO4 and Nd:YAG, producing pulses with durations between 15 and 100 ps and energies of up to 180 mJ. This system can be used as a standalone source or as a pump for a 1.55 μm KTA-based optical parametric chirped pulse amplifier, producing pulses with durations from 220 fs to 10 ps, with energies up to 10.5 mJ. Ultrashort mJ-level pulses from the OPCPA were used to study the effects of pulse energy and duration on microdroplet deformation. The velocities of the cavitation expansion and of the ablated material are found to decrease for longer pulses, indicating shorter pulses lead to faster, more violent dynamics. Using linearly-polarized pulses, cylindrically-asymmetric shock waves were produced, leading to novel asymmetric target shapes which show good qualitative agreement with smoothed-particle hydrodynamics simulations, highlighting the role of shock waves in the laser-driven deformation dynamics. Charge-state-resolved ion energy spectra were recorded using an electrostatic analyzer to study the processes that shape the energy of the ions emitted by the plasma. For lower laser energies we observe a linear relation between the mean ion energy of each charge state and its ionization number, a trend consistent with the typical electrostatic-driven ion acceleration model. For higher laser energies this relation becomes nonlinear, hinting at effects beyond the simple electrostatic acceleration model. Using the 1064 nm amplifier to irradiate droplet targets with a pair of 50-ps laser pulses, comprising a weaker first pulse with variable energy in the μJ range followed by a more energetic 5 mJ pulse, we produced novel target shapes which were keenly dependent on the energy of the first pulse and the time delay between the pulses. Furthermore, the addition of a small first pulse led to a significant reduction in the kinetic energy of the ions emitted by the plasma, with a 30-fold reduction in the most extreme case. Charge-state-resolved measurements show a reduction of ion kinetic energy for every charge state and a pronounced reduction in the yield of higher-charge-state ions, which are the most energetic. In certain pulse-pair configurations, it becomes possible to completely suppress higher charge states above Sn2+. This suppression of faster ions holds practical significance in extending the lifetime of optical components inside EUV sources when paired with ion mitigation techniques which are most effective for lower-energy ions.